Canon EF 100-400mm- Using all focus points tricky?

Canon EF 24-105mm f/4.0L IS

05/30/2008 09:35:52 AM · #1

<--- on 40D, Canon EF 100-400mm f/4.5-5.6L IS

I usually use just the centre-point for focus, but thought I'd try using all-points yesterday. I thought it would make holding focus easier, and allow me to pan on my subject while keeping her in focus even if I wanted her off-centre.

For the first two examples:
Av mode
AF was set to servo.
All focus points.
F 5.0
1/250sec
ISO 800
Focal length 235 (not allowing for 1.6 crop-factor)

For this last one:
1/320
Focal len = 190
(all else as above)

======================

So is there some trick to using all focus points in servo mode? I thought that simply the closest object would be chosen, which clearly was NOT the case in these, and many-many other shots. Sometimes the focus fell between the subject and the people in the far background, on nothing but grass.

Am I forever stuck having to centre-compose these types of shots using only one focus point? There's no feasible way that I personally can toggle focus points on the fly during games.

Message edited by author 2008-05-30 10:32:24.

scalvert

Canon EOS-5D Mark III

05/30/2008 09:51:23 AM · #2

I would guess that part of the problem is shallow DOF with moving subjects. At a distance of 30ft, 235mm at f/5 on your camera would give you a DOF of about 11 inches. If the kid is running toward or away from you, even the slightest delay between focus and capture could put your subject out of focus- especially since the servo mode could lock onto any of several moving targets.

brownsm

Canon EF 24-70mm f/2.8L USM

05/30/2008 09:54:49 AM · #3

Personally, I've had better luck using just a single focus point (usually center, then I crop afterwards), and keeping the main subject on that focus point. The servo mode tries to predict where the focus point will be, taking into account the shutter lag and how the subject is moving through the frame (focus wise). if it is focussing on different objects, it won't be able to correctly predict how the focus will change during that shutter lag. remember, sometimes those focus points will only be over the grass or background, and that will mess things up.

Personally, I don't use the feature enough to know how best to use it. that's why I take the simple way out!

eta: plus what Shannon said. In addition, your focus distance will also affect how well the servo motors can track. the closer you are, the slower the subjects need to be. Now, I'll admit, I don't know if kids can run faster than that speed!

Message edited by author 2008-05-30 09:56:21.

hopper

Canon EOS-80D

Canon EF 35mm f/2 IS USM

05/30/2008 09:59:21 AM · #4

Is it possible that due to multiple focus points - your camera never locked onto the correct target in the first place, and so was tracking the wrong subject?

Camabs

Canon EOS-20D

Canon EF 35mm f/2.0

05/30/2008 10:07:53 AM · #5

Originally posted by Strikeslip:

So is there some trick to using all focus points in servo mode? I thought that simply the closest object would be chosen, which clearly was NOT the case in these, and many-many other shots. Sometimes the focus fell between the subject and the people in the far background, on nothing but grass.

Using all focus points means that the focus will be averaged over all those points. Focussing on everything means focussing on nothing. That's no problem for a wide angle or at very small apertures (i.e. as long as you have a deep DOF, you can do about anything without going wrong).

Originally posted by Strikeslip:

Am I forever stuck having to centre-compose these types of shots using only one focus point? There's no feasible way that I personally can toggle focus points on the fly during games.

Don't toggle on the fly then. Just do what you do now, but use a different focus point.

hopper

Canon EOS-80D

Canon EF 35mm f/2 IS USM

05/30/2008 10:10:28 AM · #6

did you read this somewhere?

Originally posted by Camabs:

Using all focus points means that the focus will be averaged over all those points.

Canon EF 24-105mm f/4.0L IS

05/30/2008 10:24:47 AM · #7

Originally posted by hopper:

Is it possible that due to multiple focus points - your camera never locked onto the correct target in the first place, and so was tracking the wrong subject?

It may be jumping from target to target. I don't see how it can track an object across the viewfinder, as I'm panning along with the subject, and I don't always keep the subject in the same spot in the frome while panning, depending on where the secondary subject (ball) is.

scalvert

Canon EOS-5D Mark III

05/30/2008 10:30:25 AM · #8

Originally posted by Camabs:

Using all focus points means that the focus will be averaged over all those points.

You may be thinking of A-DEP (auto DOF), which tries to get everything in focus. Having all the focus points active does not average them, but attempts to select the "correct" one.

05/30/2008 10:32:10 AM · #9

Oh yeah, I was in Aperture Priority mode.

hopper

Canon EOS-80D

Canon EF 35mm f/2 IS USM

05/30/2008 10:32:47 AM · #10

The 40D manual states (i just looked it up) that when in servo and multi-point. The camera first uses the center focussing point, and when focus is achieved, the camera will be able to track that point even if it switches to another point and as long as it stays within the points.

05/30/2008 10:39:49 AM · #11

Originally posted by hopper:

Oh shiz, suh-weet! RTFM. Thanks hopper, I'll try that out.

brownsm

Canon EF 24-70mm f/2.8L USM

05/30/2008 10:43:33 AM · #12

Originally posted by Strikeslip:

Originally posted by hopper:
The 40D manual states (i just looked it up) that when in servo and multi-point. The camera first uses the center focussing point, and when focus is achieved, the camera will be able to track that point even if it switches to another point and as long as it stays within the points.

Oh shiz, suh-weet! RTFM. Thanks hopper, I'll try that out.

excellent! whoulda thunk to read the manual? I'll have to remember this tip (the servo tip, not the reading-the-manual one)

scalvert

Canon EOS-5D Mark III

Canon EF 24-105mm f/4.0L IS

05/30/2008 11:05:51 AM · #13

Originally posted by hopper:

The 40D manual states...

What if you're not shooting in manual mode? ;-P

05/30/2008 11:10:47 AM · #14

Originally posted by scalvert:

Originally posted by hopper:
The 40D manual states...

What if you're not shooting in manual mode? ;-P

That only gets one guffaw on the punny scale. :-P

trevytrev