DOF formula?

[BrianPhotog]

Well, my Palm is toast...left it in a pants pocket when I did my laundry. It's very clean and smells like a fresh spring day, but it's toast.

So, I can no longer do quick DOF & Hyperfocal lookups and need to know the formulas involved since pen & paper have temporarily replaced my Palm TX. I'm shooting 4x5 and 6x12 (120 rollfilm) through an old 135/4.7 Xenar.

Thanks for the help!

 

[Mike Wilde]

a very detailed document for optics

last year I found an electronic book edition of this subject on the web, but don't recall the web site. It is about a 2m pdf, so it is too big to attach here.

search for 'the ins and outs of focus' by 'harold m. merklinger'

 

[Mike Wilde]

online calculator too

DOF at a site with doug kerr as the author is another resource

 

[Richard Harris]

I am not answering but asking. Isn't depth of filed in any format dependant on the field of view. e.g. depth of field could be 1/3 of the width of view of the lens for a particular hypothetical negative size. Would this not remain constant whatever the focal length of the lens?

If you fill the frame of an negative with a face with a 24mm lens will you not have the same depth of field as if you fill the frame with a 135mm?

If I am right, does this not make an easier system of calculation or estimation in the field, given that few lenses are now marked with DOF scales especially zooms?

 

[Ralf]

The actual formulae can be found here: http://en.wikipedia.org/wiki/Depth_of_field#Depth_of_field_formulae

@Richard: The above link might also answer some of your questions.

 

[Jim Jones]

Most formulae are only a partial solution to DOF problems. Subject matter, film grain, print size and viewing conditions, and many other factors should be considered. When they are well understood, simple math done quickly in one's head may work as well as formulae on a Palm. For example, hyperfocal distance equals the diameter of the entrance pupil of the lens multiplied by a constant (which depends on the above factors, but is usually about 1000 or 2000). The convenient DOF scales on some lenses are quite optimistic for critical photography.

 

[Nick Zentena]

Why not go to the Schneinder website and download the one page DOF chart for a 135mm lens? It won't cover 6x12 but that won't be so different from 4x5.

 

[freygr]

Why not go to the Schneinder website and download the one page DOF chart for a 135mm lens? It won't cover 6x12 but that won't be so different from 4x5.

Size of the film does not affect the DOF only the focal length, F stop, and distance to the subject are the only variables.

 

[Nick Zentena]

Enlargement factor? COC choice?

 

[Ian Grant]

Size of the film does not affect the DOF only the focal length, F stop, and distance to the subject are the only variables.

While that's true in theory in practice most DOF tables take into account the degree of enlargement and assume you need less sharpness with larger negatives.

Enlargement factor? COC choice?

So you can find DOF tables for the same focal length lenses that vary quite a bit, but in the end a lot comes down to practical experience.

Ian

 

[Tom Hoskinson]

Enlargement factor? COC choice?

I keep my circle of confusion large - - this helps me stay in focus.

Fuzzy Logic

 

[Neanderman]

Enlargement factor? COC choice?

The "circle of confusion" is the size of the circle of illumination that any single point in the image forms on the film. Starting at the point of critical focus, where the CofC will be smallest, the CofC grows as you get farther away from the critical point (in both directions).

By the same token, the CofC for any point is magnified as you enlarge the negative.

It is also affected by viewing distance.

Therefore, to calculate the "effective" DOF, you need to know:

• How large will my final print be?
• How far away from the image will the viewer be?
• A somewhat abritrary notion of what defines 'sharp focus.'

A typical value for the circle of confusion is 1/1000 of the focal length of the lens.

Ed

 

[BobNewYork]

Some considerable time back here was an article in, I think, Darkroom & Photo Techniques which gave a formula for the sharpest aperture for depth of field. The "sharpest aperture" was calculated to take diffraction into consideration and was based on the bellows extension difference from near focus to far focus. I used the formula to calculate "sharpest aperture" for a whole range of bellows extensions, typed it on to a card and laminated it.

In practice, you focus on the near and measure the extension, (I use a field 4x5) focus on the far, measure and then set the bellows at the mid point between the two. The chart tells me my "best" aperture and shutter speed is established based upon this aperture. I use this all the time and it's worked well for me.

I've been looking but I can't find the magazine, (I always stored the somewhere safe, of course!:rolleyes Can't remember the author's name, but the mag. issue must have been 15 years ago or so. Any ideas?