# Depth of field



## Daniel (Jun 6, 2004)

Browsing through the photos section, i came across a great example of Depth of Field. this is Scott's Photo. Sorry Scott i should have asked before using it. Hope you don't mind.
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For those of you that are clueless about just what depth of field means.
It is the area between the nearest and farthest points that are in focus.
Now although Scotts picture shows a wonderful pen. I need you to ignore that for just a second. Look closely at the cloth it is laying on. just in front of the pen at the bottom edge of the photo.It has been crumpled up a bit so the line where it comes into focus is a little hazy but you can make out in the texture of the fabric where it is. then look behind the pen and you can make out anouther line where the focus is lost again. the space in between is the Field of View
Notice that Scotts pen happens to land squrely in that field. all of it. It is easy to have something like the clip be outside of it. or if you are taking a picture of a pen end on. the tip may be in focus while the other end is not. 
now you are asking, SO What?
well Field of View can become great fun when you know how to control it. Did you know that keeping the background out of focus will make your subject stand out? It will the human eye is magnetically drawn to the sharpest point of focus. want to make the clip on a pen stand out. leave it the sharpest thing in your photo. but how do you slice the field of view so thin that the clip is on focus while the pen is not.
AHH did that get your attention? I have to admit this is one of the primary reasons I miss my 35mm. I could set this shot up and never look through the view finder for it. so unless you have a digital camera with replaceable lenses. there will be a litle more trial and error envolved. 
  this post is intended to explain the features of the camera that control depth of field rather than the how of getting a specific picture anyway.
 the digital camera is simply put a box that has a sensor in it that reacts to light. this sensor is protected from any light by two things.
one a shutter, Most photographers seem to have little problem getting the idea of what a shutter does. the other is called an Aperture.
and in reverse. most photographers think that word belongs on the list of things you shouldn't say in public. 
So just what is the Aperture  anyway.
there are two things that need to be controled about light so that the camera gets just enough to get the right exposure.  the amount of light for a correct exposure never changes. it takes a given amount of light for the sensor to be able record a viewable image. a dim light requires longer and more. a bright light requires shorter and less.
the shutter controls how long the light gets in, But the Aperture controls How Much!
and fiddling with these two controls will do some mightly creative things for your photos. One of which is to control the Depth of Field.
  the Aperture is basically a hole.Measured In f/stops. what an f stop is is a little more difficult to go into so I will pass for the time being mainly I needed to get that point in here somewhere 
the larger the hole the faster light can pour in, requiring a faster shutter speed. but it is even neater than that. somebody along the way in the develoipment of cameras said. Hey if I make the hole smaller. just how much faster does the shutter need to be. and they proceeded to make out a scale that lists all the hole sizes and shutter speed that need to be used with them. this is known as "Correct Exposure". now this does not mean you will alwasy use Correct Exposure settings. but at least you do not need to take a Shot in the Dark, Pun intended. to get the shot you are looking for.
  Now I most refer to My Canon A-1 to denonstrate. I have Shutter Speed control and Aperture control on this camera. I also have  a neat display when I look through the viewfinder that tells me at what Shutter speed and Aperture the camera is reading for the image. I can get hand held meters as well. My digital will do the same thing but changing the settings is somewhere in all the menues which is harder to describe in writing.
  O.K. the shutter control on my Canon A-1 is a little Dial that has numbers on it, like 1000, 500, 250, notice each number is half of the previous number. this camera goes from 1000 or 1000/th of a second. with the speed getting slower by half (a 500th of a second is twice as long as 1000th of a second) all the way up to 30 seconds (not 30th 30 seconds) with each step along the way being twice as long as the last.
the Aperture control is a ring on the lens that has a bunch of strange numbers like 1.8, 2.8, 4,5.6,8,11,16,22 notice also that these numbers are not twice the value of the proceeding one. 
Now suppose I point my camera out the window at a fairly sun lit tree, which I just did. and the meter in the view finder reads. 125   5.6
this is telling me that the camera is reading the correct exposure to be
125th of a second shutter speed with a hole that is f/5.6
now think of an f/stop number as a fraction. If you replace f with one it makes a little more since. 1/5.6 or roughly one sixth. this would be a smaller hole than say an f/2 or 1/2 right. so remeber an f/5.6 is a smaller hole than an f/2 which is backwards to our thinking.
O.K. I have this camera telling me to take the picture at 125th of a second at f/stop 5.6
well I have a problem. I want a picture of the tree. I want the tree to be what you see when you look at this picture. and there are alot of buildings and cars and what not in the background.these will distract from the tree being the object of my photo. but that is composition and fodder for anouther topic. I also know that the smaller the hole my camera is using the deeper the Depth of field  is going to be. 5.6 about half closed. so i know the background is going to be in focus for a long ways. In fact I can tell you it will be in focus for over 30 feet behind the tree.there is a scale on the lens. but that is also anouther conversation. I want everything from behind the tree to be out of focus but make sure the whole tree stays in focus. 
by making the hole larger I can make less of the background out of focus. 
I can make the Hole (f/stop) larger but to do so I must make the shutter speed faster to keep the correct exposure. for every step I make the hole larger, I make the shutter speed one step faster, and the exposure is still correct.
in this case I estimate the back of the tree to be 15 feet away. and the front of it to be about 10 feet. 
  so I change the shutter speed to 250th of a second and the f/stop to 4 by reading the same scale on the lens that i did before I now have a Depth of Field that starts 10 feet form the camera and ends 15 feet from the camera. the photo will isolated the tree and it will be the only thing in focus.
what IF I could not come up with a shutter speed and Aperture
that produced these distances. I would either have to change lenses
the distances are different for every lens. or move the camera. In the case of having a fixed lens digital camera moving the camera is the only option.
for most of us that do not have all the scales etc on our lenses. My digital does not. the only way I know of figuring out depth of field for a given photo is Experimenting.
and for pens with a digital camera on your livingroom floor. experimenting is cheap. try this with film you are paying to have developed. that's how I learned it.
shoot the first photo with the camera on auto. hopefully you have some way to read the settings it is using. then adjust f/stop and shutter to adjust the Depth of Field as you want it. and yes by being very close you can get the back of the Field down to within an eighth of an inch and even less. A zoom lens also helps narrow the Field one of the characteristics of a telephoto lens is its narrow Depth of Field. a 70-210 mm zoom lens is my favorite lens for taking pictures of people at birhtday parties etc. is makes the subject just jump out at you.  
In short, the larger the hole the narrower the Depth of Field.
the larger the number in the f/stop the smaller the hole.
and the shutter needs to be faster by one step when you make the hole larger by one step....and visa versa.


