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Will new 600 and 800mm lenses autofocus on R/RP?

Started Jul 9, 2020 | Discussions thread
John Sheehy Forum Pro • Posts: 26,688
Re: Yes and no

quiquae wrote:

EOS R actually has exactly the same low-light focusing spec (EV -6 @ F1.2) as R5, and only half a stop behind R6 (EV -6.5), which suggests that it shouldn't lag much the newer bodies in low-aperture focusing, if at all. But we'll see.

AF ability is not just about light quantity. It can be heavily dependent upon light geometry; the different angles from a which a source point hits different parts of the entrance pupil and reconverge at the sensor.

So, turning up the light can't compensate for losses in AF ability due to high open f-numbers.

Just about any DSLR, for example, can focus better on big black and white lines at f/5.6 indoors than at f/11 pointed straight at in direct sun, despite much higher light levels on the AF sensor for the latter.

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