# Depth of Field



## gketell (Nov 2, 2008)

Over the years we've had many discussions about Depth of Field, what affects it and how to control it.  The October 2008 issue of Popular Photography had a great article covering it and I thought I would share it with the group.

_*Depth of field (DoF) describes the distance that's in focus in the foreground and background of a photo.  Using a shallower DoF in your photos allows you to direct your viewer's attention where you want it, while a deeper DoF captures more detail in every part of the scene.  Mastering it will help you create more artful images.  Here's how.*_

_*1) Adjust the size of your aperture.  *The f-stop plays a huge part in depth of field.  When the aperture is wide open (smaller f-stop numbers), it causes the main focal point to be in focus while the rest of the photo is somewhat blurred.  This blur is often called "bokeh."  Smaller apertures produce more depth of field._

_*2) Change your distance from the focal point.*  As you move closer to your main point of focus, the image's depth of field decreases; moving further away increases the depth of field._

_*3) Choose the right focal length for your lens. * At the same distance to your subject, the shorter the focal length (the wider the angle of view) of your lens, the greater your depth of field.  With a longer telephoto lens, depth of field decreases._​The only detail I would add to this is that the size of your camera's imaging sensor (or size of its film) also makes a difference.  Everything else being equal (same f-stop, same distance to object, same focal length), the camera with the bigger sensor will have less depth of field and the camera with the smaller sensor will have more depth of field.  This means that a point and shoot will have more of the photo in focus than a small-sensor dSLR which will itself have more than a full-sensor dSLR/SLR which will also have more than a medium format camera.

For a great write up on how image sensor sizes affect all aspects of your photography you can go to http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm.



GK​​


----------



## VisExp (Nov 2, 2008)

Great points Greg. Your comment about depth of field being inversely proportional to the size of the sensor is interesting. I was not aware of that. Is that just applicable to digital cameras or does it also apply to film? In other words would a 35mm film camera inherently have a greater depth of field than a medium format camera, all other things being equal?

One other point I might add is that the depth of field does not extend equally forward and backward of the focal point. As a rough rule of thumb the distance in focus in front of the focal point is 1/3 and the distance in focus behind the focal point is 2/3.


----------



## gketell (Nov 2, 2008)

Thanks for adding that 1/3-2/3 bit, Keith.  It is very important if you want JUST your pen in focus.  Don't focus on the middle of the pen, focus 1/3 back from the point nearest the camera.

The way that Cambridge in Colour explain the sensor size is this: _This is because *larger sensors require  	one to get closer to their subject, or to use a longer focal length in order  	to fill the frame* with that subject. 
_So basically, the bit about "everything else being equal" is the tough part.  But if you get that, then yes, a 35mm would have a greater depth of field than a medium format and a medium format would have a greater depth of field than a large format.

If you simply think of the physics of light it makes sense, too.  Blur is caused by the light from a single point hitting multiple points on the film/sensor.  That is why a smaller aperture produces a wider DoF: all the light has to come through a single "point" so the spot of the image is reproduced in only one spot.  The bigger aperture allows the single spot of the image to hit multiple points on the sensor/film which causes the blur.  Similarly, the smaller the target the "tighter" the optics have to focus the light to hit it so the less chance of multiple hits occur.  The bigger the target, the "looser" the optics therefore more blur.

GK


----------



## Sylvanite (Nov 2, 2008)

gketell said:


> ... the size of your camera's imaging sensor (or size of its film) also makes a difference.  Everything else being equal (same f-stop, same distance to object, same focal length), the camera with the bigger sensor will have less depth of field and the camera with the smaller sensor will have more depth of field.


I'm sorry Greg, but I don't believe that is correct.  With the f-stop, distance, and focal length, constant, the sensor size does not change which objects are in focus and which are not.  The subject will occupy a smaller area of the larger sensor, but the optical physics do not change with the area size being sampled.

Now, if you change something so that the subject fills the larger sensor, then the depth of field changes.  You can move closer, or use a longer focal-length lens--either of which will reduce depth of field.

Most of this discussion, however, is moot for pen photography.  Few people choose a sensor based on depth-of-field considerations.  Sensor quality, resolution, and lens quality (distortion and vignetting) are much more overwhelming factors.

Focal length doesn't really come into play either.  We typically try to fill the frame when shooting a pen.  In that case, focal-length and distance (except in extreme cases) tend to cancel out each other's effects.  That is, if you keep the magnification constant (fill the frame), a longer focal length lens at a greater distance will have practically the same depth-of-field as a shorter focal length lens closer.

