P3D Re: "Cheap" new lenses VS. "Quality" new lenses

[John Ohrt <johrt@xxxxxxxx>]

Dr. George A. Themelis wrote:

> There is certainly a big price difference with faster
> lenses being disproportionally expensive but everything I have read or
> heard says that you pay a lot of money only for the speed and if you
> are going to use the lenses (say a f1.4 vs. f1.7 vs. f2.0 50 mm) in
> the same smaller aperture (say f8), then you are not going to see any
> difference in sharpness.

I would hope not!  f/8 is usually the "ideal" f-stop for resolution in a
50 mm focal length.  The actual f-stop for best resolution should be
determined by test for any given lens and type, but for max resolution,
f/8 for a 50mm fl is a real good guess.  However, you have to sacrifice
something to get speed, and it is usually the size of the circle of
confusion at maximum aperture.

In astrophotography, you usually shoot at the most sensitive combination
of camera settings.  Assuming focus is correctly set, a big assumption
for really crisp photography at infinity, then you select aperture. 
Unfortunately, the aperture, maximum aperture, that gives you the best
sensitivity against distributed sources is almost never the aperture
that gives you the highest resoltuion of point sources and sharp
detail.  In general, you get the best detail when you select one stop
closed from max aperture.  You would want to test this for each lens but
don't be surprised if a good f/1.4 gives you more detail than an f/2
when both lens are set at f/2.

A simple test for maximum sensitivity to fine detail is to point your
camer at the polar sky region on a clear, dark night.  Take 10 minute
time exposures at various focus settings near infinity and various
aperture stops near and including maximum apperture.  The combination
that lets you see the most stars in the region wins.  The stars will
"trail", and judging the crispness of the trail is a good way to assess
the impact of the combination of focus and aperture settings.  I plan to
write a web page about this in detail.

I agree that you really pay for the extra usable aperture, but like all
sensors, nothing beats having a good "front end".

The foregoing discussion assumes the film is a constant, not
necessarrily a good assumption.  If you can consider changing films,
then you may get better resolution with a faster film and a smaller
aperture than a slower film with finer grain but a larger aperture with
larger circle of confusion.  This is especially true today as the faster
films are much more fine grained than their predecessors.

Unconsidered is the renditon of color or tone when changing film types.

This whole subject of trade offs is to me part of the art of
photography, even though you can accurastely measure much of the data.

Looking forward to comments.


-- 
John Ohrt *** Toronto, ON, Canada *** mailto:johrt@xxxxxxxx


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