Alan321 Offline Upload & Sell: Off
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My 1D2 had a focus problem. Once fixed by Canon it was able to accurately focus on the small lettering of the licence plate of a car appoaching me. With a 300mm f/2.8L lens I could get 18 or 19 pin sharp shots out of a burst of 20 at 8.5 frames per second.
However, the 100-400L cannot respond to focus instructions as quickly as the 300 f/2.8L IS can, and the 1D2 could not maintain pin sharp focus at full frame rate for a full burst. It would skip a beat every 6-8 shots - except when my 100-400 had a focus problem, also fixed by Canon. For a while both the lens and the camera had their AF problems at the same time, which made diagnosis a little tricky, but the camera would never skip a beat during a continuous burst because never realised how poor the focus was throughout the burst.
Focus tracking - as in predictive AF - is mostly about predicting what the focus distance should be for the next shot based on the current distance and the rate of change of the distance.
Predictive AF can benefit by you tracking the subject for 0.5 to 1 second before you begin shooting. Afterall, it can't predict what the future focus distance will be if it has no history of distance changes to work with. Be aware taht even the shortest release of the AF button (whether shutter button or the * button) will instantly trash all knowledge of the predictive AF data. This can be helpful if it was wrong anyway, but not if you only released the button accidentally.
On top of that is lateral AF tracking - the use of multiple AF sensors so that at least one is on target at any time, and the camera knows which one. It helps overcome the effects of a subject moving erratically, or a camera/lens not being held exactly on target by the user.
Lateral AF tracking depends on you having the right focus sensor on the right target at the moment you begin the AF. The right AF sensor is the one you selected for the camera to use, but defaults to the centre one if you activate all 45. It also depends on the expansion range being large enough to cover your wobbles and still have a sensor on target. If you wobble too much then the camera loses the plot and makes its own decision of which AF sensor to use and which target to use.
The 1D2 was the first model to have a second processor dedicated to handling the focus data from all 45 AF sensors at the same time. There was no penalty for using all 45 so long as you kept your aim pretty well. If you let the target slip out of the AF coverage area then giving the camera 45 to choose from was often messier than giving it just a few to choose from.
Starting IS can be a problem for both types of AF tracking because there can be significant IS lens motion and so the subject can appear to be moving even when it is not. Not only is this visible in the viewfinder and perhaps in the photo, but it also affects the image presented to the AF sensors. Allow a full second between starting IS and taking your shots to get the best result. However, starting IS often starts the AF too, but you don't want the AF active if you are not yet on target, so learn to half-press the shutter button to start the IS, then release it while you aim with your stabilsed view and re-press it to activate the AF when you have the chosen AF sensor on target. Be careful that you don't release it for too long or else the IS will stop.
As well as being slow to focus, the 100-400 has the oldest version of IS in the Canon range. It is slow to stabilise initially and not as effective as the better versions while it is working.
- Alan
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