Re: [Leica] DOF

[Johnny Deadman <john@pinkheadedbug.com>]

on 11/11/00 4:03 am, Robert Appleby at robert.appleby@tin.it wrote:

> If I've understood the discussion right, John is saying that DOF is a
> subjective (perceived) factor which must take print enlargement into
> account. 

Correct. Lenses actually have effectively *no* depth of field until you mush
up their images by putting them on film. Without specifying a magnification
you have no criterion for deciding what will look in and out of focus.

> Hence MF lenses have longer focal lengths but equivalent DOF
> because the negative requires less enlargement. Is that right? This is news
> to me, I always thought it referred to a given amount of out-of-focusness
> on the film plane, and hence MF lenses would have the same DOF as the 35mm
> format lenses of the same focal length.

Well I don't know if it cancels out arithmetically in the way you describe
but you've got the idea. MF lenses of the same focal length *don't* have the
same DOF unless you are blowing up the neg to the same degree, which the
standard figures don't assume.
> 
> But it does make a sort of twisted sense.
> In that case, what print enlargement do the DOF markings on the camera lens
> refer to?

On a 35mm normally 8-10x, which is optimistic for bigger enlargements.
> 
> Maybe there are really two concepts here, film-plane DOF and print DOF? The
> latter can be varied by enlarging the negative more or less, but the former
> is determined solely by subject distance, focal length and aperture.

I've never heard of the former but it's an interesting concept. It would
depend a *lot* on what kind of film you used. Since you have no
magnification criterion you'd come down to lp/mm. I'm not sure how useful a
concept it would be.
> 
> Just my idea based on reading the posts over the last couple of days.
> 
> Since I never print, the idea of print DOF had never occurred to me. I'd
> always thought of it just as being film-plane determined. And so I'd also
> always thought the drawback of MF was reduced DOF for a given angle of view
> lens.

Someone else would have to figure this out. I can see what you're getting at
here. It sems inconsistent. It's certainly my experience on LF that the
equivalent of a 25mm lens has a minute DOF in comparison to a 35mm lens with
the same angle of view.

- -- 
Johnny Deadman

http://www.pinkheadedbug.com