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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