So, in real life terms, aperture is the tool for controlling depth-of-field in a single exposure.

Distance to subject controls foreshortening, and that influences choice of focal length (but that is a different topic).

Regards,
Eric


----------



## gketell (Nov 2, 2008)

Sylvanite said:


> I'm sorry Greg, but I don't believe that is correct.  With the f-stop, distance, and focal length, constant, the sensor size does not change which objects are in focus and which are not.  The subject will occupy a smaller area of the larger sensor, but the optical physics do not change with the area size being sampled.



If the lens were the same on both cameras, you are correct, having the lens from a smaller-sensor camera would simply make a smaller image on the larger-sensor.  (As what happens when you put a Canon dSLR specific lens on a full-frame SLR.)

 But the reality is that the lenses are seldom the same.  The lenses are designed to work with the camera and sensor they are mounted on to get the image onto the sensor in the best way.  And to do this the lenses HAVE to change the optics to force the constant sized object to display the same way on two different sized sensors.  For instance, if I'm at 100 yards with a 80mm lens and shooting at f8 and I'm photographing a 50ft wide side of a house the optics MUST be different to get that same image of the house onto a 6.16 x 4.62mm digicam sensor vs putting it onto a 36 x 24mm full-frame dSLR sensor.  That change in optics changes the focal point of the lens which will affect the depth of field.

Again, this was described on the page by Cambridge in Colour.
_As sensor size increases, the depth of field  	will decrease for a given aperture (when filling the frame with a subject of  	the same size and distance).  This is because *larger sensors require  	one to get closer to their subject, or to use a longer focal length in order  	to fill the frame* with that subject.  This means that one has to  	use progressively smaller aperture sizes in order to maintain the same depth  	of field on larger sensors. _​The way they say it and the way I said it is not quite 100% but the end result is the same: smaller sensors will result in greater depth of field for shots of the same object at the same distance at the same aperture when compared to cameras with larger sensors.


----------



## Sylvanite (Nov 3, 2008)

gketell said:


> ... the lenses HAVE to change the optics to force the constant sized object to display the same way on two different sized sensors.


The change you are talking about is the focal length - which in your first post, you said you were holding constant.  

Sensor size doesn't affect depth-of-field.  Focal length does.


----------



## gketell (Nov 3, 2008)

I think we are nit-picking here.  Given the same lens, changing the focal length will give you a different sized image by moving the image farther from the lens and refocusing.  This is what I think you are describing.

But you can also use materials with different refractive properties, different numbers of lens elements, different curvatures of the lenses to get a different image size for the same distance from the lens.  So 2 different lenses can have the same focal length (distance from the middle of the lens to the image) yet produce different sized images at that same focal length.

Honestly, given the complexity of today's multi-element, multi-material, aspherical lenses I couldn't begin to tell you if those two lenses would have the same theoretical depth of field or not.  Might be a fun experiment when it stops raining around here.

GK


----------



## Sylvanite (Nov 3, 2008)

gketell said:


> I think we are nit-picking here.


Agreed.  Nobody chooses sensor size based on depth-of-field.  There are other, far more important considerations.  Once you've selected your camera body and want to photograph a pen, chances are you will try to fill the frame (to maximize effective resolution).  At that point, focal length and distance generally cancel out each others' depth-of-field effects.  The only variable left is aperture.  

So, to control depth-of-field, adjust the aperture.



> Given the same lens, changing the focal length will give you a different sized image by moving the image farther from the lens and refocusing.  This is what I think you are describing.


I don't understand what you are saying here.  All the discussion so far as assumed a constant distance and focusing hasn't come up at all.



> But you can also use materials with different refractive properties, different numbers of lens elements, different curvatures of the lenses to get a different image size for the same distance from the lens.  So 2 different lenses can have the same focal length (distance from the middle of the lens to the image) yet produce different sized images at that same focal length.


Focal length is an optical property, not a physical one.  It is not the distance from the middle of the lens to the image.  Case in point - I'm looking at a 28-70mm zoom lens that is physically longer at 28mm than at 70mm.  Two interchangable lenses with the same "focal length" will produce the same size image on the sensor regardless of lens size, number of elements, and refractive index of those elements.  



> Honestly, given the complexity of today's multi-element, multi-material, aspherical lenses I couldn't begin to tell you if those two lenses would have the same theoretical depth of field or not.  Might be a fun experiment when it stops raining around here.


