Calculating DOF

[timeUnit]

Hi!

Technical question here: how does one calculate DOF at a given film format, focal length, distance and aperture?

And question number two: most texts I’ve read state that DOF is dependent on the focal length, but I find that slightly illogical. Isn’t it so that DOF is dependent on (1) distance to principal plane of focus relative to the focal length, (2) aperture size, and to some degree (3) film format (larger film format allows longer focal length with the same angle of view/distance to PFF).

Thanks!

 

[chug]

Maybe this will help..

http://www.dofmaster.com/dofjs.html

Cheers

 

[denishr]

FWIW, I've also read that same focal length always has the same DOF. Other variables are film format and aperture, if I remember correctly.
There are many DOF calculators on the Web - including the one linked by chug above...

 

[timeUnit]

Thanks!

I couldn’t really find the answer to the second question, but the DOF calculator is great!

 

[richard_l]

Formulas used by my f-calc software:

h = hyperfocal distance
A = aperture
c = circle of confusion diameter
s = subject distance
f = focal length
N = nearest in-focus distance
F = farthest in-focus distance

h = f^2/(Ac)

N = hs/[h+(s-f)]

F = hs/[h-(s-f)]

DOF = F - N

Different formats can be accomodated by changing the circle of confusion (COC). For example, the DOF scales on Zeiss 35 mm lenses use 0.025 mm for the diameter c of the COC. This is 1/1730 of the diagonal of the 35 mm film frame (which is 43.27 mm). For a different format, you could use 1/1730 of the frame diagonal in the formulas. For larger formats this will give a greater DOF.

 

[Stu :)]

Or the lazy method....

Buy a PalmPilot or PocketPC and download the software from www.tucows.com 😀

Stu 🙂

 

[timeUnit]

That’s what I'm talking about, at equal distance, equal focal length and equal aperture, DOF will be equal and only the angle of view will change between the two cameras. I’m not confused. 🙂

What I’m asking is this:

Why are many people stating that shorter focal length lenses have longer DOF than longer focal length lenses? This seems to me untrue. The DOF is dependent on distance relative to focal length + aperture. Right?

Consider this:

Two 35 mm cameras, one with 35 mm / 1.4 lens and one with 75 mm / 1.4 lens, focused at 1 meter, both with aperture 2.8. The things that have changed are the focal length and therefore the distance relative to the focal length. The DOF will be different. If the distance relative to the focal length had remained the same, i.e. one had focused farther away with the 75 mm lens, the DOF would be the same. Right?

henning

 

[richard_l]

Yes, focussing farther away (around 2 m) with 75 mm lens will give about the same DOF as
1 m with 35 mm lens.

 

[timeUnit]

richard,

I guess that there is a value with which one can calculate at what PPF distance different focal lenghts have the same DOF at equal aperture. It’s probably right in front of me in your formulas, but my algebra isn’t what it used to be...

Thanks for your insights!

 

[denishr]

What I’m asking is this:

Why are many people stating that shorter focal length lenses have longer DOF than longer focal length lenses? This seems to me untrue. The DOF is dependent on distance relative to focal length + aperture. Right?

I guess it's subjective "feeling" or perception of DOF.
On the other hand, 135mm lens on a Leica and a 135mm lens on a Mamiya TLR (6x6 format) will have the same DOF - but, since the format is different, the perception of DOF changes....

Denis

 

[timeUnit]

I guess it's subjective "feeling" or perception of DOF.

Yes, of course. I’m not really out to discuss the use or feeling of DOF, I’m just hungry for knowledge. And since I found so many strange answers on the web stating that a focal length x has an inherent DOF, I wanted to ask around. I guess most of us agree on this one. Thanks all!

henning

 

[Hugh T]

Consider this:

Two 35 mm cameras, one with 35 mm / 1.4 lens and one with 75 mm / 1.4 lens, focused at 1 meter, both with aperture 2.8. The things that have changed are the focal length and therefore the distance relative to the focal length. The DOF will be different. If the distance relative to the focal length had remained the same, i.e. one had focused farther away with the 75 mm lens, the DOF would be the same. Right?

Things aren't quite that simple. The equations aren't linear with respect to focal length and distance. Another equation to consider is the Scale of Reproduction = (Focal Length)/(Object Distance-Focal Length) obtained fron The Leica Manual.
Using your example of a 35 mm lens focused at 1 m, the depth of field is from 0.95 to 1.06 m. To get the same image height with a 75 mm lens, the object has to be 2.14 m away with a depth of field from 2.09 to 2.20 m. To 2 decimal places, the depth of field is the same at 0.11 m. However, with the 35 mm lens focused at 10 m, the 75 mm lens would be focused at 21.43 m for the same image height. This gives a depth of field of 16.9 m for the 35 mm lens and 12.3 m for the 75 mm lens. I used the Pocket PC DOF program from http://home.comcast.net/~jonsachs/
For short focus distances, the depth of field for your example is the same for practical purposes but differs at longer focus distances.

Hugh T

 

[timeUnit]

Ok, Hugh! Thanks!

If angle of view (i.e. image height) was equal as well, one could say that all lenses are identical, and the only difference is how close you are to the subject. But I understand that’s not how it is, and I’m content with that.

The only point I have, although I find all this info very interesting, is that a given focal length does not have a DOF, it is all in relation to the distance and the aperture (plus some other things that make the equation even more complicated).

But at the distances you stated, the discussion gets more philosophical than pratical as you said. It does rarely matter if the DOF is 16.9 meters or 12.3 meters, don’t you agree? Actually, many focus rings do not even go as far as 10 meters, let alone 20...

 

[Hugh T]

timeUnit,
math is interesting (to some people) but it does all come down to practicality. If you want to get deep into the math and derivations, you can download the book by Dr. Harold Merklinger (also from Nova Scotia) at http://www.trenholm.org/hmmerk/download.html. The other difference btween the two lenses focused for the same image height is an obvious change in perspective which is the relative size of near and far objects.

Hugh