Topic: Autofocus Confusion (Read 7066 times)

Ray · « **on:** May 09, 2003, 06:01:36 am »

Perhaps this whole subject needs a thorough discussion. What actually affects focusing accuracy and speed? The quality of the body and the number of focusing points? The motor in the lens? The size of the maximum aperture of the lens? How do these factors interact?

Jim D · « **Reply #1 on:** May 10, 2003, 07:03:27 am »

Yes, Ray, I am sure that Canon knows exactly where the limitations lie in this camera/lens combination. However, this is basically the AF system that comes in a $400 film body, and both Canon and Nikon have two levels of AF systems and this is the lesser of the two. For me, AF is nice for some shots, but in reality, most of the photos I take do not require AF and I like to choose exactly the plane of focus as a creative option.

And like others, my eyesight is not what it used to be, and it would be nice to have the option to turn over to the camera this responsibility, especially given the smaller than normal screen and no focus aids like the split prism, etc. But, alas, maybe that can only be true with the 1Ds which is way outside my budget. Careful diopter adjustment and/or an auxillary diopter lens, plus large aperture lenses for a brighter screen can at least help to make the most of the situation.

I just made the switch from Nikon MF to Canon, and this is my first lens, so I have nothing to compare with yet. And truth be told, the camera nails it 90% of the time to my satisfaction. However, as I mentioned in a previous post, it has many times jumped to ALMOST perfect focus without making that last little nudge, and locked me out of an action shot in good light that was gone before I could react. And it has also locked on a completely out of focus image a few times, so a trip to Canon seems in order. They can check this front/rear thing while they have it. Otherwise I am thrilled with all the other aspects of the 10d and consider myself a lucky man indeed to have such a fine photographic tool.

RAHAF · « **Reply #2 on:** May 10, 2003, 12:01:11 pm »

Hi all,

I recently bought the 10D and I hope I get it soon enough...
Any ways, regarding the focusing problem the 28-135mm lens has, I just thought you'd like to know that I've been using the Canon 1V with this lens and have had similar autofocus problems. It either almost reaches the focusing plane and I have had to make that extra turn for the image to be in focus, or it just doesn't respond when I half press the shutter. Even in bright day light it wouldn't focus properly at all times. However, as can be expected, I have had no problems with the L lenses I owe.

Rahaf

BJL · « **Reply #3 on:** May 16, 2003, 05:38:17 pm »

I just want to back up Jonathan Wienke's idea of setting the camera to use the central AF point only, at least for "non-action" photography when you can easily afford to take an extra second or two focusing.

One reason already stated is the greater control over what gets focused on: with multiple AF points, the rule on my camera seems to be just finding the closest object; I am happy instead to point at an off-centre main subject, lock focus with a half-press of the shutter release and recompose.

Another reason is that the central AF point is the one with the best chance of being sensitive to both horizonal and vertical lines (a "cross-type sensor"?) while some or all of the others are "one dimensional" (horizontal lines only). Saves twisting the camera to focus on vertical lines, as one of my manuals suggests I do!

Jim D · « **Reply #4 on:** June 06, 2003, 06:53:41 pm »

The 10D came back from Canon today. They had it for 14 working days, so let that be an indication as to the backlog in the repair facility in NJ. I did call once at the 10 day point and found it had just made it to the bench.

I must say, the focus issues are completely gone. In fact, it will lock on objects that I would not think are contrasty enough for AF to work at all. One test was on an object in the room here which could be best described as "black on charcoal". At an illumination level of EV 2.5, the AF took longer than normal, but focused and locked, over and over. In another, a reddish pattern on white background, the original camera would not lock at all. Now it comes to focus lock as quickly as any other normal object. I am happy, to say the least, with the AF performance. I may try the "ruler test" when I get a chance, but that issue did not plague me like the one that got me to within 99% of focus but locked me out of the picture, as I described in an earlier post.

