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Re: [OM] OM macro / closeups vs medium format / TLR

Subject: Re: [OM] OM macro / closeups vs medium format / TLR
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Tue, 01 Jan 2002 17:37:34 +0000
At 15:54 1/1/02, Frieder Faig wrote:
Hi John(s),

I don`t agree here. John A. Lind wrote in an older mail, that DOF changes with (focal length)^2.
But to keep the same magnification, the distance only changes with f^1.
There is some compensation, and there might be situations where the influence of
focal lenght isn`t really much.
But I´m sure DOF changes with focallenght even when you keep the same magnification.
Altough I don`t do the math now. Just my thoughts.

Frieder Faig

This is difficult to convey in print. Perhaps some graphics will show it better. Some time ago I put together a spreadsheet to get a feel for how all this works. Also created some graphs to show the significant results. I've exported these charts to image files and put them into a web page with a little text to explain them. This page is *unlinked* and stands alone on my site (you gotta use this URL):
  http://johnlind.tripod.com/science/sciencedof.html
At some point in the future this page will likely get integrated somehow into the rest of the information about lenses.

Photo.net also has an excellent tutorial with a similar chart showing what happens to DOF behind the rear DOF boundary:
  http://www.photo.net/photo/optics/lensTutorial

The confusion (pun intended) may be about my comments regarding the effect shifting to a larger film format has on DOF. If I change from my OM-1n with 50mm lens to my M645 with an 80mm lens and use the same aperture and focus distance, the depth of the DOF shrinks slightly. While the field of view has not effectively changed, nor has the size of the subject (at critical focus distance) in the viewfinder changed, but magnification _on_film_ has changed. Why? I'm now filling the same percentage of a larger piece of film with the subject. In order to "fill" a larger piece of film with the subject material, the magnification on film *must* increase. That's why the focal length increased to maintain the same field of view and perspective. One might think an allowable increase in the maximum acceptable circle of confusion diameter for the larger film format would compensate. Less enlargement is required for same size print or projection. It does, but not completely. DOF shrinkage occurs at (focal length)^2 and DOF growth occurs at (max acceptable CoC)^1. However, both focal length and CoC growth are linearly increased with the increase in film format. This leaves a (focal length)^1 shrinkage in DOF. Think of it as two steps backward (focal length) and then one step forward (CofC).

-- John

-- John


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