Marechiare wrote:
> Yes, but that was not a telescope, that was a microscope; there was no
> need to catch as much light as possible (at least for that price).

Catching more light, all else being equal, is a good thing in just about any
photographic application.

> The
> very reason for the existence of the big apertures must be - better
> resolution, I believe.

No.  Bigger apertures allow for more light gathering.  More data on whatever
you're measuring is usually a good thing.  It costs money to make optics
larger, whereas cutting down the light is relatively simple and cheap.
Therefore manufacturers brag about the largest apertures a lens can have.
When you buy a telephoto lens, the f/4 ones are going to cost more than the
f/5.6 ones, and the f/2.8 ones will be hideously expensive or nonexistant.
Even at 300mm focal length, a f/2.8 lens would require a optical diameter of
107mm (4.2 inches).  That's a big and therefore very expensive piece of
glass to get just right.

> My point (possibly wrong) was that "diffraction
> could be the major issue at f/14", not to say at f/22.

Several people have now told you that diffraction effects don't effectively
matter until at least around f/22.  Why do you persist with this point
especially since you said you're not that familiar with this issue?

There is only one way I've found to have diffraction effects visible in the
final picture at lower F stops.  That is with lots of extension tubes.  For
example, I have a very nice 135mm lens.  I did some experiments with it at
1:1 magnification and a little beyond, and the results were washed out even
at apertures as wide as f/11.  Note that this lens was not designed for
macro use.  I also didn't flip it around, which possibly would have helped
(I didn't have the necessary mounting adapters and didn't feel like rigging
up something for that test).


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