Re: [APML] Picturing a star

["bert katzung" <katzung1@home.com>]

Hi Glenn (& son):
That is fascinating stuff! Thank you! I wonder what the color correction
characteristics of those IMAX cameras is like. Apo, achro, or what?
Bert


----- Original Message -----
From: "Glenn Shaw" <gshaw@alaska.net>
To: <astro-photo@seds.org>
Sent: Sunday, 05 August, 2001 4:14 AM
Subject: [APML] Picturing a star


> To all the list:
>
> My son who has all this training in optics told me that my idea that
> photographing a star was the hardest thing to do is nuts, clarified his
> position in the following (pasted in from a communique from him):
>
> Now hold on there a minute...
>
> I did NOT say exactly that you were nuts about a star being the best
> test of optics. I DID say that a point test is only ONE of several
> issues to be concerned about.  The point spread function, which is just
> a fancy name for the image of a point source at infinity, is broadly
> regarded as one of the most fundamental measures of potential image
> quality (note, however, that most optical designers actually work with
> the modulation transfer function, which is just the Fourier transform of
> the point spread function).  However, to evaluate a photographic lens,
> which is USUALLY designed to reproduce scenes that are not just
> individual point sources, requires more than just good point-to-point
> imaging.  It also requires high contrast, etc.  Very few people would
> choose a low-contrast lens with superb point imaging over a
> high-contrast lens with only good point imaging IF they are taking
> pictures of scenery, people, etc., as is done most commonly.  Therefore,
> it is simply not fair to make statements like "lens A is better than
> lens B" based ONLY on a point-image test.  You really need to at least
> qualify such a statement with the words "for point imaging" or similar.
>
> Now don't misunderstand me...I'm NOT saying that achieving point imaging
> is easy, but rather that it's easier to achieve decent point imaging
> when you don't have to worry about all the other myriad of details that
> can ruin a photographic lens.  But YES - I agree that the ultimate test
> of the resolving power of a lens is photographing stars.  AND one of the
> ultimate tests for aberrations is point imaging across the field, as you
> suggested doing.  Furthermore, one of the most difficult (i.e.,
> impossible) problems in optical design is to achieve even decent point
> imaging with a wide field (especially if you want fast optics, as
> astrophotographers always do).  Just remember that there are other
> issues besides point imaging that are often equally important for a lens
> used in typical photographic applications.
>
> A really interesting example is the lenses designed for the IMAX
> cameras.  These lenses are full of all sorts of aberrations and would do
> a really crummy job of photographing stars.  They are, in fact, often
> designed with EXTRA on-axis aberrations simply to achieve similar
> imaging quality across the field.  Why?  Because the human visual system
> does a great job of detecting CHANGES in image quality across a field,
> but a rather poor job of detecting absolute image quality.  Therefore,
> what you get in an IMAX film is equally mediocre image quality across a
> really huge field, adding up to a spectacular visual experience for a
> human observer!  In this case, within reason, the absolute point-imaging
> ability of the lens is quite far down the priority list of the lens
> designer.
>
> glenn shaw



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