Archived posting to the Leica Users Group, 2001/05/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
At 9:25 AM -0700 5/7/01, Mark Rabiner wrote:
> ><Snip>
>>
>> I need to think about this for a while. As you might guess, I don't
>> buy your explanation, I think there's a fallacy. While I look up the
>> theoretical analysis I'll see if I can also do my own tests. I'd love
>> to shoot the same object with a long lens and a short lens wide open
>> and at f22 to see if there's a difference in diffraction losses.
>>
>> Byron.
>
>I'm real interested because i tend to stop real down when i use wide angles.
>When i use wide angles i want EVERYTHING in focus. F11 is normal, f8
>makes me a
>little antsy, and F16 gives be that warm fuzzy feeling.
>Telephotos however for me (and many) are all about edge; a thin area of
>sharpness with everything falling off in both directions; shooting
>wide open or
>stopped down one or two.
>To all of a sudden realize that i should be doing the opposite to get the best
>out my optics really throws me!!!
>Shoot wides wide open but stop way down with teles? forgetaboutit!!
I've discussed this a bit more with Byron off-list, and I've done
some more reading in a couple of optical tests.
The situation (from MY point of view :-)) is this: What I said about
the quantum nature of light with respect to diffraction is not
correct; there are quantum issues, but it's not as clear cut as I
made it out to be. I have some information that supports the concept
that diffraction is a function of absolute aperture size, meaning
that wides have more diffraction problems at the same aperture than
teles, but there are quite a few factors in practical photography
that muddy this. Also, another important concept in classical
geometrical optics is the Airy disk, which along with the Rayleigh
criterion results in the concpt that for any given aperture there is
a maximum resolution. This states that only the f-number is a factor
in diffraction, not the focal length. But this also deals in a
limited case, having little to do with real world photography. In the
end, wides still have more problems at small apertures, but not as
rigidly as the 'focal length divided by 4' rule would indicate. I've
been trying to track this down, but while I can find many references
to it, I can't find any explanation of it.
The 12 and 15 don't do well at f/11 or beyond, the 21ASPH and 21/3.4
don't seem to be as severely affected at f/11, but f/16 doesn't do as
well, and with long teles, f/16 doesn't seem to give much worse
results than f/5.6. In LF photography, f/22 seems fine for wides, and
f/45 not too bad for longer lenses, but the enlargement is a lot less
with the bigger film.
The above is just what I've notices with MY lenses under MY shooting
circumstances. You'll just have to try your own stuff to see what
you're happy with.
I'm going to look at this some more, but at present there seem to be
too many factors involved for me to see anything definitive.
Meanwhile, I'll shoot as I usually do; I don't stop down more than
necessary, but 'necessary' is quite flexible. So I'll just shoot, and
if I need an exposure of f/16 with the 12 on an M6, so be it. Maybe
next time I'll have the Hexar with me.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com