300mm focus over-ride conundrum

I have the earlier 300mm f2.8 Canon lens. Sometimes it permits

manual focus over-ride and sometimes not. I cannot figure out any

reason or setting which causes the difference. A friend suggested

Al servo would not permit whilst One shot would - not so it can be

on or off in either mode - apart from "must be broke then" can

anyone shed any light ? No manual as it's thirdish hand.

I believe that the first 300mm f/2.8 needs to have the shutter button half pressed in order to use manual focus, like the 85/1.2. I know you cannot manual focus without a camera attached.

I'll check that out - though I'll have to consider the implications of controlling focus by the thumb button on the back of the camera - maybe that's where the problem lies. Thank you.

You can't override with AI Servo engaged.

True - not while actually focussing - but one can manually focus in A1 Servo if focus button (as opposed to shutter press) is not pressed. Is there a difference between 1D and 1D MkII on this or any custom function which might affect it ?

<p>I don't have any electronic FT-M lenses (like your original 300/2.8) so I've never really paid attention to this. I know that some bodies do have CFs to control how electronic FT-M works; I don't know the details. There are Web sites with lists of CFs for various bodies and maybe one of them would have more details.</p>