EXCERPT page containing first few paragraphs. 2021-12-08 01:18:02
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General commentary on the Metabones SpeedBooster.
Not applicable to DSLRs
A Nikon or Canon DSLR could in theory use the SpeedBooster concept with medium format lenses (such as Leica S). But the mirror box gets in the way, so unless and until Nikon or Canon makes a full frame mirrorless DSLR (Nikon F mount or Canon EF mount), the concept does not apply to DSLRs. According to designer Brian Caldwell:
The problem with Leica M lenses is that they have a significantly shorter working distance than DSLR lenses. The glass in the Speed Booster is packed very tightly, so that there is just barely enough wiggle room to do a factory adjustment on infinity focus. If we tried re-design in order to accommodate Leica M lenses the image quality would drop like a rock.
We did debate whether to make the Speed Booster compatible with the Canon 50mm f/1 lens to provide a route to f/0.70 heaven. However, this would have required making the outer element of the Speed Booster much larger than it currently is, and this would have caused compatibility issues with other much more popular lenses. In addition, the Canon 50mm f/1 is discontinued, rare, and expensive, and even if you transformed it to a 35mm f/0.7 you might not be able to find a camera sensor capable of accepting an f/0.7 cone of light (~+/- 45 degrees for the extreme marginal rays). For all these reasons we decided to scrap the idea of trying for f/0.7.
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