Lens design has come a long way in the past 20 years, but the relationship between focal length and depth-of-field is a function of magnification, not construction.  Interchangable lenses of the same focal length will exhibit the same depth-of-field (all else being equal).

Regards,
Eric


----------



## Daniel (Nov 3, 2008)

Greg, Thanks for the added info. I did notice at the get together nobody really had to have what depth of field is. This is a striking comparison to just a few years ago when hardly anyone knew what it was. Going deeper into the details of just how to get it the way you want it is good info to have.


----------



## gketell (Nov 3, 2008)

Sylvanite said:


> So, to control depth-of-field, adjust the aperture.



Given a fixed camera body and lens combination, Absolutely.



Sylvanite said:


> Focal length is an optical property, not a physical one.



It is both.  For a simple, single element lens the definition of focal length is the measurement from the center of the lens to the point the image of the object in front of the lens appears in focus behind the lens (the focal point).  But that measurement will change with both the optical qualities of the "glass" that the lens is made out of (compare the "coke-bottle eye-glasses to the same correction "high refractive" lenses; the latter are as much as 75% thinner) and the physical grind of the lens (shallower lenses have a focal point further from the lens than do more heavily curved lenses).



Sylvanite said:


> It is not the distance from the middle of the lens to the image.


Ok, here we need to be specific and differentiate lens vs LENS.  The first is a single element piece of "glass"; the second is a complex camera LENS made up of many lenses working together.

In the case of a lens, it is indeed middle of lens to focal point.  In the case of LENS it is not.  (http://www.bobatkins.com/photography/technical/measuring_focal_length.html)



Sylvanite said:


> Case in point - I'm looking at a 28-70mm zoom lens that is physically longer at 28mm than at 70mm.  Two interchangable lenses with the same "focal length" will produce the same size image on the sensor regardless of lens size, number of elements, and refractive index of those elements.



If the camera LENS makers decides it should.  But it doesn't have to.  For instance take any of the camera LENSes designed for the Canon APS-c sized sensors like those used by the 10D, 20D, 30D, 40D.  If you put that lens onto a full-size sensor camera like the Canon 5D or the Canon 1Ds then you will get a smaller image on the sensor than if you put a LENS designed for the full-size sensor.  And it won't matter what you set the LENS to, the image will ALWAYS be too small for the sensor because it was designed with a image that focuses at a different focal-point and therefore different focal-length so can't come to focus with the image bigger than it was designed for.



Sylvanite said:


> Lens design has come a long way in the past 20 years, but the relationship between focal length and depth-of-field is a function of magnification, not construction.



I humbly disagree based on training and all the reading I've done over the past couple of days for this thread.  ((Thanks for the great discussion leading to better understanding of how all this stuff works.))



Sylvanite said:


> Interchangable lenses of the same focal length will exhibit the same depth-of-field



OK, being specific again.  Using a single camera body with its specific sensor and using LENSes designed for that specific camera/sensor, this is a true statement.  But this whole discussion started because we were talking about different cameras with different sized sensors with their own LENSes set at the same focal-length.  In this case, DoF may not be the same between a picture taken by camera A and those taken by camera B even if all the settings were the same.



Sylvanite said:


> (all else being equal).



And that is probably the rub, right there.  When dealing with systems as complex as these cameras are becoming, what exactly does "equal" mean??  On one of the pocket cams I have it has a 7x zoom in a camera only 7/8" thick.  They accomplish this by having a mirror behind the LENS and the sensor on the bottom of the camera facing up.  Now you have to add the refractive property of the mirror's glass and the reflective property of the mirror to the equation of the camera's LENS system.  blech.

Back to your original point: there are very very few people who will decide on a camera based purely on the depth of field of the sensor/LENS combo.  Once you have chosen your camera body (with associated sensor) and the LENS you will use, your most effective way of controlling the depth of field is, indeed, the aperture.  But if you DO want to be able to have lots of good Bokeh with your background and foreground thoroughly out of focus then you don't want a small sensor.  That is all I was pointing out in my original post.

GK


----------



## Sylvanite (Nov 4, 2008)

gketell said:


> Ok, here we need to be specific and differentiate lens vs LENS.  The first is a single element piece of "glass"; the second is a complex camera LENS made up of many lenses working together.


I am specifically talking about modern camera lenses here, not eyeglass lenses, magnifying glass lenses, or pinhole camera lenses.  For modern camera lenses, "focal length" refers to the magnification at the "focal plane" (where the sensor or film lies).



