Then again, AFAIK, EOS lenses don't transmit focal length (virtually) stepless and some don't transmit it at all.
EOS lenses do transmit the focal length (if focal length means 'zoom setting', which it does if i'm not mistaken). On my RT with TBE I get a very precise readout of the current zoom-setting, in millimeters IIRC. AFAIK they do however not transmit the focussing distance.
Oops, yes, you're right. Some EOS lenses don't transmit *focusing distance*, but I think all of them do transmit their FL. The question remains, what *exactly* do they transmit? The FL as marked on the zoom ring/lens barrel or the *true* focal length they have at the actual focusing distance? These can be substantially different, due to the optical design of most EOS lenses. Most of them change (decrease) their focal length at close focusing distances.
Does one need to know the focal length or the focussing distance for in camera IS? Or both?
I'm not sure. I think it depends on what kind of shake you want to compensate. As long as there is no angular shake, it should be sufficient to stabilize the sensor position in x and y. (not along the optical axis, obviously, as this would affect the focus) But if there is angular shake, I don't know. Stabilizing the sensor in x and y should still help, though.
Thomas Bantel
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