Your comment about speed boosters has nothing to do with mirrors, just adapting and focusing the light captured by a full frame lens onto a smaller sensor. Clearly lens focusing distances factor in, but mirrors don't necessarily do so. I'm also not convinced about your focal length assertions. A 50mm lens remains a 50mm lens. However the field of view could be potentially the same as a 32mm lens designed for a M4/3 sensor. As for DOF, it will be thin at the widest aperture.

Without using intermediate adapters (with lenses) I have a modified 55mm f/1.2 Canon FL lens on my EOS APS-C body. I'm never short of light but the DOF wide open is razor thin. From a field of view perspective it is equivalent to a 88mm (portrait) lens on a full frame.

It's all about the frame of reference and the results. So far as I understand it, what the speed booster does is provide a couple of extra equivalent stops from concentrating the light at any aperture setting. How useful it is I have no idea.

For example, I use my film age Olympus 90mm f2 macro on my former Olympus E-M5 and now my E-M1mrII. The 90mm macro was a full frame lense and on a micro 4/3rds camera has a field of view the equivalent of a 180mm f2 in full frame terms. The adapter only provides the distance required to place the lense / camera in the proper relationship. The lense aperture has to be manually set, there is no autofocus (What is manual focusing?), and exposure is only proper shown if one shoots in aperture mode or manual mode. Olympus indicates it is only sharp to their standards between f4 and f11. Their standards are pertty tough since I commonly shoot with it at f2.8 and occasionally at f2 and I see very little degradation of quality.

Remember, there's a 2X crop factor on Micro 4/3 cameras. So a 50 without the SpeedBooster has the full frame equivalent field of view of 100. With SpeedBooster, the focal length is 32mm, but the full frame equivalent field of view is 64mm.

Maximum aperture is limited by the inner barrel diameter of the SpeedBooster to f/0.8, regardless of the attached lens.

The SB works on the principle of minification, the opposite of magnification. So the image gets smaller, yielding a wider field of view. The light intensity is increased by 1.33 stops. And the modulation transfer function curves show sharper, higher resolution performance than you would have without it.

One benefit of mirrorless is focus accuracy. There is never any need for lens-to-body calibration. The sensor handles focus! So razor thin DOF isn't a problem if you can autofocus on the right thing, OR manually focus using focus peaking or focus magnification features. Besides, on Micro 4/3, you have a two stop increase in DOF without the SpeedBooster, and about 2/3 stop increase in DOF with it.

All in all, it's the only dSLR lens "adapter" that makes sense. If I had Canon EF-L series lenses, I'd get one. With Nikon lenses, you lose any and all the automatic features of your lenses. And I have Nikon lenses from long ago.

Sorry for the somewhat offtopic responses :-)



I've been itching for an excuse to buy a mirrorless body that does 4K video. I did marry the Canon line of lenses, so I think I'd like to give this a try. I'm planning some trips to places that don't allow flash photography, e.g. Vatican and I'd like to go armed for bear for handheld available light so the combination of a super fast lens and in body stabilization looks like a winner in those situations.

I need to study what is available. the Olympus OM-D E-M1 Mark II looks interesting but they haven't been known for good quality at higher ISO settings. The Panasonic LUMIX GX8 also looks like a good choice. If I do give it a try, I'll be sure to share my results.