At 14:17 09.11.01, C.H.Ling wrote:
I don't understand why you need to crop before compare, if so, which to
crop? Crop the 4x5 to fit 2:3 or crop the 35mm to fit 4:5?
You neep to crop to define exactly the ratio between the film sizes (and
therefor also the ratio between the focal lengths you consider "normal",
and the COCs). You can end up anywhere between 3.5 and 4.4 by cropping only
one frame. In my previous example I used a somewhat arbitrary factor of 3.0
because it fit the COCs available in the DOF calculator and the standard
view on "normal" lenses for 35mm and 4x5 (50mm and 150mm). I could've used
the the COCs given for the film formats, but then I would've had to define
a "normal" lens for 35mm as 30mm and I didn't think that was very
intuitive. In hindsight....
Can't catch what you mean, anyway a longer lens can give softer out-focus
effect. You don't need a medium or large format. In the 35mm system. The
longer lens has faster transition from sharp (in focus) to soft (out-focus).
Yes, for the same film format a 50/2 has larger DOF than a 100/2 (keeping
f/ constant) and the 50/2 has even larger DOF than 100/4 (keeping the lens
opening constant = 25mm). ***THIS ONLY APPLIES WHEN KEEPING THE FIlM SIZE
CONSTANT***. Change the size of film and scale the COC with it, so that the
100/4 becomes the new normal lens, and the DOF is suddenly the same again.
It's easy to find a 300/5.6 for 4x5, or a 150/2.8 for 6x6, but for us OM
users it's hard to find an 85/1.4 (DOF would be the same for all practical
purposes). I think this is where your practical experience seems to
contradict theory.
I think we're the only ones reading this now, so feel free to hand me some
numbers off list if you want.
Regards,
Thomas Bryhn
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