Just to clarify a few things. While stars are theoretical points of
light when they pass through a lens/mirror they form a diffraction pattern
(Airy disk). The Airy disk is inversely related to aperture size. The
bigger the objection the smaller the pattern. So, a larger objective lets
you resolve such things as binary stars.
John
On Mon, 13 Sep 1999, Chuck Norcutt wrote:
> Chuck wrote:
> >>> Stars are mathematical point sources of lights. No matter what the
> focal length of the lens their images cannot be magnified.<<<
>
> and Acer said:
> If I understand this correctly, it means that whether I use a 200mm lens
> or a 28mm lens, the size of a star will be the same, a point?
> ---------------------------------------------------------------
>
> Yes, you've got it. It doesn't matter if you're using the world's
> largest telescope. None are large enough to be able to image a star as
> anything other than a point. (except for that star we happen to call the
> sun)
>
> The significance of this is that some otherwise normal rules in
> photography don't apply. With a normal object if you changed from a
> 50mm lens to a 200mm lens you would magnify the image 4 times. If the
> physical opening of the lens stayed the same, the same amount of light
> from the subject would now be spread over an area 4x4 = 16 times larger
> than at 50mm. From another standpoint, a 50mm lens with 25mm aperture is
> an f/2. A 200mm with a 25mm aperture is only an f/8. With a normal
> object the exposure time would have to be increased or the aperture or
> the film speed increased to compensate for the reduced focal ratio.
>
> However, the star doesn't care whether the focal ratio is f/2 or f/8.
> As long as the physical aperture of both lenses is 25mm the exposure
> time is the same.
> So, for stars, only physical aperture, film speed and exposure time
> matter... and exposure time will be limited by sky fog.
>
> Now, as soon as you get to the moon, planets, nebulae, etc. you are
> talking about normal, extended objects and the normal rules of
> photography once again apply.
>
> Chuck Norcutt
> Woburn, Massachusetts, USA
>
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