Canon autofocus information

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Archive 2005 · Canon autofocus information

riccardo
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p.9 #1 · Canon autofocus information

Lens is back!
I haven't had much time to test it well, just took few shots, but it seems to me that now it's pretty much precise than before. Actually the fine tuning did something. As soon as possible I'll take and post some test shot.
Imho, anyone having such a problem should send the lens and the camera to the service. I payed (warranty is over) 25 euros for the job, not that much!

Sep 28, 2005 at 09:39 AM

WillWeb
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p.9 #2 · Canon autofocus information

RDKirk wrote:
mtl337 wrote:
If the precision AF is activated by a switch, wouldn't it be nice if there was a hack that would 'trick' my 20D into activating the cross-type sensor for my f/5.6 or larger aperture lenses ....?

Why does it have to be f/2.8 lenses (besides the effort on Canon's part to force me to purchase $$$ glass)?

When that happens, though, the camera is using an erroneously shallow depth of focus figure for its calcuations. There would certainly be a slow-down of focusing at the very least because it will be trying to determine a higher degree of precision at a lower level of illumination that the system was designed for. If the lens starts out greatly defocused, it will probably do the in/out rack, fail to find the focus point because it's calculating too slowly, and give up.

OK, so lets say it's an f/4 lens instead of a 5.6 - would focus be significantly slower than it would be with the 2.8 lens the precision sensors were designed for? What about the case of, say, a 200 mm f/2.8 lens on a 1.4x TC? A Canon TC will report the combination as 280 mm f/4 and the camera will focus based on that full dof. A third party TC will report 200 mm f/2.8 but the camera will think there is 1 EV less light. Will it succesfully focus based on 1/3 dof of 200/2.8?

All this AF is new to me - I'm in the process of transitioning from FD to EOS on a budget, so I appreciate these opportunities to learn.

Oct 02, 2005 at 04:39 PM

csavery
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p.9 #3 · Canon autofocus information

There was some question about why it needs to be f/2.8 for the higher precision sensor arrays. Keep in mind that the max aperture determines the actual amount of light hitting the sensor before the shutter fires and the aperture is closed down for the shot. So during the focus the need for 2.8 would be determined by the sensitivity of the sensor array. Presumably the higher precision sensor needs more light to make that more precise determination - perhaps in relation to sensor noise, for example.

I mean, failing that, it's a great way to move people to more expensive glass for better results but that can also result in people buying 3rd party glass to get there at the expense of losing sales in the slower glass. I would never that doubt companies make options in their best interest but there is also plausible reasons why it may be so just due to the electronics.

Oct 02, 2005 at 05:47 PM

WillWeb
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p.9 #4 · Canon autofocus information

csavery wrote:
There was some question about why it needs to be f/2.8 for the higher precision sensor arrays. Keep in mind that the max aperture determines the actual amount of light hitting the sensor before the shutter fires and the aperture is closed down for the shot. So during the focus the need for 2.8 would be determined by the sensitivity of the sensor array. Presumably the higher precision sensor needs more light to make that more precise determination - perhaps in relation to sensor noise, for example.

I mean, failing that, it's a great way to move people to more expensive glass for better results but that can also result in people buying 3rd party glass to get there at the expense of losing sales in the slower glass. I would never that doubt companies make options in their best interest but there is also plausible reasons why it may be so just due to the electronics.

But an f/4 lens on a sunny day gets more light to the sensor than an f/2.8 lens does on a cloudy day - let alone indoors. So I don't think it's a matter of available light. I know that split-image circles in a manual focus screen go dark when the aperature is too small; this can be a problem with long telephotos. I'm wondering if it's an optical effect similar to that - the high resolution sensors with their longer baseline just can't see anything unless the lens is at least f/2.8. If it's just software it should be hackable.

Oct 03, 2005 at 11:34 AM

jeffmedkeff
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p.9 #5 · Canon autofocus information

WillWeb wrote:
But an f/4 lens on a sunny day gets more light to the sensor than an f/2.8 lens does on a cloudy day - let alone indoors. So I don't think it's a matter of available light.

You are correct. The autofocusing system measures phase differences. The phase difference available to the autofocus sensors is greater with faster lenses. It is strictly a matter of geometry, not of the availability of light or the sensitivity of the sensors.

Oct 11, 2005 at 01:30 AM

Linen
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p.9 #6 · Canon autofocus information

Great thread thank you!

Oct 12, 2005 at 02:32 AM

ben egbert
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p.9 #7 · Canon autofocus information

The 24 dollar question is if the 1 series fixes these problems? Obviously all cameras have tolerances, so the question is how much improvement can one expect?

I have a low keeper rate of birds in flight with 20Dand 500 f4. I have enough experience to know it is mostly AI servo tracking. The camera tends to grab the wrong thing, usually water, trees or clouds rather than the bird. I have used centr only and all points.

I shoot in demanding situations, usually too far away, often poor light etc. I have all the gear I need, tripods, Wimberley etc. I have lots of practice. (just trying to avoid going off topic here)

So would a 1D-mk2 or 1D-mk2n make a significant improvement in AI servo mode captures?

Dec 13, 2005 at 07:57 PM

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p.9 #8 · Canon autofocus information

ben egbert wrote:
So would a 1D-mk2 or 1D-mk2n make a significant improvement in AI servo mode captures?

As for my own experience in shooting friends playing soccer with 20D and 1D MkII. MkII peforms much better in AI servo. It's much faster in any situation. The improvement will be more then significant.

Dec 21, 2005 at 10:37 PM

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