How to determine acceptable DOF at maximum aperture?

I have been trying to find the proper equations to help me calculate a few things related to DOF and subject magnification. I hope someone can point me in the right direction. Here is what I want to figure out:

1. the camera to subject distance that will yield a one foot acceptable DOF when shooting a person
   
2. the percentage of the frame that a person will fill at this distance
   

Interesting approach. I'd mark 6' on a wall & see what numbers work with it.

frugivore wrote in post #13276053
Here is what I want to figure out:

1. the camera to subject distance that will yield a one foot acceptable DOF when shooting a person
   
2. the percentage of the frame that a person will fill at this distance

You have missed one extremely important point here. That is the choice of the distance that you use between the camera and subject to obtain a desirable perspective. Changing the camera-to-subject distance will change the size relationship between a person's nose and ears, for example.

frugivore wrote in post #13276053
Once I have these distances, I'd like to determine how much of the person will fill the frame. I'm guessing that this is a relatively simple calculation, such as sensor width divided by focal length divided by object (person) height. Am I in the ballpark?

In my opinion, this whole exercise that you've described is rather useless.

While it would be good to know what aperture (f-stop) to choose for various portrait setups, there are a lot of different variables than what you've come up with.

1. The first thing to do is establish the camera-to-subject distances that you want to work with. This will control the perspective in the image.
   
   
2. Then, define the type of portrait you want to make (waist-up, head-n-shoulders, tight on the face, etc.).
   
   
3. With these two variables, you can then determine the focal length you'd want to use for framing the portrait.
   
   
4. Once you have the three items above, then you can calculate the depth of field that you would like to have and that would let you define the aperture (f-stop) that you'd want to use.
   

Skip, the "Perspective" policeman strikes again!

As long as the aperture is the same between focal lengths, your subject will always be framed practically the same no matter what focal length you use if you're attempting to obtain an identical DOF (of course, perspective changes as you move your position between focal lengths). Hope that makes sense

Use this calculator to generate a list of angles of view for each focal length:
http://www.howardedin.​com/articles/fov.html
The tangent of each angle will give you the relation of field of view height to camera-to-subject distance.

tzalman wrote in post #13279332
Use this calculator to generate a list of angles of view for each focal length:
http://www.howardedin.​com/articles/fov.html
The tangent of each angle will give you the relation of field of view height to camera-to-subject distance.

Thanks! That's just what I was looking for Elie. So I guess you've recovered your account?

SkipD wrote in post #13278348
In my opinion, this whole exercise that you've described is rather useless.

While it would be good to know what aperture (f-stop) to choose for various portrait setups, there are a lot of different variables than what you've come up with.

1. The first thing to do is establish the camera-to-subject distances that you want to work with. This will control the perspective in the image.
   
   
2. Then, define the type of portrait you want to make (waist-up, head-n-shoulders, tight on the face, etc.).
   
   
3. With these two variables, you can then determine the focal length you'd want to use for framing the portrait.
   
   
4. Once you have the three items above, then you can calculate the depth of field that you would like to have and that would let you define the aperture (f-stop) that you'd want to use.
   

Skip, while I agree with you that this would be a better method for portraits in most cases, the purpose of determining the values I inquired about is for use in event photography. My thinking is that in this type of environment, one would want to use the largest aperture of a given lens to do two things:

1. gather as much light as possible since it light will be at a low level (discounting artificial light for now)
   
2. put the background out of focus as much as possible since in many cases it will not integrate well with the subject
   

An extremely useful photography calculator web page would be this Photography Calculators web page. It has a section titled Dimensional Field of View Calculator which would be better than an angle of view calculator (which this page also has) for your purpose. There is also a depth of field calculator on the same page.

If you've got an iPhone... the app "pCAM" is a valuable tool.

has a calculator that will show you how much of your subject will be visible at each FL etc.

Combo program with DOF calculactor and FOV calculator, on-line version is found at http://fcalc.net/onlin​e/

They also offer a version that does not require on-line net connection, for a $10 fee.

SkipD wrote in post #13279605
An extremely useful photography calculator web page would be this Photography Calculators web page. It has a section titled Dimensional Field of View Calculator which would be better than an angle of view calculator (which this page also has) for your purpose. There is also a depth of field calculator on the same page.

Perfect Skip! I just did a quick test with 24mm and 50mm FLs.

Focal length: 50mm
Distance for 1' DOF: 10'
Horizontal FOV: 7' 2.4"
Camera: 5DII

Focal length: 24mm
Distance for 1' DOF: 6'
Horizontal FOV: 5' 7.5"
Camera: 7D

So if I had a 7D with the 24L and a 5DII with the 50 f/1.4, and I wanted to get a full body vertical shot at max aperture, I would use the 5DII. Granted, I don't think I'd task myself with memorizing all the values that I get, but it would give me a general idea about which combinations to use for what.

frugivore wrote in post #13279961
So if I had a 7D with the 24L and a 5DII with the 50 f/1.4, and I wanted to get a full body vertical shot at max aperture, I would use the 5DII. Granted, I don't think I'd task myself with memorizing all the values that I get, but it would give me a general idea about which combinations to use for what.

Something you really need to realize about depth of field is that the calculators' results are all based on printing ALL images to a standard size, viewing them from a standard distance, and the assumption that the viewer has a specific visual acuity. Once you change those variables, the calculation results change. The bottom line is that the calculators for DOF should only be used to give you a feel for what would work. Forget the double-digit precision of the calculation results.

frugivore wrote in post #13279961
Perfect Skip! I just did a quick test with 24mm and 50mm FLs.

Focal length: 50mm
Distance for 1' DOF: 10'
Horizontal FOV: 7' 2.4"
Camera: 5DII

Focal length: 24mm
Distance for 1' DOF: 6'
Horizontal FOV: 5' 7.5"
Camera: 7D

So if I had a 7D with the 24L and a 5DII with the 50 f/1.4, and I wanted to get a full body vertical shot at max aperture, I would use the 5DII. Granted, I don't think I'd task myself with memorizing all the values that I get, but it would give me a general idea about which combinations to use for what.

So why not stand at 9' with the 7D and 24mm f/2, get 6.3' x 7.9' FOV with 1.6' DOF ? ? ?

My DOF calculator (link in my signature) will give you field of view measurements after you enter the camera format, focal length and distance.

Here is one way: http://www.amazon.com …Field-Guide/dp/B000YN0VWU