On 25/09/15 09:54, NY wrote:
"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 time between pressing the shutter and the exposure. It will be the
same for a digital as a film SLR as there is no difference in the
process. However if using a digital 'live view' or a 'mirror-less'
camera then the shutter has to close and the sensor be set before the
exposure can begin. 'Electronic first curtain' eliminate this delay.





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).

SLR use 'phase detect' autofocus which is faster than hunting for edge
sharpness as the system knows which way to focus (ie near or far). Some
newer mirror-less such as the Fuji have phase detect sensors on the main
sensor (in SLRs it is a separate system).



€¦snip€¦

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.


Try manual pre-focus: set the focus point to where the subject is likely
to be, concentrate on tracking alone, press shutter when in range.



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! ;-)


--
DJC
(–€Ì¿Ä¹Ì¯–€Ì¿ Ì¿)