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+--- Thread: Equivalence thinh too complicated for me BC can you help ? (/showthread.php?tid=341)
Equivalence thinh too complicated for me BC can you help ? - toni-a - 04-16-2017
Any volunteers for explaining this ?
https://www.youtube.com/watch?v=XcX4D9Dq2pI&feature=youtu.be
Equivalence thinh too complicated for me BC can you help ? - obican - 04-16-2017
At around 2:47, it switches from being informative to wrong.
Equivalence thinh too complicated for me BC can you help ? - wim - 04-17-2017
Hi toni-a,
Without seeing the video (which I normally do not, too time-consuming), the answer is that it's very simple really.
A speedbooster shortens the FL while keeping the maximum physical lens opening the same.
If the magnification factor of a speedbooster is 0.7x or 0.71x, that effectively means that the aperture gains a stop, and the lens gets about 30% shorter in FL (multiply both aperture and FL by the speedbooster factor).
F.e., a 50 mm F/1.4 now becomes a 35 mm F/1.0. Of course, thereafter, for equivalence, you still need to take the crop factor into account. However, you now have a faster, shorter lens to start with.
If your crop factor is 2, like it is with MFT, you still need to multiply with a factor two. IOW, the equivalent after applying the speedbooster factor for the same lens now becomes a 70 mm F/2.0. However, that is still a stop wider than the default equivalent 100 mm F/2.8, i.e., the lens without speedbooster, even though the equivalent FL is shorter now.
Of course, you do need a smaller sensor to not get optical vignetting, as a lens with speedbooster also only covers a smaller area over the sensor.
In effect, the maximum sensor that can be used without vignetting for a speedbooster with factor 0.7x or 0.71x, is a 1.4 crop factor APS-C sensor with a FF lens, or an APS-C lens with MFT sensor (2X crop factor, obviously you can also use a FF lens with an MFT sensor).
Personally I own a glassless adapter, and two speedboosters, the Metabones 0.7X and 0.64X, which gives me all options
. The difference between 0.71x and 0.64x doesn't look like a lot, but it provided an additional 1/3 of a stop over the 0.71x version.An extra advantage of a (decent) speedbooster is that it also increases resolution, and diminishes lens faults (to a degree).
HTH, kind regards, Wim