Quote:
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Originally Posted by netwolf
Dennis,
In a Telescope we usualy use a lense (barlow) either postive or negative to increase or reduce the focal length.
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You’re getting there, well done.
Yes - a
Barlow will
increase the focal length (aperture stays the same) and a
Reducer will
reduce the focal length (aperture stays the same).
Quote:
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Originally Posted by netwolf
Dennis,
In a Camera lense we change the apperture size to do the same thing usin a iris. I got it.
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However…
In a camera lens, when you
change the aperture (open up or stop down the Iris), the
focal length stays the same. That is, a 200mm lens wide open at F4 and the same 200mm lens stopped down to F22 still has a focal length of 200mm. The Focal Ratio changes due to the Iris effectively reducing the aperture of the lens.
Cheers
Dennis