Manual Focus

When using a manual only Pentax lens (e.g. K or M), there seems to be no difference in the way the camera works whether the focus switch is set to MF or AF. So, apologies if these are daft questions, but is there anything wrong with leaving the setting on AF, and/or is there any advantage in switching to MF? I'm using a Pentax KP, the OVF, centre point focus, recompose when required.

Thanks in advance for your helpful replies.

Philip

Some new electronic lenses like the DFA* f2.8 70-200 can be switched between auto and manual focus so manual can be used without using the on-camera switch, and some lenses focused by the screw drive can be overridden manually just by rotating the focus ring. I would guess then that a manual only lens would work the same way with a permanent disconnect between camera and focus mechanism.

It used to be that using AF on MF lenses would activate trip focus (the shutter would fire only when the AF module thought the image was in focus). No idea if that's still the case, but it was handy for macro in particular - could just rock back and forwards through point of focus with the shutter held down and it would fire when ready.

If you leave it on AF with a manual lens, sometimes the tiniest movement backwards or forwards will stop the shutter button from operating since it will only fire when the green hexagon is lit. YOU can't tell the difference but the camera can. If you have it on AF and accidentally try to manually focus older AF lenses you will be manually forcing the screw drive mechanism to operate, right back as far as the AF motor in the camera. You could damage it, so best to set the switch accordingly unless you are doing 'catch in focus' with a manual lens like mattie mentions, above.

Adding to what David has said :

You can change a setting in camera menu from Focus Priority (default) to Release Priority. The latter setting will allow the shutter to fire irrespective as to whether the camera has achieved AF Lock or not.

While this setting is aimed at autofocus lens usage, it also has an effect when using a manual focus lens (K;M;A), in that if you have the setting on Focus Priority and leave the AF switch on, the shutter will not fire unless the AF system has a lock. Changing the setting to Release Priority means with a manual lens the camera will fire even when the camera thinks the subject is unfocussed. Same goes for an AF lens.

Unless you want to use the "Catch in Focus" feature, you are best switching the camera AF switch to M.