As a side note, the return paperwork was brief as to the work performed. It only said "adjusted focus, adjusted software".

jwjohnson · « **Reply #5 on:** June 16, 2003, 11:31:31 pm »

I just got my 10D last week. After reading all the posts about the focusing problems I decided I needed to test it out. I used the Bill Atkins test. I did it outdoors after the sun had gone down. Tripod, remote release, and single AF point. I tried all my lenses (16-35, 24-70, 100-400) I shot about 4 images with each lens, manual and AF. There was some variation in focus from shot to shot, but the AF always did a better job than manual. I also did the test at the same time with my (just sold) D60. Similar results, but I found the AF and Manual shots from the 10D were better focused than the D60.

Jim D · « **Reply #6 on:** May 08, 2003, 09:08:09 pm »

I have had my 10d about a month now, and have of course been following the threads on autofocus. I tried the test described by Bob Atkins in another thread in this forum using the same test conditions. However, my camera seems incapable of coming to focus on the narrow vertical line in the center of Bob's chart. The system will sometimes hunt, sometimes stop close to focus but never lock, and sometimes show lock with the lens severely out of focus. These results are with the 28-135 IS lens and the chart illuminated to EV 8 according to my spotmeter. I have performed the Clear Settings step numerous times, then select the center AF target and perform the test.

Subsequent testing using a real object like a dark picture frame against a light colored wall seems to indicate that none of my AF points will come to focus lock on vertical lines or boundaries. But they all seem to work reliably on horizontal edges. And with just about any other "real life" object, all of the AF points, when selected one at a time, will fairly quickly give me focus lock down to about EV 2.

My next step is to call Canon CS, but thought that with all the attention that this topic has received lately that someone else may have had the same experience. I know that photographing test charts makes about as much sense as photographing newspaper against the wall, and if the camera works on real objects, then maybe this is a waste of time. Except that during the tests, the camera focus-locked several times on an image that wasn't even close to focus and this alarms me. And the fact that others have repeated Bob's test with apparently no problems reported other than the front/rear focus tendency.

Any suggestions?

neil · « **Reply #7 on:** May 08, 2003, 11:25:33 pm »

Wow, that's a new one. How did you come to need that???

« **Reply #8 on:** May 09, 2003, 06:44:58 am »

You're right, it is new. I just came up with it when thinking about the hysteria that seems to be going around about supposed 10D focusing problems.

I started thinking about how if the manual focus point and the autofocus point coincide then the issue is user error, not camera error. But, how to be sure of this? Most cameras designed for autofocus lenses no longer provide a means for accurate visual focus, such as a microprism or split image rangefinder.

I then thought about how autofocus uses phase and contrast differences to sense focus point. How could one set up a test that did the same? I sat pondering the issue with a 10D in my hand and idly pointed it at the TV screen. I immediately saw the moire pattern. This lead to about an hour of testing, which lead to Reichmann's Third Law of Dynamic Focus Testing.

I hope someone finds this test useful.

Michael

Ray · « **Reply #9 on:** May 09, 2003, 11:53:13 pm »

Well, Jim, you've done a good job in laying the groundwork for such a discussion. Your problems with the 28-135 IS zoom seem similar to my experiences using this lens with the D60, and I wonder how much is due to the design of the lens and how much is due to the design limitations of the D60 body.

Being an amateur, I don't have a wide range of lenses to choose from. If I'm standing there trying to photograph something/somebody in less-than-bright light and the lens either starts hunting, locks onto the wrong point, or doesn't do anything at all, I don't have the luxury of being able to fish out of my camera bag an F2 135mm prime or an F1.4 50mm prime in order to appreciate the immediate difference.

I can only assume that the more expensive camera bodies have better auto-focusing; the more expensive lenses have better autofocusing and the 'faster' lenses have better autofocusing. But I have little idea of the relative significance of each of these factors. For example, would the 28-135 zoom autofocus as well on a 1Ds as an F2 prime does on a 10D?

If you're having trouble testing the 10D's autofocusing capability using the 28-135 zoom, why not try a bigger aperture lens?

Jonathan Wienke · « **Reply #10 on:** May 10, 2003, 09:36:57 am »

One thing I found somewhat useful on my EOS Rebel TI (it has a 7-point AF system i believe is similar to thew 10D's) is to manually select a single autofocus point, so that the camera is not trying to decide which point to use for focusing. Otherwise the camera will frequently achieve perfect focus on something other than the intended subject.

