The lens filter size, is to a great extent, dictated by physics. The F
number of a lens is roughly the ratio of the focal length to the diameter
of the front element , for lenses other than wide angles.
So if you want to make a 180mm f2 the front element has to at least be
90mm. Of course, the filter thread has to be wider than the element so
you make it an even 100mm.
If you limited your lens line so you had nothing with a filter size larger
than 72mm your 500mm offering could be no faster than about F7.
Giles
RobBurn@xxxxxxx wrote:
> readily available filter sizes. That's good planning. (Must have been some
> other dude who came up with the 100 mm size.)
>
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