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alundeb wrote:
The focal length reduction and the f-stop gain cannot be separated.
The focal length reduction will probably not be as much as 1.6, but I will use that number for the examples.
The 24-70 mm f/2.8 with a 1.6 focal length reducer becomes a real 15--44 mm f/1.8 lens
When mounted on a crop camera like the EOS M, it gets the equivalent FOV, DOF and photon noise characteristics as when mounting the 24-70 mm 2.8 on a FF camera
A 800mm f/5.6 lens with a 1.6 focal length reducer mounted on it becomes a real 500 mm f/3.5 lens. It is identical to an ordinary 500 mm f/3.5 lens.
When a 500 mm f/3.5 lens is mounted on a crop camera like the EOS M, it gets the equivalent FOV, DOF and photon noise characteristics as when mounting a 800mm f/5.6 lens on a FF camera.
If you believe that the 500 mm f/4 L IS is a 800 mm f/4 L IS on a crop camera, then I will step out of that discussion.
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kalieaire wrote:
Actually, there is no f-stop gain. Aperture bokeh remains the same as whatever f-stop aperture it is set at regardless of whether or not there is a focal length reducer between the lens and the camera. However, the T-Stop changes, please tell me you know what a T-Stop is w/o having to google it.
FYI
I Own Sony NEX cameras, a regular EF-NEX adapter and a Metabones EF-NEX SpeedBooster. When I mount the EF 35 mm 1.4 L on the SpeedBooster, the camera reports maximum aperture 1.0 and used wide open I get 1 stop brighter exposure than when using the lens on the regular adapter.
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