Also, the AF points are more sensitive to horizontal lines than vertical ones. On the 1Ds, which has a 45-point AF system, only 7 of the AF points are sensitive to both horizontal and vertical lines, and only if the lens is f/2.8 or faster. All the rest are horizontal-line sensitive only. My Rebel manual doesn't seem to broach the subject, but I'm willing to bet the same sort of limitation applies there. Try adding a horizontal black line to the center of the test chart, so that you are pointing the center AF point at a (+) instead if an (I), and manually select the center AF point. You may notice a significant difference in results.

Jim D · « **Reply #11 on:** May 10, 2003, 10:03:46 am »

Aha! I thought so. I was thinking I might do this anyway, so thanks so much for the input and confirmation. This starts to get into "reverse engineering" which I do not want to do. But understanding the limitations of the system without pronouncing any judgement on those limitations I think is an important part of owning such a sophisticated piece of equipment. You pretty much get what you pay for.

Jonathan Wienke · « **Reply #12 on:** May 10, 2003, 11:35:55 am »

Another reason autofocus systems work better with a faster lens is that a faster lens has a brighter image. More photons on the AF sensor translates to faster, more accurate focus lock. So there are 2 significant benefits to faster lenses.

Pete · « **Reply #13 on:** May 10, 2003, 05:54:08 pm »

This might rankle all you 28-135 proponents, but ...

what I never liked about this lens (aside from the f/5.6 at 135 mm) is that, once you achieve focus, you cannot change the zoom (thereby the crop and/or composition of the photo) without having to refocus.

Perhaps the last post has something to do with this???

Ray · « **Reply #14 on:** May 10, 2003, 09:39:11 pm »

Well, I shall try Jonathan's excellent idea of selecting just the one focus point. If that doesn't result in a significant improvement, I think I'm going to get myself a 50mm F1.4 or F1.8. I'm reluctant to duplicate focal lengths, since I'm a frugal sort of bloke, and I really find zooms much more versatile than primes. However, standard lenses seem to be excellent value and the faster and more accurate focusing might allow me to justify the outlay.

Now, I wonder if that slightly faster F1.4 is going to be worth the extra cost?

Jim D · « **Reply #15 on:** May 16, 2003, 08:41:21 pm »

I may have missed something in my 10D manual, but I cannot find any information on the sensitivities of each of my focus zones. However, my own tests indicate that the center zone, which I used for all testing and for most AF shooting, is not very sensitive to vertical lines. It seems much more sensitive to horizontal lines though. The upper and lower zones do seem to have the most sensitivity to vertical lines.

I think it was Jonathan who reminded us of the AF's similarities to the Elan 7, and I saw somewhere a pattern where it showed the H/V sensitivity of each zone. As I remember, the center zone showed a cross, but you couldn't prove it by me.

chesty · « **Reply #16 on:** May 21, 2003, 12:32:14 pm »

Yep, gettting this quality out in such a short time is excellent. And there will always be the 1% of items that are not up to snuff.

Let us know how the camera does when you get it back.

Ray · « **Reply #17 on:** June 06, 2003, 08:50:52 pm »

Jim,
I'm glad the focusing issue is now fixed, but this episode raises some other disturbing issues, doesn't it. You took the trouble to have the matter sorted out, took the trouble to do the ruler test, decided the focusing wasn't up to expectation. Others might have accepted the situation and just assumed it was a normal limitation of the camera design. (You get what you pay for, sort of thing.) If the focusing had been a shade better, but not as good as it now is, you might have accepted the situation yourself. I wonder how many degrees of mis-calibration of this sort can apply.

Gerry · « **Reply #18 on:** June 09, 2003, 08:43:04 am »

Tested my autofocus on 10D over the weekend, works fine with a 28 to 80, and Vivitar 100 to 400.

jiacone · « **Reply #19 on:** June 18, 2003, 03:05:26 pm »

What is the Bill Atkins test?

Author Topic: Autofocus Confusion (Read 7066 times)

RAHAF

neil

Ray

Gerry