Archived posting to the Leica Users Group, 2004/02/16
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At 2:31 PM -0700 2/16/04, Tim Atherton wrote:
> > What happens is that that when you mount your 50 mm lens on a camera
>> with a 1.5 multiplication factor, the 50 gives you the magnification and
>> field of view of a 75mm lens - BUT - it still has the depth of field of
>> a 50 mm lens because you are still using the central portion of the 50
>> mm lens...or something like that. ;-)
>
>i.e there isn't a change in depth of field?
>
>this is where I get confused with LF lenses - and the principle is the same.
>
>Using the same lens with different formats (8x10 to 4x5 or 35mm to
>chip/sensor) doesn't change the depth of field does it? My 250mm lens is a
>moderate wide angle on 8x10, but telephoto on 4x5. But the depth of field
>hasn't changed if I go from the 8x10 back to the 4x5 back - or has it?
>
>tim
The additional factor, which is quite important is that the
acceptable circle of confusion decreases as you decrease the format
size. Therefore, if you use a 250mm lens on 8x10, and find an
acceptable DOF for a given aperture, and then you use the same lens
on 4x5 at the same aperture, you'll find that the acceptable DOF is
now less if you make the same print size, because you have to enlarge
the neg 2x as much. Your acceptable COC has decreased, so you have to
stop down more to get the same apparent DOF.
And so it goes on down the line, until you end up with digicam lenses
of 5mm that you have to focus if you want acceptable results. If you
make an 8x10 print from one of these cameras, you have to enlarge by
40x, so your little 5mmx7mm CCD image had better be damn sharp, and
you have to work with a miniscule circle of confusion.
It's like taking an 8x10 neg, blowing it up to 8m x 10m, and viewing
it from the same distance that you normally look at an 8x10 print,
and then expect it to look as sharp and clear as a good 8x10.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
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