Autofocus points

Janice

Janice

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Janice
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I generally use manual focus, so I'm sorry to sound such a dork, but if I was using autofocus and had it set to use ALL autofocus points......what if I was taking a picture of a crowd of people which stretched a long way back.
Say a couple of the focus points fell on the front people, some fell on middle people and some fell on far away people.
How does the camera focus in that situation, which overrides which?
 
which ever is easiest for it to focus on id imagine...... thus it gives you no control

you can set the camera to select one point for autofocus, thus meaning it focuses on what you want not on what it wants, remember your camera is basically stupid.

ive never let the camera decide for me, i use the centre focus spot and then recompose the image before firing the shutter
 
I believe it focuses on the closest object if all points are selected and all are covered with people, or if its bad / low light and the closest point is too dark, it would be the one with the greatest contrast range, to enable focus lock. I think.
 
My guess it that it would be set up to focus on the nearest people. You can check this wherever you're sitting right now... select all AF points and focus around the room with objects at different distances from the camera. You'll find that it fairly reliably focuses on the closest object. I tend to use all AF points though, when (for instance) tracking flying birds,(when you'd have no hope of keeping one AF point on the bird), and Servo mode to keep focus once locked on.

For the situation you describe Janice, I'd use one AF point, focus on the desired point in the crowd, then control the DOF you want by use of a suitable aperture.
 
I *think* it focuses based on a quick equation, of which are likely to cover most of the scene.

So if you have 9 AF points (in the case of the 30D), and 6 of them are in the distance, 1 in the middle and 2 in the foreground..It'll take the 6 in the distance.

I *think* anyway. It's supposed to be 'clever' (for want of a better phrase)
 
I've never really understood the whole multipoint AF thing either Janice. I have never understood how a camera can focus on multiple points at once. Hence the reason I only ever use the centre focus point. I used to frequently get the wrong area of my shot in focus till I switched to just using the one AF point. There must be something in it though coz the better the camera the more AF points it has! Can anyone explain? :shrug:
 
Doesnt the camera choose which one to focus on, and focuses on it??
or
Does it look at them all and choose a best fit?
 
Grendel said:
I've never really understood the whole multipoint AF thing either Janice. I have never understood how a camera can focus on multiple points at once. :shrug:
Click to expand...

LOL. It can't. It only focuses on one, which is why in 'normal' shots without undue movement you should choose one AF point.

The 1 series cameras have 45 AF points, although you have a couple of options for less. Use all the AF points and servo (continuous) focus when you're tracking fast moving objects in the viewfinder. In this mode the AF points pass active AF from one to the other as necessary (once you're locked on) to achieve continuous focus till you take the shot. :)
 
Marcel said:
I *think* it focuses based on a quick equation, of which are likely to cover most of the scene.

So if you have 9 AF points (in the case of the 30D), and 6 of them are in the distance, 1 in the middle and 2 in the foreground..It'll take the 6 in the distance.

I *think* anyway. It's supposed to be 'clever' (for want of a better phrase)
Click to expand...

Bod this may be true in the 'none creative auto everything' modes (which hopefully no-one here uses), where all the settings and decisions are made for you, but I'd seriously doubt it applied to Manual, AV or TV modes, in fact I'll fairly confidently say it doesn't . The only possible benefit would be that the camera would choose an aperture based on this info to decide depth of field which is the last thing you want - you want to be making those decisions yourself.
 
I probably quallify as a -> :geek: , but here goes...

DSLRs all :thinking: use passive autofocus (they use the available light to decide what's happening). Autofocus assist is where extra light (flash/IR) is beamed out to illuminate the scene.

How the A-focusing works is that as the lens searches from one end stop to the other all of the N points are simultaneously evaluated (assuming all are selected).

Each AF point is divided into a number of segments vertical and horizontal (not sure how many in each segment >5 probably). As the lens travels from close->infinity the contrast between adjacent segments is evaluated. When this reaches a maximum (above a certain theshold depending on the level of ambient light) then the scene is deemed to be in focus.

A nice edge is best, as you've probably noticed. This is because a sharp difference in contrast can be detected.

If you assume 5 segments in a horizontal axis then what the AF point on the camera "sees" the lens tracks is something like this:

55555
44566
33477
22288 <- Sharpest difference in contrast = focused
33566
55555


This also explains why in low light conditions the camera "hunts". Also why you can't focus (easily) on a sheet of paper, but it is a doddle to focus on its edge. Low contrast situations are hard for the AF to work on.

