Old 20-12-2007, 01:18 PM
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ving (David)
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silly noob question...

... and one that i should by rights know the answer to...

200mm lens with a front lens size of 52mm @ f/9

600mm lens with a front lens size of 72mm @ f/9

are they going to offer the same brightness at the same shutter speed or will the greater sixed front lens offer more light gathering ability?

I really should know the answer!

i know how it works for telescopes, but how does it work for camera lenses?
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Old 20-12-2007, 02:42 PM
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The greater the aperture the fainter you can detect in a given exposure.

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Old 20-12-2007, 03:08 PM
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ving (David)
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like i said, i know how it works in telescopes... say a refractor where a aperture of 80mm has better light grabbing ability than a 60mm. but the aperture of a camera lens is controlled at the back of the lens (and yes still controls the light handling), but how about the front of said lens?

is a 300mm f4 lens set at f4 going to capture as much light as a 300mm f2.8 set at f4? if so then whats all that extra glass at the front doing exactly? they take different filter sizes because the front of the f2.8 is soooooooooooooo much bigger.

my head is going to explode i think.

taken the above 2 300mm lenses as an example, we know the aperture which we control and which controls the light is at the back of the lens right? why not then make the f2.8 the same physical size as the f4?
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Old 20-12-2007, 08:51 PM
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Inverse Square law have something to do with it??

To get f/9 @ 600mm you need a wider aperture to have same light gathering ability as a 200mm f/9, but at a higher magnification. Higher quality components are needed to make the field sharp and free of vignetting @ f/2.8.
My guess is that they use a much bigger lens so that the manufacturer can control the vignetting in the lens, much like a field stop in an eyepiece.

I'd have to look into that a bit more i think to make sure!
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Old 20-12-2007, 09:06 PM
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acropolite (Phil)
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Light wise, F9 is identical on both lenses, otherwise your exposures would be all over the place. An image with two different size lenses @ say f8 will have identical exposure (i.e. same amount of light) at a nominated shutter speed, only the FOV and DOF will differ.
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