Vitaliy wrote:
>>> You can get it all in focus using a large aperture setting (say, 14)
>>> but it comes out sort of blurry/noisy.
>>
>> Really?  Most lenses are sharper at the higher F-stop.
>
> Not true, you get more of the image in focus but you get a sharper
> image at the lower F-stop settings.

Not really true either.  There are several interacting and competing factors
going on here.

First, no lens is perfect.  And even if you could get a optically perfect
lens, this perfection only holds true for the center of the image.  There
are some inherent distortions as you get away from the center.  Like
anything, real lenses are a series of compromises (hopefully) cleverly
traded off by skilled engineers.  The compromises to off center and other
tradeoffs get worse with wider aperture.

Second, there is such a thing as "single slit diffraction".  Every
electromagnetic wave will bend a little as it passes near objects, with
"near" being related to the wavelength.  Another way to put this is that
there is no such thing as a sharp shadow.  Even if the thing making the
shadow is perfectly sharp and straight, the light waves are not simply
blocked on one side of a mathematical line and passed on the other.  They
are bent to varying degrees near the obstruction.  The wavelength of visible
light is so small that we normally don't see this effect at human-size
scale.  AM radio is a good example of this effect put to use.  The earth is
the obstruction, and the the "ground" wave will bend with the curvature of
the earth so that you can receive transmission even though the earth is
blocking the direct line between the transmitter and your radio.

There are other effects too I won't go into.  The net result is that for
most lenses there is a arpeture setting where the sum of these effects is
minimum and you get the best "sharpness" (a term that by itself is a lot
more complicated to quantify than you may think).  For common camera lenses,
f/8 is usually a good guess for best sharpness if you don't have specific
data.


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