Re: [Leica] Re: DOF...OK...Think I've got it.

["M.E.Berube" <MEB@goodphotos.com>]

At 01:46 PM 11/9/00 -0700, John Collier wrote:
>Please look in your library for books such as Ansel Adams' "The Camera" and
>other similar books. You can read it there for yourself.
>
>DOF is determined by just two factors: final image size and relative
>aperture used.

Pg. 48-9 "The Camera" on Lenses:
"There are two other factors that affect the depth of field: the focal 
length of the lens (change to a shorter focal length if you need more depth 
of field) and the subject distance (move away from the subject to increase 
the depth of field). These three factors, aperture, lens focal length, and 
subject distance, give us considerable flexibility in managing depth of field."

I think the part that I was not taking into consideration in my thinking it 
out was camera to subject distance.

>Perspective is a function of viewpoint only.

Here too I knew what I was thinking in my head, but could not frame to 
pronounce it rightly so that you all knew what was in there as well. 
(scary, dark and cold in there mostly.)

If you move camera position and wish to keep your main subject at a 
constant size in your composition, you need to change focal lengths. Doing 
so will compress or un-compress the perspective distance of any background 
elements. Again I was not taking into consideration mentioning the camera 
to subject distance in my previous posts.

>Lens choice does not enter in at all. Take a picture with the subject
>filling the frame with your 90 and 35 using the same aperture (you will have
>to move closer with the 35). Print to the same size and you will get the
>same depth of field.

Yes moving closer to maintain the same subject size in the VF limits your 
DOF. Agreed. (Illustrated best by the focus scale representing greater 
distances closer together on the infinity side of the focus dial of every 
lens.)

In clarification (to late?) Here is what I was thinking in first making 
mention of Allan's claim that FL has nothing to do with DOF:
at f/16 my 40/2 Summicron-C gives me a DOF = about 5 ft to inf.
at f/16 my 90/4 Elmar gives me a DOF = 25 ft to inf.
(at f/16 a 90 Elmar has 20ft less minimum DOF available. The extra 20 ft. 
focusing latitude of the wider lens comes in handy when you are worried 
about focusing on a fast moving toddler. You just give up cropping in camera.)

>Ansel has his answers illustrated and will be much easier to comprehend than
>anything I can cram into my cable line.
>
>John Collier

Now I can't find any reference to how cropping a printing in a wet darkroom 
can change the DOF that is static and set on the negative. (This might be 
in "The Print" which I don't have yet. But a cursory look in "The Negative" 
and my USArmy Photographic Applications Course manuals didn't yield it 
either.)

How would Printing to the same (or different) size effect DOF at all? The 
focus of the negative in the enlarger is a flat plane, and has no depth to 
manipulate. Anyone know of a better forum for this discussion?

Carpe Luminem,
Michael E. Berube