Archived posting to the Leica Users Group, 2004/02/16

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Subject: RE: digital lenses was: Re: [Leica] Digital Bessa RF
From: C.R.Marshall@uwsp.edu
Date: Mon, 16 Feb 2004 16:07:50 -0600

Confusion and possible misinformation follows...
(I am trying to think through this as I write.)
Please correct as needed...

Depth of field is the range in which the circles of confusion are small
enough for something in the image to seem sharp.

A 35mm lens set to a given f-stop and focused at a given distance will
produce the same pattern of circles, regardless of the size of the
film/sensor.

The smaller the film/sensor, the greater the magnification needed to
make, for example, an 8x10 print.

Greater the magnification of the image yields greater the magnification
of the circles of confusion which should give shallower apparent depth
of field. (Is this true? It is possible that with a larger print, you
just increase the viewing distance.)

For a smaller sensor, a shorter lens is needed for the same field of
view.  

It seems like the greater enlargement needed for the small sensor should
partially offset the effect of the wider lens needed for that small
sensor.

It is quite possible that one of the effects is linear and the other is
quadratic (or something else). Is there someone on the list that can co
the math?

C.R. Marshall

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