> If the camera LENS makers decides it should.  But it doesn't have to.  For instance take any of the camera LENSes designed for the Canon APS-c sized sensors like those used by the 10D, 20D, 30D, 40D.  If you put that lens onto a full-size sensor camera like the Canon 5D or the Canon 1Ds then you will get a smaller image on the sensor than if you put a LENS designed for the full-size sensor.  And it won't matter what you set the LENS to, the image will ALWAYS be too small for the sensor because it was designed with a image that focuses at a different focal-point and therefore different focal-length so can't come to focus with the image bigger than it was designed for.


No, that is not correct.  If you put a Canon EF-S lens on a 5D or 1Ds (if it even works - I don't know if the body will reject the lens or not), you won't wind up with a smaller image.  Let's take a specific example:

Put a Canon EF-S 18-55mm  f3.5-5.6 IS Lens on a Canon 5D (on a tripod), zoom it to 50mm focal length, set the aperture to f8.0, focus, and take a picture.  Now, without changing anything except the lens, switch to a Canon EF 50mm f1.2 L USM Lens.  Keep the aperture at f8.0, focus on the same spot, and take another picture.  The image will be the same size, and have the same depth of field in both shots.  The difference will be that the first picture will exhibit significant vignetting, poor corner sharpness, and pretty bad chromatic abberation, pincushion distortion, and barrel distortion in the corners.  The second picture will be bright, sharp, and clear across the entire photo.

Likewise, if you perform the same experiment with the same two lenses, but use a Canon 40D body instead (for which both lenses were designed), you will get the same size picture both times.  The difference will be far less dramatic, but the "L" lens shot will be a little better.



> ... we were talking about different cameras with different sized sensors with their own LENSes set at the same focal-length.  In this case, DoF may not be the same between a picture taken by camera A and those taken by camera B even if all the settings were the same.


Again, if focal length, distance, and aperture are the same, then depth-of-field is the same.  Let's take another example:  
Take a camera with a APS-C sensor, the Canon 40D (on a tripod), put a Canon EF 50mm f1.2 L USM Lens on it., set the aperture to f8.0, and focus on an object that fills the frame.  Take a picture.  Now swap out the camera body with a Canon 5D (35mm sensor).  Using the same lens and same aperture, focus on the same spot and take another picture.  The two images will have the same depth-of-field.  The difference is that the second shot has a wider field of view than the first.  If you keep the distance, focal length, and aperture constant (as stated in the original post), then the depth-of-field is the same, regardless of sensor size.

Now let's turn that example around:
Put the Canon 5D on the tripod, with the Canon EF 50mm f1.2 L USM Lens on it, set the aperture to f8.0, and focus it on an object that fills the frame.  Take a picture.  Now change both the camera and lens to the Canon 40D with a Canon EF-S 18-55mm f3.5-5.6 IS Lens.  Set the focal length to 31mm ,aperture to f8.0, and focus on the same spot.  The image will fill the frame.  Take a picture.  Both pictures will have the same field of view, but the second picture will have greater depth-of-field -- not because the sensor is smaller, but because we used a shorter focal length lens.



> Back to your original point: there are very very few people who will decide on a camera based purely on the depth of field of the sensor/LENS combo.


I hope there are none, because the other considerations (sensor sensitivity, noise, resolution, and lens quality requirements) so far outweigh depth-of-field as to make it meaningless.



> But if you DO want to be able to have lots of good Bokeh with your background and foreground thoroughly out of focus then you don't want a small sensor.


No, what you want is a good lens, with a wide maximum aperture, and a quality sensor that you can set to the appropriate sensitivity (ISO) with minimum noise so you can use the aperture you need to get the desired depth-of-field at the shutter speed you want to use in the existing lighting conditions. 

Then again, how often do you want a shallow depth-of-field when photographing pens?  Usually, we want _greater_ depth-of-field.  I'm not, however, going to suggest people run out and buy a Canon 1Ds Mk III just to get a little more depth-of-field in their pen pics.  I fear this discussion has become way too complicated for this forum, especially when the answer still boils down to "control depth-of-field via aperture".

Regards,
Eric


----------



## gketell (Nov 4, 2008)

Sylvanite said:


> No, that is not correct.  If you put a Canon EF-S lens on a 5D or 1Ds (if it even works - I don't know if the body will reject the lens or not),



you will break the camera because the EF-s LENS is longer so will strike the sensor of a full-frame camera.



Sylvanite said:


> you won't wind up with a smaller image.  Let's take a specific example:



You are getting off the original topic of different cameras with different lenses.  But let's continue for a bit.



