When you attach a Speedbooster to any lens, it gains a stop of light gathering capability, and effectively shortens the focal length by 0.71x. So your Canon 50mm F1.4 will behave roughly like a 35mm f/1. On a GH3, to calculate full frame equivalence you need to multiply the focal length of a lens by 2, so 2 x 35mm = 70mm.

Here is where it gets complicated: you have the light gathering ability of an f/1 lens, yes, but not the depth of field. You still have the depth of field of a 50mm f/1.4 lens that was cropped down to 70mm equivalence, or expressed another way, a 70mm f/2.8 lens.

So to summarize, your 50mm f/1.4 lens with a Speedbooster on a GH3 will look similar to how a 70mm f/2.8 lens looks on a full frame camera, except that you have the light-gathering capability of an f/1 lens.

What your saying is that, in order to have an actual depth of field of f1.4 with the speedbooster - I would need to have, say, a 50mm f.07?

Now I am really confused. You are saying that, essentially, the 50mm f1.4 would effectively preform exactly like a 50mm f1.4 on my M43 camera with the lens turbo adapter - the only difference is maybe dynamic range.

@Vitaliy_Kiselev -- EV values will not be equal, provided that it was shot at the same shutter speed, f-stop, and ISO. The APS-C body will be 1 stop brighter with the Speedbooster refocusing the image circle onto a smaller sensor. Of course ISO performance on smaller sensors generally isn't as good as on full frame sensors, so the image may be a little noisier depending on complex things like pixel pitch and sensor technology, but it will be brighter.

@Vitaliy_Kiselev -- I always thought that ISO was standardized across differing sensor technologies? That a photo taken, for example, at F/2.8, 1/100s, and ISO 400 will have the same EV whether it's taken on a 20MP full frame camera or a 20MP APS-C camera? I understand what you are saying about total number of photons hitting the same number of pixel sites, and I know that the gain values will vary to achieve the same ISO value, but once the ISO, shutter speed, and f-stop match, the EV has to match too, right?

If that is true, then using a focal reducer on the APS-C body to make the image circle smaller and brighter than on the Full Frame camera should have the effect of boosting the EV by one stop. Of course you are still getting the same number of photons and you aren't magically creating light, and the only reason that it looks brighter is for ISO 400 on 20MP APS-C to match ISO 400 on 20MP full frame, the APS-C body needs to increase the gain...

Please correct me if I'm misunderstanding how ISO works and how it relates to actual sensor gain values.

The math can get confusing, so start with this: these focal reducers usually have a focal reduction factor of 0.71x.

To determine the focal length: Multiply the lens' focal length (e.g. 50mm) by the reduction factor (e.g. 0.71x):

50 * 0.71 = 35.5mm

To determine the light gathering ability: Multiply the lens' aperture (e.g. f/1.4) by the focal reduction factor:

1.4 * 0.71 = 0.994 (round this to f/1.0)

Depth of field does not change, but as the field of view widens, the image can have the appearance of a shallower depth of field.

Once you use a focal reducer, it makes more sense.