How does camara decide which AF point to use?

How does a camera decide which autofocus point to use? With 11 to choose from why would one be any better to use than the others?

Mine appears to go "Eany meanie miny moe" unless I set it to centre spot then focus and re-compose.

becuase it's closest to the part of the scene you want in focus ???

I like this theory! However how does the camera know which bit I want to focus on?

It doesn't and that's the problem. So, if you use centre focus only you can focus, lock, recompose, shoot and get it right every time.

I've never done any experiments with it but I would wonder if it changes with the mode you select (for those cameras with action/landscape/macro/etc)?

truthfully it measures contrast and brightness and looks for sharp edges/lines and then uses a mathematical algorithm to adjust focus to the point that is most likely to be a subject within the af zones..
in other words voodoo or witchcraft. Stick to the centre spot and recompose method.

Except at airshows using AF-C.

I defy most people to pan so accurately as to keep a fast jet hurtling past at 400 mph bang in the centre AF spot.

If you can't, and I certainly can't, what happens is that you get it slightly wrong, and the lens hunts to infinity and you lose all sight of the aircraft in the VF.

Auto-AF-spot every time at airshows.

The background is almost invariably featureless, in which case the camera is much better at choosing what I'm looking at than I am.

It's the only case, mind you. Otherwise, I'm with Don. You have to be a bit careful with extreme wide-angles, because the recompose can dramatically change the subject distance from the focal plane.

Do you mean AF-C Chris?

I found this simply not up to taking fast aircraft shots with my K20 and 150-500 or DA* 300.

I've done race cars using the pre-focus in manual and simply timing the shot... I don't care much for airshows, but have taken some plane shots when I went skydiving.

I think the issue boils down to predicting the spot and releasing the shutter just as the plane gets there.
but for sure predictive continuous af will yeild more keepers than my way. I compare my style more akin to a military sniper, as opposed to the machine gunning marines. I'm usually content to make one shot one kill, so to speak.
Switching brands was another option to get more keepers in the past.. let us hope the new kr/k5 will change all that!

I'm normally a centre spot and recompose focusser. Except when taking shtos without even looking through the viewfinder

I've never shot an airshow, but I did try shooting a few greyhound races last year and failed miserably. Lack of light didn't help.

Magic Pixies (tm).


Following scientific development from the Sirius Cybernetics Corporation (makers of the famous Neutrimatic Drinks Dispenser), the Magic Pixies (tm) in your camera will in less than 0.1 microsecond carefully analysis the scene, cross-reference it with your brain to determine what part you want in focus then randomly pick another part of the scene, focus instantly on that, then wait until the perfect moment has passed before allowing the shutter to open.

These Magic Pixies (tm) never need to be serviced, as they feed exclusively on your rage.





... or in other words, Centre-Spot and recompose is the way to go. Manual Focus is quite nice, too...

My little sony compact has smile recognition.


I'm suer it will be possible in the future to have object recognition, so that at an airshow you can tell the camera to focus on what it thinks is a plane flying

I'm a portrait photographer.
you want to impress me?
Show me a camera that can recognize ugly and fix it.

I do mean AF-C, yes, although I wouldn't say I use it all the time, and in fact I only use it to any significant extent at airshows.

Haven't used either of the lenses you refer to - most of my recent airshow stuff has been exclusively with the 55-300, although I'm getting fairly desperate for something longer.

I'd describe AF-C on the K10D as 'not bad' rather than good, but it's definitely better than AF-S if the camera-to-subject distance is changing very fast. And the more you can do to help it, by getting the thing prefocused as close to the right distance as possible (I couldn't do without quickshift either), the less work it has to do so the quicker it is.

It's a black art really, with lots of frustration, any attempt to do FJ work with the K10D. Hopefully I'll have a K-5 and a Bigma by the time the next Farnborough comes round.

That said, even if I'm using AF-S with aircraft, the same argument about Auto-AF-spot selection applies.

This isn't really a big deal at all - it can already do that. Aircraft in flight are usually set against a relatively featureless background, and there's usually nothing in the foreground for it to get confused with. So as I said above, Auto-Af-point selection achieves exactly that.

What would be a lot more useful, would be something that could tell the difference between birds and twigs, so that as a small bird hops around in a tree, and you follow it with the camera, it keeps the bird in focus.