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From: phl@drusd.UUCP (LavettePH)
Newsgroups: net.rec.photo
Subject: Re:Re: Camera Vibration
Message-ID: <1257@drusd.UUCP>
Date: Fri, 15-Mar-85 12:08:14 EST
Article-I.D.: drusd.1257
Posted: Fri Mar 15 12:08:14 1985
Date-Received: Sat, 16-Mar-85 04:56:42 EST
Organization: AT&T Information Systems Laboratories, Denver
Lines: 69

>I do not see how I can parse this as a true statement.
>The obvious meaning -- that a 60mm lens puts twice as much
>light on the film as a 50mm lens -- is false.  How much light
>reaches the film depends on only two things: the amount of light
>on the subject and the f-number of the lens.  Focal length
>has nothing to do with it.
>
>Phil, what did you mean?
>
> Andrew Koenig

Let me give you an example instead of a lot of formulas.  If you know a back-
yard astronomer, he or she can give you a demonstration.

On a good clear night assume three telescopes are set up.   The first is a small
spotting scope with a 50mm diameter objective; the second, a spotting scope with
a 90mm objective; the third is a *small* astronomical telescope with a 200mm 
objective.  The objective is the front lens.  The three scopes are set up for
the same power and trained on a cluster of stars.  M13 is a good choice.

Through the 50mm you will see a faint green round smear.  Through the 90mm you
will see a smaller, but brighter, green round smear surrounded by a pattern of
stars extending out to the diameter of the smear you saw through the 50mm. 
Through the 200mm you will see a pattern of bright stars the same size as the
smear you saw through the 50mm and with more stars than you saw in that cir-
cumferential band of stars you saw through the 90mm.

The overall increase in *brightness* resulted from the change in effective ap-
erature (f-stop) caused by the change in objective sizes while holding the same
effective power of magnification.  The disappearence of the smear resulted from
both the increase in resolution and the reduction in the size of the Airy-disc
caused by the increase in objective diameter.  The image you see through the
200mm is brighter, it is better resolved and the brilliance (the definition
between the light and dark areas - photographers call it contrast) is much im-
improved.

Lack of sharpness in prints or slides can be caused by any number of other
problems and all of them should be considered before anybody lays out big bucks
for that magic camera or lens that will solve all his or her troubles.  

Assuming the problem is not in the slide projector or darkroom:

Poor focus - inexact focus by the photographer or poor correlation between the
             image projected on the film vs on the viewfinder screen

Poor stability - flimsy tripod
               - defective lens mount
               - shakey photographer
               - mirror slap
               - low shutter speed

Poor definition - dirty or chipped lens or filters
                - low quality lens or filters

Poor contrast - too many elements in lens design causing reflections
              - not enough or poorly designed light baffles in lens and camera
              - forgetting to use a lens hood
              - high speed, low contrast film

The longer the lens, the more the problem is magnified :-)

There is no difference between telescope and photographic lenses except for the
outward appearance.  You can (and I regularly do) hang a camera on the back of a
scope or mount an eyepiece on a camera lens.  Some camera lenses make fine tele-
scope eyepieces.  My 1000mm, f11, Celestron C90 will resolve the whiskers on a
muskrat at 50-75 yards.  When I want to resolve the feather detail on a Canadian
goose at 100-150 yards I use my 2000mm, f-10, C8.

- Phil