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## Scott (Jun 6, 2004)

Hi Daniel,

No problem using the picture!  The only thing is that the link only led to my album, not the picture.  Tell me which pen, and I'll fix up a good link to it.

Nice explanation of depth of field!  It's not an easy thing to describe.  Now we need to go through the steps of figuring out how to affect the depth of field on our digital cameras!

Scott.


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## Daniel (Jun 7, 2004)

Scott,
 the photo is reda.jpg
It is on the first page of your album. lower left corner. Unless you know of a better example. I saw a picture of a spider web once that was probably the best example I have ever seen. the effect the Depth of field had on the photo was fantastic. anyway I can't find it again and yours caught my eye.
  I know I can overide the Apateure adn shutter on my camera I just haven't physically gone through the motions. I will try to get soem comparison photos so that people can see the difference in blurring the background. otherwise you can phsically measure from your camera for distances. say take a Shot (this is pure experimenting with your camera) at soemthing that has apecific points of reference. lay a bunch of pens on the ground or soemthing then shoot withthe camera very close to the floor. hope the idea is getting across. take a photo with your camera position marked with tape or something. now load the pic on your computer adn look at what pen is the first and last in focus. keep those measurments in a notebook or somewhere. name the photo with the information or something. once you have a full lighting set up going the depth of field will be the same time adn again withthe same camera. the problem will be if you have a zoom lens and change that setting. soem cameras also have a preview ability that lets you see the Depth of Field before you shoot but this does not work very well for me. Mainly the idea is to experiment knowing what you are trying to achieve. if you know you want the focus to cut off just in back of the pen. then you can easily figure out ways to devise that when you know what factors influence it. it may take a few shots. but there is nothing like taking that photo just the way you intended it to look.
someone recently mantioned wrecking wedding photos with overlit backgorunds. it reminded me of a shot I took of my niece in her Prom Dress. Talk about keep sakes. i was the only one around with a camera and she was adamant about standing right in front of a sun lit window. so I was called to duty just before she would be heading out the door. one chance one shot and it needed to last a lifetime. No pressure. well I got that shot and the lace in her white dress is visable. But it would have been a wasted frame had I not known jsut a little bit about cameras. and I mean a little bit I had one trick I knew to fool the camera and it worked like a charm. a copy of that picture is on my nieces refrigerator 17 years later. it is one of two that are her favorite pictures ever taken. I took them both. Had I just pointed and shot you wouldn't have been able to see her face.


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## jeff (Jun 7, 2004)

To get the link to a photo from the album, click on it so it opens in the popup. Right click on the popup window, then click properties. On the properties, just copy the Address and paste it wherever you need it.

http://www.penturners.org/oldalbums/Scott/reda.jpg


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## Daniel (Jun 7, 2004)

AHA anouther secret that I can tuck under my belt. but you know this means I will only remeber that it can be done. not how to do it. all this cindness has acomplished is that one day you will recieve yet anouther e-mail from me saying. O.K. how do I.....


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## Bev Polmanteer (Aug 15, 2004)

Thank you Daniel, for explaining the depth of field in terms that most of us can understand. Now if womdonw comes up with some magic tricks for digital cameras, we're all set!  Thanks again!


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## Alan Tansey (Sep 2, 2004)

A good overview.
I tend to try to explain DOF by making the eye analogy: if you try to read something in the distance, you squint, and suddenly you can read it. Therefore, smaller the hole (spuinting), the more depth of field. The camera is 'squinting' to see everything. And if you have to squint, then you have to concentrate longer to see it. So does the camera: the more it 'squints', the longer it takes to see the image: shutter speed has to decrease.

Also, the 'Correct Exposure' is based on an 18% gray scale. Good for referencing the zone system, but in color photos, this means that things will tend to be a bit underexposed. With the sunlit tree example, going off the reading from the camera, the tree bark will probably be a bit dark. If you want to see the detail of the bark, you've got to let in more light (less squinting or more concentrating). You will ofcouse sacrifice the deep colors of the tree leaves.

Also, in your example of the Prom Dress backlight, you're right, the A-1 without an AE lock would have washed out the image. However, if you used the Olympus OM2 with matrix metering (or most Nikons today), you wouldn't have needed that trick. Alternatively, my T90 has a spot meter where this wouldn't have been an issue either.

But really, all this is mumbo-jumbo in today's world of digital cameras. If you want a narrow DOF, set the camera on 'action'. Wide? set it on 'Landscape'. And if that fails, or you forget, take about 5 minutes and 'sharpen' or 'blur' the image in photoshop or like program.


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