"DJC" wrote in message ...
For a SLR the time is limited by the speed of the mirror.

I'm intrigued by this statement. Surely the mirror remains down at all times
except for the brief instant either side of the shutter opening.

The autofocus time is governed by where the lens is currently focussed (ie
was it previously focussed close-up and you're now focussing on something in
the distance) and on how complex the picture is - how much the object that
you are focussing on can be distinguished from the background, which governs
whether the mechanism needs to "hunt" either side of the focus point to
select the focus that gives the sharpest focus (which I think is often
judged by the camera as being the highest contrast on an edge-detector).

All of that is done with the mirror down. Once the focus is correct, the
photographer presses the shutter release. Only then does the speed of the
mirror come into effect, governing the delay between pressing the button and
the shutter opening.

Some cameras even have a setting (on or off) which will only release the
shutter when the AF says "in focus" to prevent you taking a photo while it's
still trying to achieve best focus.

I've had limited success with taking photos of flying birds (eg puffins on
the Farne Isles returning with the beaks crammed full of sand eels) because
the buggers move extremely fast and you only see them for a few seconds, so
framing and focussing in that time is very hit-and-miss, as is following
them in flight when you are hand-holding a camera with a long lens (I really
ought to take a tripod so I can concentrate on following without having to
also bear the weight of camera and lens). Either you choose a small
focussing zone in the centre of the frame, in which case the bird *will* be
in focus, but only if you can position it dead-centre in the frame (or
wherever you've positioned the focus zone). Or else you set a larger focus
zone in which case the AF focuses on (I presume) the closest object that it
sees in that zone, which may be the wing-tip of another bird, or even (if
the bird is flying very low) part of the ground. Continuously-adjusting
focus rather than single shot (only when you initially half-press the
shutter release) is essential.

I never know whether it's better to use a shorter lens (eg 200 mm which is
equivalent to 300 in a 35 mm camera) and choose birds which are close enough
to fill that field of view, where there may be a fair spread of birds at
different distances, or to use a longer lens and go for birds further away
where the spread of distances will be smaller. Obviously a closer bird will
pass through a greater angle as it flies past me and I follow it.

This really is a very demanding test of autofocus - and a situation where
manual focus almost certainly isn't much use because the human eye can't
react quickly to keep an object in focus where the distance is rapidly
changing, and if you choose a fixed distance and wait until the bird's path
takes it to that distance, you are dependent on reaction speed which makes
things very hit and miss.

If only puffins could be made to follow designated flight paths so you could
position yourself where you were a constant distance away as they fly! ;-)