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Old 01-02-2012, 09:21 AM
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Terry B
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Join Date: Mar 2006
Location: Armidale
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Quote:
Originally Posted by The_bluester View Post
Thanks, that sorts me out nicely. And as a bonus, corrects my thinking in another area, that of the focal plane.
snip

Only leaves me one thing to ponder, if the image size formed at the focal plane by a given diameter objective is dependant on focal length (And therefore ratio) why does the image size not change when you stop a camera down?

I thought I would leave that question in although I reckon I have thought of the answer myself while typing it. Anyone who knows better please correct me.

snip

!
Your reasoning is correct. Recucing the f stop on a lens is effectively reducing the diameter of the lens. The focal lenght remains the same.
One important thing to remember is that the resolution of a lens is proportional to the diameter of the lens and the frequency of the radiation being imaged(radio waves need much bigger scopes than optical for the same resolution).
If the diameter of the scope gets too small then the resolution will become the limiting factor in image quality rather than other factors like seeing and pixel size. This is why bigger diameter scopes are needed.
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