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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