Patent: Lots of fast RF prime lens optical formulas
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Well there are some interesting lenses such as the 28-70 f:2.0 (which can use the IBIS of R5 / R6), the 15-35 L f:2.8 which gets IS (with the EF mount only the f:4.0 has IS), ditto the 24-70 f:2.8 IS. The 70-200 f:2.8 IS is extraordinarily compact, not sure of course whether Canon could have produced the same lens in EF but chose to have all it's L lenses in 70-200 be internal focusing only. The 35mm f:1.8 is interesting because the large glass is camera-side and the front lens is small, contrary to the EF 35mm f:2.0 (not quite sure what to make of that, but interesting). I am personally waiting for the 70-135mm L f:2.0, as far as I know nothing similar in EF. To your point I think that the benefits of RF in terms of optical design decrease as one goes to longer telephoto lenses, with only the electronics of RF (4 additional pins, faster data transfers, interaction of IBIS and lens IS) and a slightly wider diameter allowing for some innovation. The biggest difference s of course the distance to sensor, without a mirror the glass can extend closer to the sensor. It is surprising that more wide angle lenses have not come out yet because that is where I would expect the RF to deliver a bigger impact, e.g. for the successor to the 11-24mm, or a 14mm / 17mm / 20mm prime (the latter two were present in the FD lineup but never had an EF equivalent).
If the “back focus” distance between a lens’ rear element and the camera’s sensor is too far, the light entering the rear element is condensed and is harder to do aberration correction on. Lenses counteract this by making the front element (and whole lens) larger and bulkier. But by moving the rear element closer to the sensor (i.e. a short back focus), Canon spreads out the light rays passing through the rear element, making it easier to reduce aberration. (https://petapixel.com/2019/06/17/canon-this-is-why-rf-lenses-are-outstanding/). Does this answer your question (full disclosure I am just a camera enthusiast, not an optical engineer).
Thanks for the explanation. It makes a lot more sense now. As far as I understand is that the enhancements by the RF mount mostly result in increased image quality. About that RF 70-200mm, the fact that it is a pumper zoom put me off from it. Had the Nikon 200-500mm F5.6 which is a pumper zoom as well and it got quite dirty inside when you use it a lot outdoors. The possibility of placing larger elements more to the back with the RF mount makes me quite interested in what a RF 400mm F2.8 or 600mm F4 can shave off in weight. Both Sony and Canon have managed to shave quite some weight off of their latest super long telephotos by placing the elements more to the back and therefore be able to shrink them quite significantly.
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