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Path: utzoo!watmath!watcgl!dmmartindale
From: dmmartindale@watcgl.UUCP (Dave Martindale)
Newsgroups: net.rec.photo
Subject: Re: Re: Camera Vibration
Message-ID: <1539@watcgl.UUCP>
Date: Sun, 24-Mar-85 17:46:29 EST
Article-I.D.: watcgl.1539
Posted: Sun Mar 24 17:46:29 1985
Date-Received: Mon, 25-Mar-85 02:12:52 EST
References: <1257@drusd.UUCP>
Reply-To: dmmartindale@watcgl.UUCP (Dave Martindale)
Organization: U of Waterloo, Ontario
Lines: 63
Summary: 

In article <1257@drusd.UUCP> phl@drusd.UUCP (LavettePH) writes:
>>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
>

The problem is that you are discussing things using atronomical terminology
when the audience is photographers.  Your 50mm and 60mm figures are lens
diameters, but photographers are used to seeing millimeters being used
to measure only focal lengths.  If you'd used the more normal photographic
terminology of f/stop, it would have been clear.  For example, assuming
that the lens focal length was 100mm, the 50mm lens is then f/2 and the 60mm
lens is f/1.7.

>
>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.

Well, yes, but the performance of these telescopes is limited by diffraction
(if I understand correctly).  The analogous situation in photography is that
a lens that is stopped down all the way will produce a somewhat unsharp
image, and that opening it up several stops will produce much better
"brilliance".  But continuing to open it up will produce less sharp images
again, due to aberrations in the lens that get worse at larger apertures.
Often, a f/1.4 lens will produce a poorer image than a f/1.7 or f/2 lens
when both are used wide open, in direct contradiction to what you are
trying to tell us.

>There is no difference between telescope and photographic lenses except for the
>outward appearance.

Not really true.  How close does your telescope focus?  How light is it?
These are important in photographic lenses.  So is cost.

Telescopes and photographic lenses obey the same optical laws, but different
tradeoffs are taken in their design, producing final results whicy may
differ.