My question concerns things like the ultra wide angle adapter for the Sony Alpha/Nex/e-mount line 16mm f/2.8 prime pancake lens, which changes the focal length of said lens from 16mm to 12mm. Since aperture sizes are expressed as a fraction of the focal length, does the effective f-stop change when the focal length is changed by such an adapter?

There is a product called Metabones Speedbooster, which is a lens mount adapter that allegedly increases the speed of a lens while widening its field of view. Is the speed increase a trivial consequence of the focal length change or is there some other principle at play there?

The ratio f/2.8 means the diameter of the entrance pupil is equal to the focal length divided by 2.8.

The key thing to note about the above is that *the entrance pupil is the image of the aperture stop as seen through the front of the lens*, the ratio **does not** depend on the physical size of the aperture itself.

A rear-mounted 2x teleconverter, such as you would use with a telephoto lens, changes the focal length whilst the entrance pupil remains the same size, so the aperture number goes from 2.8 to 5.6.

A front mounted tele/wideconverter will change the focal length, but it will also change how large the aperture appears, meaning the entrance pupil changes in line with the focal length so the f-ratio stays the same.

A rear mounted focal reducer such as the Metabones SpeedBooster will give you a brighter f-ratio as the entrance pupil remains the same size whilst the focal length gets shorter.

October 20, 2014 08:22 AM

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