Digital BW, The Print

Seth wrote:

>Clayton-
>
>DOF has nothing to do with sensor size.  A 200mm lens on a full frame has a
>given DOF.  If you put it on a sensor with a 1.5 factor it now has a 300mm
>EFFECTIVE focal length.  It still is a 200mm lens and maintains THAT DOF.
>Just ONE of the advantages of using a smaller sensor --if everything else is
>equal.  Since it also has the SAME f/stop you gain that.  No need to use an
>extender or a longer, heavier lens.
>
>Seth 
>
>  
>
DOF is an arbitrary rule. If one wants to compare optical systems there 
has to be a common size for the image produced with the systems. That 
can be a a final print of say 20 x 15 inch. Based on the quality in that 
print size you could define the DOF for the optical systems, whether 
that is a Pentax 67 II or an Olympus C8080. You don't measure DOF on the 
sensor or on the lens without counting to which degree the picture has 
to be enlarged and can be enlarged. Your 1.5 factor sensor needs a 1.5 
factor to produce the same size print again. It depends on the pixel 
count and lens quality whether that gives the same quality. If the angle 
of the lens has to be equal, the focal length has to get shorter, with 
the same glass diameter it could become a faster lens for that smaller 
image and DOF would be equal to the larger sensor + longer and slower lens.

Enjoying the DOF of today's digital compacts is overlooking the fact 
that the optics are quite slow for their focal length, but that is also 
why they can produce such high zoom factors if compared to 35 mm lenses. 
A true portrait lens on a Canon Pro1 should be an 0.5 or something like 
that and DOF would be much more like what we are used to. Whether the AF 
in compacts can cope with that is another matter.

A friend inherited an Ernostar 125 mm 1.8 that was used on an Ermanox 
with 6.5 x 9 cm film. A camera like Erich Salomon used before his Leica. 
The lens the Sonnar is based on. My friend's uncle used that lens with a 
custom adaptor on a Mamiya 645. On the web I discovered that David 
Burnett bought the same lens not so long ago to use it on a Speed 
Graphic with some vignetting. One could argue that the DOF should be the 
same on both cameras but only when the print from the Mamiya is 2.5 
times smaller in size.  The equivalent lens on a Canon Pro 1 would be an 
8.6 mm lens with an 0.12 aperture (if that is possible).

Ernst