Sylvanite said:


> Put a Canon EF-S 18-55mm  f3.5-5.6 IS Lens on a Canon 5D (on a tripod), zoom it to 50mm focal length, set the aperture to f8.0, focus, and take a picture.  Now, without changing anything except the lens, switch to a Canon EF 50mm f1.2 L USM Lens.  Keep the aperture at f8.0, focus on the same spot, and take another picture.  The image will be the same size, and have the same depth of field in both shots. The difference will be that the first picture will exhibit significant vignetting,



What is vignetting but the light from the image not hitting the entire sensor?  If the image covered the entire sensor then there would be good light all the way to the corners.  Since the image is hitting in the middle of the sensor the edges and corners are dark, hence vignetted.  



Sylvanite said:


> poor corner sharpness, and pretty bad chromatic abberation, pincushion distortion, and barrel distortion in the corners.  The second picture will be bright, sharp, and clear across the entire photo.



Because the image hits the full sensor, not just the middle.




Sylvanite said:


> Likewise, if you perform the same experiment with the same two lenses, but use a Canon 40D body instead (for which both lenses were designed), you will get the same size picture both times.  The difference will be far less dramatic, but the "L" lens shot will be a little better.



Not according to the manufacturers or the experts who have written about this in detail.  See the many references I've already posted.  With the smaller sensor you have a 1.6x "magnification factor" which is caused by the fact that the smaller sensor is only picking up the center portion of the image created by a lens for a full-size sensor so an "L" lens set at 50mm appears to be an 80mm effective setting on the 40D.  So to get the same size image you have to change the focal length of the "L" lens down to 50mm/1.6=31.25mm if you want the same image.  And going to a wider image results in a change to the DoF, specifically you get a more DoF.  See my previous reference.



Sylvanite said:


> Again, if focal length, distance, and aperture are the same, then depth-of-field is the same.  Let's take another example:
> Take a camera with a APS-C sensor, the Canon 40D (on a tripod), put a Canon EF 50mm f1.2 L USM Lens on it., set the aperture to f8.0, and focus on an object that fills the frame.  Take a picture.  Now swap out the camera body with a Canon 5D (35mm sensor).  Using the same lens and same aperture, focus on the same spot and take another picture.  The two images will have the same depth-of-field. The difference is that the second shot has a wider field of view than the first.
> 
> You will have a much broader field of view in the picture.


Exactly!!!  You don't have the same picture at all.  That isn't what I've been talking about at all.  I'm talking about instances where you get the same picture on different cameras/LENSes.




Sylvanite said:


> If you keep the distance, focal length, and aperture constant (as stated in the original post), then the depth-of-field is the same, regardless of sensor size.



So are you talking focal-length as set on the LENS or effective focal-length of the camera+LENS combination?  If you adjust the camera/LENS so the effective focal-length remains the same as you move the camera body/LENS around, then yes, DoF stays the same.  But if the as-set focal-length remains the same then the effective focal-length changes and your DoF changes with it.




Sylvanite said:


> Now let's turn that example around:


Again, not what we were originally talking about, but OK.



Sylvanite said:


> Put the Canon 5D on the tripod, with the Canon EF 50mm f1.2 L USM Lens on it, set the aperture to f8.0, and focus it on an object that fills the frame.  Take a picture.  Now change both the camera and lens to the Canon 40D with a Canon EF-S 18-55mm f3.5-5.6 IS Lens.  Set the focal length to 31mm ,aperture to f8.0, and focus on the same spot.  The image will fill the frame.  Take a picture.  Both pictures will have the same field of view, but the second picture will have greater depth-of-field -- not because the sensor is smaller, but because we used a shorter focal length lens.


So this whole discussion really has been a potAto, potAHto conversation.  We've been talking past each other.  You are absolutely correct, if you CHANGE the focal-length of the LENS when you change the camera/LENS then you will have a different DoF.  My entire conversation has been around using the same focal-length on different sensors which would yield different results.



Sylvanite said:


> I hope there are none, because the other considerations (sensor sensitivity, noise, resolution, and lens quality requirements) so far outweigh depth-of-field as to make it meaningless.



For what you and I are doing, sure.  But we can't speak for everyone out there.



Sylvanite said:


> No, what you want is a good lens, with a wide maximum aperture, and a quality sensor that you can set to the appropriate sensitivity (ISO) with minimum noise so you can use the aperture you need to get the desired depth-of-field at the shutter speed you want to use in the existing lighting conditions.



Quite.



Sylvanite said:


> Then again, how often do you want a shallow depth-of-field when photographing pens?



