Since you're using Canon cameras, the speed of the AF is totally dependent on the autofocus mechanism in the lens (Canon does not use in-body AF motors). It will depend on the type of motor usedRing USM is the fastest that Canon makes, I don't know how Sigma's HSM compares. It will also depend on the optical design and focus design of the lens..
So the answer for AF speed is that you need to evaluate each lens design separately. I've never seen any table that compares focus speeds..
In the Canon system, AF accuracy depends on both the camera and the lens. The camera decides what edge to focus on and measures the amount of misfocus. In AI Servo mode the camera also tracks the edge and projects the amount of misfocus expected at the time of exposure. The misfocus amount is communicated to the lens, which then calculates what kind of adjustment is needed based on the current focus distance and, for varifocal lenses, focal length..
AF speed and accuracy is not something that is readily measured. There is no easy answer to your question. It's something that you have to determine from experience..
As for the specific question about upgrading from your XT/350D, the main thing that you'd get is probably better accuracy when using the center AF point with lenses of f/2.8 or faster. The XT/350D uses only standard-precision AF detectors, but the better models have a triple-precision center AF detector (for vertical edges) when using lenses of f/2.8 or faster...
Thank you for your reply!.
Today I use my 350D and a Sigma AF 18-200/3,5-6,3. Since I recently picked up photographing action sports like skateboarding and surfing I realized that my equipment isn't good enough..
When shooting series in AI Servo mode most of my pictures are out of focus.Would a Canon 70-200 4 L, Sigma 70-200 2,8 or Tamron 70-200 2,8 do a better job?..
The canon 70/200 4L non IS will be the fastest focusing .......my experience has been that is slows down the focus of the lens.
Bill,Jr'I kind of like the Earth, it's where I keep all my Stuff.'Website; http://www.pbase.com/wboth125 Lake Wylie, SC..
When shooting series in AI Servo mode most of my pictures are out offocus..
Just a check before you spemd a lot of money on a lens..
Are you sure that you have a focussing problem and you aren't just seeing motion blur?.
Do you have focussing problems when shooting individual shots?Chris R..
I don't have the focusing problem when I shoot with "One shoot" in non-action nor action photos - only in series..
Regarding the earlier post about 4L: I misread your post and thought that you wrote that 4L wouldn't be the fastest to focus...sorry...
I don't have the focusing problem when I shoot with "One shoot" innon-action nor action photos - only in series..
The I think that the problem is with the way that you are using AI Servo, not with the lens. Changing the lens won't make the problem go away..
I don't know whether the 350D has a particular AI Servo problem, so it might be worth posting on the Canon forum 350D forum.Chris R..