AI Servo, AF Points and IS

[drounding]

I have a two part question that I'm hoping the more experienced can explain for me....

I have a Canon 70D and take some fast action photos. I have it set up for BBF, AI Servo, centre AF point only, IS switched off and I'm using a high shutter speed.

Q1. Whilst holding the focus button down with the subject initially in the centre AF point. If the subject now moves out of the centre AF point does the focus track the subject or does the subject have to be kept in the centre AF point for AI Servo to work correctly?

Q2. Although it is not required due to the high shutter speed, what effect would IS being enabled have on the AI Servo focusing?

 

[Phil V]

Hi Duncan
1, it depends what mode your AF is set to, it's covered in the manual but it's not that straightforward; the simple answer is that you can set it like that.

2, IIRC Depending on the lens and camera, there's a hit to AF speed when using IS, however it's measurable but you never feel it. It's certainly not something I've ever noticed but I believe it's there in the specs.

That didn't really help as much as it should have.

 

[cmcm789]

On my 60D (which has a less complicated Af system) AI Servo always starts by selecting the subject in the centre point. After this if you have selected the centre point only, it will lose focus once you go outside the centre point boundary and then latch on to the next subject in the centre point. If you have all points activated, it will initially lock the subject with the centre point and then track it across the others as the subject moves around the frame.

That's interesting about IS slowing shutter speed. I normally leave it on on my 55-250, but for sports I might just turn it off as it's not needed. Any help with AF speed and accuracy would be good.

 

[HoppyUK]

I have a two part question that I'm hoping the more experienced can explain for me....

I have a Canon 70D and take some fast action photos. I have it set up for BBF, AI Servo, centre AF point only, IS switched off and I'm using a high shutter speed.

First thing to say is that camera set-up and technique are vitally important for good servo-tracking performance. Not sure what options the 70D has without checking, but the 7D for example has loads of them and they need to be set right, and differently for various situations. Time spent on this will pay back with greatly improved hit-rate.

Q1. Whilst holding the focus button down with the subject initially in the centre AF point. If the subject now moves out of the centre AF point does the focus track the subject or does the subject have to be kept in the centre AF point for AI Servo to work correctly?

Yes and no. With phase-detect AF, the AF points don't move - unlike some contrast-detect systems that can actually recognise and follow a subject as it moves around the frame, eg face-detection. With phase-detect, if multiple points are selected, and one or more points are always on the subject at any given time, then it will track it. But if the subject is small enough to fall in-between AF points, it won't.

The best technique for action tracking is usually a single AF point (preferably the centre) often with expansion points enabled. Then plenty of practise to keep that AF point nailed.

Q2. Although it is not required due to the high shutter speed, what effect would IS being enabled have on the AI Servo focusing?

No hard and fast rules. With many fast-moving subjects, the priority is keeping the AF point reliably pinned and IS helps a lot with that by stabilising the viewfinder image. IS does not slow down AF and if anything it improves AF accuracy by stabilising the image it's working with, but that's only a theory and I've never seen any real evidence. The downside of IS is if the tracking movement is unpredictable and zig-zagging around, this can fox the IS into actually working against you. Fine for panning motorsport in mode-2 say, but maybe not for BIF'ing.

The best solution to all these issues is a monopod (highly recommended ) with IS off.

 

[drounding]

Thanks for the pointers. Time to get the manual out and check on the additional AF settings for a start I guess.