Me personally?  Every time.  I want the pen in focus and everything in front of the pen and behind the pen soft so that the viewer's eyes are drawn to the pen.




Sylvanite said:


> Usually, we want _greater_ depth-of-field.  I'm not, however, going to suggest people run out and buy a Canon 1Ds Mk III just to get a little more depth-of-field in their pen pics.  I fear this discussion has become way too complicated for this forum, especially when the answer still boils down to "control depth-of-field via aperture".
> 
> Regards,
> Eric



Yup, I agree.  It was fun having the discussion though.

GK


----------



## Sylvanite (Nov 5, 2008)

gketell said:


> For instance take any of the camera LENSes designed for the Canon APS-c sized sensors like those used by the 10D, 20D, 30D, 40D. If you put that lens onto a full-size sensor camera like the Canon 5D or the Canon 1Ds...





> you will break the camera because the EF-s LENS is longer so will strike the sensor of a full-frame camera.


In that case, let's not actually do it, but the thought experiment still holds.



> What is vignetting but the light from the image not hitting the entire sensor?  If the image covered the entire sensor then there would be good light all the way to the corners.  Since the image is hitting in the middle of the sensor the edges and corners are dark, hence vignetted.


Yes, that is what vignetting is.  All camera lenses exhibit some degree of vignetting.  Really good lenses have so little vignetting that it is not discernable across the entire sensor (or film frame).  Poor lenses will have as much as 1 (or even 2) f-stops worth of exposure difference from the center to the corner of the frame.  Other image quality measurments (such as chromatic abberation, corner focus, and distortion) are likewise less pronounced near the center of the image than near the edges.

However, a lens considered poor when casting an image on a 35mm frame might still cast a pretty good image on a 22mm frame.  Think of it as using the "sweet spot" of the lens.  This is one of the valid reasons for choosing an APS-C size sensor over a 35mm sensor.  You can get good image quality out of cheaper lenses.  In fact, that is the seminal idea behind the EF-S lens series.  Make lenses that physically fit the same mount, but use lighter materials and less expensive designs that don't strive for image quality outside the smaller frame.



> Not according to the manufacturers or the experts who have written about this in detail.  See the many references I've already posted.


I'll refer you back to your chosen source http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm.  This page discusses the "crop factor", which is the portion of the focal plane that the sensor samples.  A larger sensor samples a larger area and therefore has a wider "angle of view".  It does not change how the lens works.



> With the smaller sensor you have a 1.6x "magnification factor" which is caused by the fact that the smaller sensor is only picking up the center portion of the image created by a lens for a full-size sensor so an "L" lens set at 50mm appears to be an 80mm effective setting on the 40D.


"Appears" is a slippery word here.  A 50mm lens is a 50mm lens, no matter what camera you put it on.  The whole "magnification factor", or "35mm-equivalent lens" is smoke and mirrors.  In essence, you have to preface each statement with "if you were using a 35mm camera, this would be 'like' using a..."  A 50mm lens on an APS-C camera is "like" using a 80mm lens on a 35mm camera.  A 50mm lens on 2 1/4 square format camera is "like" using a 28mm lens on a 35mm camera.  A 50mm lens on a 8x10 format camera is "like" using a 6mm lens on a 35mm camera.  A 50mm lens on a Canon 5D body is "like" using a 31mm lens on a Canon 40D, a 85mm lens on a 2.25"x2.25" camera, or a 450mm lens on an 8"x10" camera.  These statement are all about "filling the frame", which is about the field of view - a combination of lens and film frame.  It is a mistake to think the lens changes when you put it on a different body.



> Everything else being equal (same f-stop, same distance to object, *same focal length*), the camera with the bigger sensor will have less depth of field and the camera with the smaller sensor will have more depth of field.





> So to get the same size image you have to *change the focal length* of the "L" lens down to 50mm/1.6=31.25mm if you want the same image.  And going to a wider image results in a change to the DoF, specifically you get a more DoF.


Which is it?  Are you changing the focal length or not?  If you change the focal length, you'll change the depth-of-field.  If you don't, you won't.  Depth-of-field depends on focal length, not sensor size.



> So are you talking focal-length as set on the LENS or effective focal-length of the camera+LENS combination?


I am speaking of the actual focal length of the camera lens, not the so-called "effective focal-length".  

