DOF calculations/preview....cou​ple of questions

So, I use this DOF calculator:

http://www.dofmaster.c​om/dofjs.html

I am wondering a few things:
1) Why does it ask for the distance of the subject? Why would that matter
2) In addition to #1, it seems changing the distance of subject only changes the near limit of acceptable sharpenss. Why is that?
3) Does anyone calculate their DOF's in the field?

DOF preview, I'm just not sure how to use it effectively. I press the button but have a hard time interpreting what I'm seeing. Any pointers?

1. Depth-of-field is determined by distance to the subject (among other things)
2. See #1
3. I don't

Re. DOF Preview, when you look through the viewfinder, the lens is wide open. The aperture doesn't stop down until you take the picture. The DOF Preview closes the aperture to the selected aperture while keeping the mirror down. Put on your 50mm on, Av mode, select f/5.6. Focus on a nearby object. The background will be quite blurry when looking through the viewfinder (because the lens is wide open). Press the DOF preview button, and the lens will close down to f/5.6. The view will become darker, BUT you'll notice the background isn't as "fuzzy"

merlin2375 wrote in post #9275669
2) In addition to #1, it seems changing the distance of subject only changes the near limit of acceptable sharpenss. Why is that?

It's probably because you are entering subject distances greater than the hyperfocal distance for that aperture and focal length. When you do that it will show infinity as the far limit of acceptable sharpness, then show an increasingly distant near limit of acceptable sharpness as you increase the subject distance.

Give it a try. Using the default values of 55mm and f/16, put 30 feet for the subject distance. Note both the near and far limit of acceptable sharpness. Now put in 31, then 32, noting the results. Now you are approaching the hyperfocal distance. Now enter 32.1, 32.2, 32.3 etc, up to and beyond the hyperfocal distance, which is 32.8'. Once you get to 32.8' the far limit will become infinity and any subject distance further than that will only affect the near limit of acceptable sharpness. This is because you have exceeded the hyperfocal distance, which is the distance that will give you the greatest depth of field for that aperture and focal length.

Anything before or after that point results in a shallower depth of field.

Mike

merlin2375 wrote in post #9275669
So, I use this DOF calculator:

http://www.dofmaster.c​om/dofjs.html

I am wondering a few things:
1) Why does it ask for the distance of the subject? Why would that matter
2) In addition to #1, it seems changing the distance of subject only changes the near limit of acceptable sharpenss. Why is that?
3) Does anyone calculate their DOF's in the field?

DOF preview, I'm just not sure how to use it effectively. I press the button but have a hard time interpreting what I'm seeing. Any pointers?

Most of Canons L primes still have DoF scales on the lens though not as detailed as older lenses or Leica and Hasselblad Zeiss they still can come in very handy.

airfrogusmc wrote in post #9276713
Most of Canons L primes still have DoF scales on the lens though not as detailed as older lenses or Leica and Hasselblad Zeiss they still can come in very handy.

...only when shooting FF/135 format film. The CofC assumptions for the scales are only suited to that size format.

Wilt wrote in post #9276763
...only when shooting FF/135 format film. The CofC assumptions for the scales are only suited to that size format.

I always forget about the smaller format.

Thanks for the help. I guess I was just confused because my logic was the only inputs should be camera-based. In other words if you shoot a Focal Length X, Aperture Y, on crop factor Z here is your DOF, regardless of anything else.

airfrogusmc wrote in post #9277453
I always forget about the smaller format.

Braggart!

Wilt wrote in post #9277708
Braggart!

size does matter.