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2x zoom lens with same absolute aperture instead of extender ?

Started Aug 16, 2015 | Questions thread
OP mermaidkiller Senior Member • Posts: 1,450
Re: 2x zoom lens with same absolute aperture instead of extender ?

MitchAlsup wrote:

mermaidkiller wrote:

Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?

Because the entrance pupil of the zoom lens is inside the optical train, Zoom lenses have to have a larger front lens than the linear aperture, typically by 20%-25% so you would be holding 5" of glass at the front of the 200-400. Thus, the cost will not be what you are anticipating.

I measured the 70-300L aperture (entry pupil, the *actual* value) by lighting a flashlight through the eyepiece and then measuring the diameter of the projection circle. That appears to be 56mm, exact the same size as I measure the front lens.

The same is with the 200mm f/2.8L. But the 24-105L is indeed much smaller than the front lens diameter.

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