Let's suppose we made test prints of our pictures, measuring 24x35mm (like 35mm contact prints).  The Canon 5D shots would require no digital magnification (a 1.0 multiplier).  The Canon 40D shots would require 60% (a 1.6 multiplier) enlargement.  Since all we're doing is manipulating the print size, it does not affect depth-of-field.  The point is that there are two kinds of magnification going on here.  Optical magnification is done by the lens.  Digital magnification is done to enlarge the 22mm APS-C sensor image to 35mm.  Depth-of-field is related to the optical magnification but not the digital magnification.  "Actual" focal length is a measurement of the optical magnification.  "Effective" focal length is a comparison term that combines optical and digital magnification and is therefore more subject to confusion.



> If you adjust the camera/LENS so the effective focal-length remains the same as you move the camera body/LENS around, then yes, DoF stays the same.


Are you now saying that a 31mm lens on a Canon 40D, it will produce the same depth-of-field as a 50mm lens on a 5D (both "effective" focal length 50mm)?  That's the exact opposite of what you've said before and is incorrect.  Depth-of-field is a function of _actual_ focal length, not "effective" focal length.  Let's perform another thought experiment.

Take a Canon EF 200mm f2.0 L IS USM Lens (chosen simply because it has a fixed focal length and a tripod mount) and mount it on a tripod.  Attach a Canon 40D body, focus on a subject, and take a picture.  Now, without changing anything else, remove the 40D body and attach a 1Ds Mk III body.  Take another picture using the same ISO setting and exposure (specifically f-stop).

Now, you will tell me that the first picture had an "effective" focal length of 320mm, based on the fact that the 40D has a smaller sensor, and therefore a narrower angle of view.  Nevertheless, nothing about the lens changed, and nothing about the light passing through it changed.  The first picture will exhibit a tighter "crop" than the second one, but they will have the exact same depth-of-field.

If you disagree, then please explain how a different sensor changed what was in focus and what was out of focus when the image on the focal plane is identical.



> But if the as-set focal-length remains the same then the effective focal-length changes and your DoF changes with it.


I don't know what you mean by "as-set focal-length".  Can you elaborate?



> So this whole discussion really has been a potAto, potAHto conversation.  We've been talking past each other.


Perhaps.  We may mean a different thing by "size" (and no nearsighted blonde jokes, please).  When I say any lens of the same (actual, not effective) focal length will produce the same size image, and the same depth-of-field, I mean the size of the image cast on the focal plane.  I don't mean the the size proportional to the angle of veiw (a.k.a. size in the frame, or "crop factor").



> You are absolutely correct, if you CHANGE the focal-length of the LENS when you change the camera/LENS then you will have a different DoF.


Whether you change the camera or not, when you change the actual focal length (or more accurately, the optical magnification), the depth-of-field changes.



> My entire conversation has been around using the same focal-length on different sensors which would yield different results.


Again, this may be conflict in terminology (which is why I dislike the attempt to couch everything in 35mm equivalent terms).  If you use the same _actual_ focal length lens on different size sensors, your photos will have different angles of view, but the same depth-of-field.  If you use the same _effective_ focal-length (which is a different actual focal length), at the same distance, your photos will have the same angle of view, but different depth-of-field.



> I want the pen in focus and everything in front of the pen and behind the pen soft so that the viewer's eyes are drawn to the pen.


That is probably the best statement that has come out of this discussion!  If you would permit me, I'd like to elaborate a little, because unlike most everything else we've said, this point is truly useful to pen photography.  

When you want the viewer's eye drawn to a particular subject, one useful tool is to keep that subject in sharp focus while everything else is (at least slighty) blurry.  In photographic terms, you want the depth-of-field to encompass the subject, but not the foreground nor background.  The goal is not minimum depth-of-field, nor maximum depth-of-field, but rather the correct depth-of-field to isolate the pen.

As Keith pointed out, depth-of-field extends farther beyond the point of focus than in front of it (because depth-of-field varies with distance).  So, after choosing your camera, and arranging your photo, you want to focus on a point somewhat closer than the median distance, and pick an f-stop that will bring the depth-of-field out to just encompass the pen.

Thinking about how you want your pen presented, and composing your shot to achieve that goal makes a far greater difference to your photo than the brand and model of your equipment.  Or, as an old instructor of mine used to say "what's behind the camera is more important than what's inside it".



> It was fun having the discussion though.


Yep.

Regards,
Eric


----------



## marcruby (Nov 5, 2008)

I'm definitely not clear on the logic of this size of sensor thing.  I was (and still am) a user of multiple formats.  The only thing size of sensor affects is what is a 'wide' angle lens on the particular format.  So a 50mm (for a 35mm camera) and an 80mm (for a medium format) have almost the same working distances and almost the same depth of field.  That much I'm sure of.

