which AF mode (Canon)

MatBin

MatBin

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matt
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Here's the scene, a person walking towards you, reasonably slowly. Poor light, you have to use a large aperture, so minimal dof, cant use flash.
You need to get the face sharp but the central AF point will give bad framing, cant use any other AF point, no time to alter it. Do you use single shot, or AI focus, by the time you have caught focus on the face (single shot mode) and dropped the lens a tad to reframe the person has moved forward a bit so the face is now out of focus if using one shot. AI will refocus but does it keep the face in focus, no point in using AI servo as it re-focuses as soon as you reframe.

Matt
 
I would agree, you could try switching to manual focus & pre-focus on a spot then hit the shutter button when the subject hits the spot
 
On the 50D I think you can asign the little joystick to AF point selection so compose the shot and nudge the joystick to quickly select the side AF point.

Or if you can pre-select it that might be better? Or even resort to auto-AF select and the camera will grab focus for you.

Failing that, if you're quick enough with your re-compose and you've not got rediculously wide aperture then they shoudl still be in focus?

Or go wide angle, focus on the subject and crop to suit like artyman suggested first!
 
MatBin said:
Here's the scene, a person walking towards you, reasonably slowly. Poor light, you have to use a large aperture, so minimal dof, cant use flash.
You need to get the face sharp but the central AF point will give bad framing, cant use any other AF point, no time to alter it. Do you use single shot, or AI focus, by the time you have caught focus on the face (single shot mode) and dropped the lens a tad to reframe the person has moved forward a bit so the face is now out of focus if using one shot. AI will refocus but does it keep the face in focus, no point in using AI servo as it re-focuses as soon as you reframe.

Matt
Click to expand...

I'm a bit confused. Is this a hypothetical situation, or one you expect to deal with? The reason I'm puzzled is that it only takes a split second to select the focus point you want and press the shutter release, but you reject this because there isn't enough time, and yet there is sufficient time to change the AF mode?

Otherwise just use AF auto select, prefocus for the distance or use central point and crop as others have suggested.
 
MartynK said:
I'm a bit confused. Is this a hypothetical situation, or one you expect to deal with? The reason I'm puzzled is that it only takes a split second to select the focus point you want and press the shutter release, but you reject this because there isn't enough time, and yet there is sufficient time to change the AF mode?

Otherwise just use AF auto select, prefocus for the distance or use central point and crop as others have suggested.
Click to expand...

One I expect to deal with soon.
AF mode can be selected beforehand if it will solve the problem, wondered if AI will adjust to keep face in focus if I have focused on the face and re-composed or if it will focus on body (new location of the central AF point on the person) thereby taking the face (eyes) out of the very small and critical focus point.
From other similar shots I have seen AF auto doesnt hit exactly the point I want to focus on, its very hit and miss (even on a 50D)
Pre focus is rubbish on an auto focus screen (split prism was so much more accurate) and I'm not sure exactly where I want to take the shot to enable pre-focus to work and be super accurate.
Guess I'll just have to try and redefine the selected focus point beforehand or use central and crop.
I want to use a large aperture to blur the background and not use wide angle as this will create a larger dof and require a bigger crop to get what I want.
Thanks for the input though.

Matt
 
It does sound particularly clandestine! Or maybe just street photography?

If you do stick to the central AF point and recompose tactic I guess the other thing to consider is having a fast enough shutter speed to avoid blur from you waving the camera around while you try to shoot quickly before the subject moves too far out of your focus area. Maybe check your ISO is high enough to allow this?

Still think the selective AF point or auto-AF points is the best way forward.



Oh, by the way, welcome to TP! It's not as crazy as some other forums I can think of. :D
 
Manual focus or changing your focus points, that is the best way.
 
Thanks for clarifying this.

I would definitely choose manual AF point selection. It's practically instant. MF was a lot easier on film SLRs, yes, but this was because the viewfinders were much bigger and brighter than the ones on a crop DSLR, not due to any inherent shortcomings in the latter. A lot of us did use the split image and microprism collars, but many preferred the simple ground glass screen too.

I struggle a bit with MF on a crop camera because my middle aged eyes aren't really up to it any more, but it's perfectly feasible and I do use it at times.

Can you practice this scenario with a volunteer replacing the future subject? Might be the best way to decide what works for you.
 
Definately AI Servo (I think that's the Canon term).

And you just really do need to get used to changing focus point on the fly. It's quicker to do this than keep changing focus modes.
 
one cheat would be to use live view...and enable face detection...?? not something my 40D has.

if you have the person in a flat plane, and their body and face is in shot, then surely they would be in the same depth of field (body and face) if you're zoomed to only see the face then there is no problem (unless they have a HUGE nose!)
 
If you know they are going to walk towards you, you can pre-select the AF point to use.
I would suggest single shot, then manual focus/override.
 
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