Finally... it is possible to have more than 1 point active at the same time if more than one area "triggers" sets the I'm in focus now flag. In which case the camera is "happy" since as far as it cares the scene is focused "click"

Of course it's not quite that simple, but the clever CPU's hanging off the back of the AF point do all sorts of algorithmic munging to work this out


Hope that helps
 
Cheers Oldigt - that actually makes perfect sense - I'll cut and paste that for future reference when we're teaching the Mongs here at work basic camera-handling...
 
it does mate, but thats the distance of what's in focus......from the focus point.
 
Marik said:
surly the distance of what is in focus, depends on the aperture
Click to expand...

Yes of course. I did say it wasn't that simple :')
Your DSLR AF system "sees" what you see through the viewfinder. We all know that
we focus with the lens wide open, then at shot time the aperture closes to the desired
Fn and then "click" (one shutter drops, closely followed by the 2nd).

Now I'm getting into a grey area here, so this is somewhat lesss certain. :shake:
I know the AF system also works out the "distance" from camera to X (by comparing the separation of 2 sensor points - bit og Triganometry) then I "presume" whacks that info through some algorithm to work out if those points are going to be in focus at the desired Fn.

Works something like this... clicky
 
I think it works out a mean point of focus - depth of field isn't really a consideration as the focussing takes place at WOA.
Also most DSLR have different pre-sets that you can specify - closest-subject priority, area mode, dynamic modes etc, all with several sub-settings that you have to play with.
I'll be honest that apart from the basic settings, I prefer to use single-area mode and manually select which cursor to use depending on the framing I want.
Only once have I successfully used the predictive multi-AF settings that allow you to 'lock-on' to a moving subject and keep it in focus while tracking and shooting multiple images - more to do with not previously having the very latest lenses as much as the camera settings not being correctly set up.
 
Thanks for helping to clarify guys - Oldigt, your knowledge is impressive :notworthy:

So am I correct in saying that for "general" photography (landscape, portrait etc) it is best to use a single AF point, prob the centre one but for motorsport photog or anything that is a moving target it is better to use multiple AF and servo focus?
 
I'd still use single AF point, but with servo focus (for motorsport)
 
Marik said:
I'd still use single AF point, but with servo focus (for motorsport)
Click to expand...

I agree, when I've tried shooting bikes in the past with multiple AF points I find that it tends to focus on the background or the track rather than the bike itself. So I tend to use a single AF point and vary which AF I use point, depending on the track/corner I'm shooting and which way the bike is leaning with respect to myself, either towards me or away from me.

And after all this I still manage to get them out of focus :)
 
yeah good advice, Not entirely sure how the multiple AF thing works, but i'd hazard a guess at it focusing on the big section of stuff. for example if you're shooting a bike against a busy background using multiple things, it'll pick up the background. anyway.. off to town with a loevly young lady :D


PS. Andy, you're shots are great
 
Thanks for all the advice guys, and sorry if I've hijacked your thread Janice, it's just a subject that's been confusing me for some time!

So if the general advice is "use one AF point", is there ever a situation where it IS better to use all the AF points? I have to say, since switching to using just the centre point I've never had a situation where I thought the focus was off (other than the ones where I know I've cocked it up myself!), whereas when I was using all the AF points I quite often found the wrong area of the shot in focus.

Sorry if I'm going on a bit here, I'm just really keen to understand this one! Thanks guys!
 
Urrm nope. :thinking: ... not that I can tell. Maybe the 1D chaps have it easier here?

I've a bad habit of chopping peoples legs off if I use the center AF for full length peeps shots as I focus on the head (why not), but then forget to re-frame so the whole body fits. To fix this I tend/try to use the "topmost" focus point which usually means that the feet or at least knees are in the frame :)
 
oldigt said:
Urrm nope. :thinking: ... not that I can tell. Maybe the 1D chaps have it easier here?
Click to expand...
The 1D gives you the option to use the centre point but also to auto expand to the surrounding 7 (I think it's seven ) points as well. I used it the last couple of outings I had and it works great :)

A lot of the time though I'll pick an off centre point to make sure I'm getting focus on the rider rather than the bike
 
I think using a single AF point depends on the size of the moving subject. You should be able keep one AF point on a moving car for instance in the viewfinder, and it's speed and direction are predictable.

Photographing the Red Kites though, their flight patters were very erratic and unpredictable and they weren't filling the frame well. I selected all 45 auto focus points with servo, and got a high percentage of sharp shots.
 
To my understanding just using one AF point makes the AF faster because its only bothering about one point and not 9, which in sports is awsome. I always use just one AF point for anything really never use 9. to my point of view the camera has too much control over the AF if I use 9. But look at my photos! :gag:
 
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