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

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Subject: RE: [Leica] Digital Bessa RF
From: "Frank Filippone" <red735i@earthlink.net>
Date: Mon, 16 Feb 2004 14:17:10 -0800

Using the same lens with different formats (8x10 to 4x5 or 35mm to
chip/sensor) doesn't change the depth of field does it?

Correct.
- ---------------------------
My 250mm lens is a
moderate wide angle on 8x10, but telephoto on 4x5.

Yes because you are taking a small angle of view..... the 4x5 is 1/4 the
size of a 8x10, so the apparent angle of view is 1/2.  Meaning you get  the
effect of a telephoto lens on 4x5 but normal on 8x10.
- --------------------------------

But the depth of field
hasn't changed if I go from the 8x10 back to the 4x5 back - or has it?

Nope.. no change..... same DOF.  As long as you do not change the aperture
nor focal plane ( plane if absolutely perfect focus.. otherwise known as ...
don;t touch the knobs.)


The generic answer.......
A lens throws a cone of light onto the focal plane.  If you use just a tiny
bit of that cone, you get the effect of a telephoto lens, a small angle of
view.  If you use the entire cone, then you get the effect of a WA lens, a
large angle of view.  The projected area that you are putting the film in
front of is a different size, giving the apparent difference of tele or
WA.... but you never changed the lens, so there is no difference in the DOF
of the shot.  Only the angle of view changed.....

Clear as mud?

Frank


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