lens quality

In rec.photo.equipment.medium-format Ilya Zakharevich wrote:
[A complimentary Cc of this posting was sent to
jjs
], who wrote in article :
If the last element touches the sensor, there is much less reflection
on the boundary, since the refraction coefficients are approximately
the same.

How close is "close"? Here are some estimates: if you consider a ray
going from higher-n matter to lower-n matter (e.g., glass to air), and
the angle of reflection is larger than the critical one, then you get
full reflection.

So, in English - is it better to have the rearmost lens as close to the film
as possible?

??? Better *for what*?

I repeat: if the distance is much smaller than wavelength, this would
practically eliminate reflectance from the sensor boundary. It may
have some other side effects; I'm in no position to comment on them...

Umm.. you should also in such a case consider polishing deffects as the
last element needs to be rather perfect.


Yours,
Ilya

--
Sander

+++ Out of cheese error +++

In message .com,
"Siddhartha Jain" wrote:

I don't want to start a holy war here (or one's already underway) but
how come I took decently sharp images with my Canon 300D at f16 with a
Jupiter 21m 200mm f4 lens?

F/16 is not a very small aperture for 200mm.
--


John P Sheehy

David J. Littleboy wrote:
I don't want to start a holy war here (or one's already underway) but
how come I took decently sharp images with my Canon 300D at f16 with a
Jupiter 21m 200mm f4 lens?


MTF50 due to diffraction is roughly 800/f number, so that's a 50% term at 50
lp/mm (at f/16, the contrast will be zero at 100 lp/mm). The Nyquist
frequency for the 300D is 2048/(15.1 x 2) = 68 lp/mm. So you should be
seeing some loss of contrast, but there's still plenty of contrast to show
Moire. (Note that most real lenses* have pretty poor contrast at 50 lp/mm,
so I'd think that you'd not even notice the difference between f/11 and
f/16. I'd think you'd be seriously unhappy with f/32 with the 300D, though.)

*: Actually, this is true of normal and wide lenses. Longer primes can be
quite good.)

David J. Littleboy
Tokyo, Japan

The shape of the diffraction mtf curve, and the shape of the spot
profile of diffraction is quite a bit different than that of defocus or
most geometrical aberrations (even with same cutoff or spot HWFM). So
it is not as easy as just comparing cutoff or spot diameter when
comparing diffraction to defocus or other aberrations. The defraction
effect is a general softening, rather than the 'blur' of other effects.