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Some time ago I tested my lenses in APS-C and full frame cameras, and concluded that the bigger sensor made my lenses provide sharper pictures (keeping constant the field of view, that is, comparing a great 50mm on APS-C against a great 90mm in FF).
But my camera has an APS-C sensor. Cue the MetaBones SpeedBooster, an adapter for mirrorless cameras that is, in theory, an optical equivalent to moving to a full frame sensor. Yes, I know it sounds silly, but it works. Read about it here.
When I say it works, I mean it does give you the depth of field of a full frame camera (that is, you get shallower DoF at the same aperture). And bokeh seems to be just as good as it is with the bare lens on a dumb adapter. But will it also be as sharp as a full frame sensor? Let's test it.
There are 3 cameras here, and I know this is a problem: FullFrame is the 5D2 (21 Mpix), APS-C is the 550D/T2i (18 Mpix), and APS-C with SpeedBooster is the NEX-5N (16 Mpix). This puts FF at an advantage, and the SpeedBooster at a disadvantage. But well, unless I get a NEX with higher Mpix count (like a NEX-7), this is what I can do (I could test APS-C without SpeedBooster on the NEX, but I think I did enough already).
Scroll down to see the results.
The speedbooster gives me an extra stop of light, and the FoV and DoF of a full frame camera, with no negative effect on sharpness or bokeh, so it's a great buy. I think it gives me an advantage in terms of sharpness too, but it's quite small, definitely smaller than moving to a real Full Frame sensor.
TO ELABORATE A BIT...
The SpeedBooster delivers a small sharpness increase. Plus, the DoF and brightness advantages make it a winner. But it's not as big an improvement as actually going Full Frame.
For example, take the 90mm Elmarit with the SpeedBooster. It is slightly sharper than both the 50mm or the 90mm on APS-C without SpeedBooster. But it is not as sharp as the 90mm on Full Frame. If all I have is the NEX-5N and I have to take a portrait, 90mm+SB is slightly better (sharper in the center, about the same in the corners) than 50mm with the standard adapter. But 90mm on FullFrame would be a lot better.
But the DoF and brightness advantages make the SB even better. I can take that portrait with the following settings:
* APS-C with standard adapter, 50mm lens set at f/2
* APS-C with SpeedBooster, 90mm lens set at f/2.8
With the same ISO and shutter speed, I'll get the same FoV, DoF and exposure in both cases. Now, looking at the tests, it's easy to see which option has the lead: because I can close the lens a bit, the one with the SpeedBooster is sharper both in the center and in the corners! Not as good as actually going Full Frame, but an improvement for sure, with no downsides whatsoever.