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>>296493 >WHY does Canon not make EF-S compatible with full frame? Physical limitations. Unlike ever other manufacturer's designed-for-APS-C lenses, EF-S lenses actually stick further back into the camera than regular EF lenses, which means that a full-frame mirror will smack into the back of the lens if you could mount it.
To make EF-S lenses work on full frame, they'd have to come up with a whole new system for moving the reflex mirror than the tried-and-true "flip it up" method, and that would add a lot of complexity, cost, and possibility of failure.
(One way I could think of is they could slide the mirror sideways out of the way and then close a second shutter over the viewfinder. That would be slower and a lot more complex than just flipping up the mirror, and would add another potential point of failure. Another way would be to do the EOS RT or EOS 1 RS method and have a non-moving mirror, but that costs you a significant amount of light and dims your viewfinder)
And, well, they'd rather people moving up to full frame just bought new lenses. So there's not a lot of incentive for them to go through that hassle.
>>296502 The idea was that Canon could get better image quality for cheaper by pushing the rear element a bit closer to the sensor. But yeah, the other manufacturers (including the third-party lens makers who make 1.6x-crop lenses for EF mount) seem to get along fine with their default focusing distances, so in retrospect it seems like Canon probably would've been fine without making the EF-S standard.
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