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Originally Posted by MrObvious
Thanks for responding Andy.
you said;
Thats correct. The key thing is that you keep the focal length constant. IOW, it works like a camera. A smaller F ratio will automatically imply a bigger aperture.
It's when people compare two scopes of the same diameter/aperture and then claim the one with the lower F ratio is a brighter scope, that is the problem I'm referring to.
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You are welcome.
I think we've hit the point of agreement. The confusion probably comes from the fact that camera lenses hold focal length constant while varying aperture (finally got that spelled right). My Nikkor 35mm f1.4 is a beautiful lens with a vary large aperture. Much nicer than say, the f3.5 model which is much smaller.
The comment about brightness only has meaning
at a constant focal length.
Quote:
Originally Posted by MrObvious
If however most people are likely to buy a scope buy its lens diameter and compare two scopes of equal diameter main lens/mirror. Then claims of one being brighter than another because its F ratio is lower are just plain wrong.
To see what I mean your link states:
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This fast field photo visual astrograph is perfect for the new mega pixel digital 35mm cameras and CCD cameras. The great speed will enable the imager to record faint nebulosity’s invisible in slower instruments.
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This is misleading. It's claiming its due to the F ratio, but its really a due to the lens diameter. |
Not misleading -- it is bright
for a 500mm focal length scope. I know people who have been amazed at the data they collect from this scope in a small amount of time. But they are looking for a 500mm/f2.2 field of view, not high magnification. The more honest statement would be, "the great speed for its magnification."
Quote:
Originally Posted by MrObvious
I've had salesmen tell me that this 200mm scope is brighter than this 200mm scope because it has a faster mirror. This propagates the misconception.
If he said this scope provides wider views for the same size eyepiece, that would be correct and not misleading.
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Assuming 200mm is aperture, absolutely correct.
Quote:
Originally Posted by MrObvious
Summing up:
Two scopes of equal main lens size give exactly the same image brightness for the same magnification. (this is obvious)
Two scopes of the same focal length but different F ratios will have a different sized main lens. (this too is obvious)
Conclusion:
It's the size of the lens that is the key parameter, not the F ratio, when determining brightness.
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I agree. Light gathering power is ultimately determined by aperture. But the design decisions that go into f-ratio make the scopes different and therefore useful for different purposes. But as you say, not a priori better.
Clear skies,
--Andy