Some questions about autofocus speed

[fishprof]

I’ve been lurking here for quite a while and have been learning a great deal from these discussions. I have a few questions about autofocus speed and the relative importance of the lens versus the camera body. I shoot a 7D and I’m primarily using one of two lenses – a Canon 300mm f/4 L with or without a Canon 1.4X TC III or a Sigma 150 mm f/2.8 macro with or without a Sigma 1.4X TC. I shoot mostly wildlife, primarily birds, and macro, especially butterflies, dragonflies, and other smallish nature subjects. I shoot about 50% hand-held, about 50% monopod mounted and I almost always use the central autofocus spot on static subjects. My vision is not very good so I rely almost exclusively on the camera’s ability to autofocus my subject quickly and accurately.

1. I have a sense (but no real measurements) that the addition of the TC to either lens slightly slows autofocus compared to the “naked” lens. Under identical shot conditions and lens/camera settings, is there a reason to expect that addition of the TC would slow autofocus lock? Maybe it is just my imagination?

2. I’m thinking about upgrading to a 7DII next year. Will the new and improved autofocus system in the 7DII make a difference in improving the speed of autofocus acquisition with my current lenses? In other words, given identical shot conditions and camera/lens setting, will the 7DII be faster to autofocus than the 7D or is speed going to be limited by the lens system I have in use?

3. I have heard it said that the image stabilization (IS) system actually slows down autofocus lock. One suggestion I have heard (or read somewhere) is to turn off IS when shooting birds-in-flight to allow the camera to lock focus more quickly. Is there any truth to this?

Thanks for your help.

 

[Loswr]

1. Adding a TC causes slower AF, intentionally based on the camera's firmware. A 1.4x slows movement of the focusing group by 50%, a 2x slows it by 75%. It's not as bad as it sounds, because with the TC mounted, the focusing group doesn't need to physically move as far. Still, it's usually noticable in practice with all but the fastest-focusing lenses.

2. It will likely speed up AF lock a little bit, newer models generally have improved AF algorithms, and the addition of metering data used by the 7DII may help as well.

3. I don't believe IS slows down Af lock, if anything it would speed it up to the extent that it provides a more stable image to the AF system. When shooting BIF, a fast shutter speed is usually required – fast enough to reduce or eliminate the benefit of IS. Leaving it on at high shutter speeds shows no effect on IQ with the lenses I've tested (but that does not include the 300/4L IS, and that lens has an early IS system that's not tripod sensing and may behave 'worse' than more recent iterations of Canon's IS). You can mount the camera on a tripod, use a fast shutter speed with and without IS engaged and test it yourself.

Hope that helps...

 

[Deleted member 91053]

I had the 300 F4 L IS for quite a while and loved it, I only sold mine because I got the 300 F2.8. As to the IS if you are putting it on a tripod or shooting anything moving then turn it off otherwise you will miss a lot of shots and get a noise like a coffee grinder! I am not knocking the IS on the 300, it is just that it doesn't like any significant movement.
Unlike Neuro (sorry Neuro!) I disagree about IS and AF performance. The difference is not large but it can be significant on things like songbirds flitting about in trees. If your IS is already running then there is no difference on static subjects, however if it isn't then the lens cannot focus until it is. I know IS fires up almost immediately but it is not instantaneous. For example yesterday I got 3 shots of a Bittern standing, then it was off! That was 3 frames at 10fps so if there was a split second delay while the IS fired up how many frames would I have got? The other thing I find is that if subjects are moving then, as you track them, you are fighting the IS - hence my coffee grinder comment above. On more modern IS systems (like the 4 stop IS on my 800 F5.6 L IS) this is far less pronounced.
I am not saying don't use IS - it can be VERY handy - just save it for when it is really needed and the subject is not moving too much.
I don't know if the 7D2 AF will be faster but, from what I read it hunts a lot less and tracks far better than any previous APSC camera - all the comments about the 7D2 AF seem to be very positive.
I sympathise with your eye problems as I have a cataract in my right eye so I rely heavily on the AF as well! Hopefully mine will be operated on soon!

 

[candc]

Those are good questions and I would like to know the answers myself. In addition to the tc intentionally slowing down the af speed doesn't it also impact the af speed and accuracy by reducing the effective aperture therefore reducing the amount of light available for the af sensor to work with?

 

[sanj]

I’ve been lurking here for quite a while and have been learning a great deal from these discussions. I have a few questions about autofocus speed and the relative importance of the lens versus the camera body. I shoot a 7D and I’m primarily using one of two lenses – a Canon 300mm f/4 L with or without a Canon 1.4X TC III or a Sigma 150 mm f/2.8 macro with or without a Sigma 1.4X TC. I shoot mostly wildlife, primarily birds, and macro, especially butterflies, dragonflies, and other smallish nature subjects. I shoot about 50% hand-held, about 50% monopod mounted and I almost always use the central autofocus spot on static subjects. My vision is not very good so I rely almost exclusively on the camera’s ability to autofocus my subject quickly and accurately.

Nothing wrong with relying on camera's AF. That is what it is for! And when shooting 10fps there is no way to rely on the eye to judge focus.


1. I have a sense (but no real measurements) that the addition of the TC to either lens slightly slows autofocus compared to the “naked” lens. Under identical shot conditions and lens/camera settings, is there a reason to expect that addition of the TC would slow autofocus lock? Maybe it is just my imagination?

Yes it does, Neuro has given you the math.

2. I’m thinking about upgrading to a 7DII next year. Will the new and improved autofocus system in the 7DII make a difference in improving the speed of autofocus acquisition with my current lenses? In other words, given identical shot conditions and camera/lens setting, will the 7DII be faster to autofocus than the 7D or is speed going to be limited by the lens system I have in use?

Faster maybe. How much faster? Very little. But overall it will be much better focusing device: Tracking, spread out better focus points etc.

3. I have heard it said that the image stabilization (IS) system actually slows down autofocus lock. One suggestion I have heard (or read somewhere) is to turn off IS when shooting birds-in-flight to allow the camera to lock focus more quickly. Is there any truth to this?

Don't think so. But even if it was, it would be so little that it does not matter.
All are unscientific personal thoughts.

Thanks for your help.