The other thing you are leaving out is that the smaller 'sensor' can't bear the same amount of enlargement.  Most digital systems interpolate to enlarge, which introduces a certain amount of distortion, much like grain in a silver negative is the limitation on image clarity.  There's quite a difference between a 35mm shot enlarged to 11x14 and a contact print of an 11x14 negative.  Not that I'd recommend using an 11x14 camera to take a picture of a pen.

Marc


----------



## gketell (Nov 5, 2008)

I think it is safe to say that we won't be coming to agreement on this.  We seem to each be arguing about different things and we can go on forever without coming to consensus since we are talking about different things.

It was fun.  Now it is time for something else.
GK


----------



## Sylvanite (Nov 5, 2008)

gketell said:


> I think it is safe to say that we won't be coming to agreement on this.  We seem to each be arguing about different things and we can go on forever without coming to consensus since we are talking about different things.
> 
> It was fun.  Now it is time for something else.
> GK



I wish we could come to agreement on what we're holding constant, and what we're changing, and what the terminology means.  Unfortunately, without that, you're right that we aren't going to agree (when I suspect we probably mean the same thing, after all).

And, I agree we should move on. 

Regards,
Eric


----------



## Sylvanite (Nov 5, 2008)

At the risk of opening up this argument again, I'll reply - but I'll try to be brief, and I'll back up my statements with quotes from Greg's authoritative source.



marcruby said:


> I'm definitely not clear on the logic of this size of sensor thing.  I was (and still am) a user of multiple formats.  The only thing size of sensor affects is what is a 'wide' angle lens on the particular format.  So a 50mm (for a 35mm camera) and an 80mm (for a medium format) have almost the same working distances and almost the same depth of field.  That much I'm sure of.


Yes, and no.  The only thing size of sensor directly affects is "angle of view" (sometimes also called "field of view").  To quote cambridgeincolor.com:

_The lens focal length does not change just because a lens is used on a different sized sensor-- just its angle of view.  A 50 mm lens is always a 50 mm lens, regardless of the sensor type. _

What is "wide-angle" and what is "telephoto" is, to some extent, a matter of convention.  It is roughly based on the diagonal length of the film (or sensor) size, but has historically been shifted to more convenient numbers.  Basically, for 35mm film (24x36mm), the standard neutral focal length is 50mm.  Shorter focal lengths are considered wide-angle.  Longer ones are considered telephoto.  To achieve about the same field of view as a 35mm camera with a 50mm lens, you would use the following (numbers taken from the calculator on cambridgeincolor.com):

A 32mm (approximately) lens on a camera with an APS-C sensor,
A 90mm lens on a 60mm square format camera,
A 230mm lens on a 4x5 format camera, or
A 450mm lens on a 8x10 format camera.
For each format, shorter lenses are considered wide-angle, longer ones telephoto.

The angle of view will be about the same, but the depth-of-field will not.  Again, quoting cambridgeincolor.com:

_As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance).  This is because *larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame* with that subject._

In other words, sensor size does not directly affect depth-of-field, but to achieve the same angle of view with a larger sensor, you have to either get closer, or use a larger focal length lens - either of which will reduce depth-of-field.

If you want to switch formats but maintain the angle-of-view, perspective and depth-of-field, you need to hold distance constant and change both focal-length and aperture.  Again, I'll refer you to cambridgeincolor.com for their online "Depth of Field Equivalents" calculator.



> The other thing you are leaving out is that the smaller 'sensor' can't bear the same amount of enlargement.  Most digital systems interpolate to enlarge, which introduces a certain amount of distortion, much like grain in a silver negative is the limitation on image clarity.  There's quite a difference between a 35mm shot enlarged to 11x14 and a contact print of an 11x14 negative.


Yes.  Resolution is a very important difference.  Other differences include diffraction, noise, dynamic range (sensitivity), lens availablity, and cost.  I've argued that these other considerations generally far outweigh that of depth-of-field, especially for pen photography.

In general (but not always, because technology and manufacturing quality play a large role), larger sensors will be more capable than smaller ones.  One last quote from cambridgeincolor.com:

_larger sensors generally provide more control and greater artistic flexibility, but at the cost of requiring larger lenses and more expensive equipment.  This flexibility allows one to create a shallower depth of field than possible with a smaller sensor (if desired), but yet still achieve a comparable depth of field to a smaller sensor by using a higher ISO speed and smaller aperture (or when using a tripod)._

Regards,
Eric


----------

