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Old Sep 5, 2005, 2:07 PM   #1
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I don't know, do ya think I have a focus problem?

Tripod (no mirror lockup), all shot at f4.0, at 70, 100, 135, 200 mm, same distance from target. Auto levels applied, resized, saved as jpeg... no sharpening. Center focus point only--defocused first, then let autofocus work with shutter press...

First time posting images--sorry if I made any mistakes...

This could explain my frustration with low percent of keeper shots...

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Old Sep 5, 2005, 8:02 PM   #2
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Now those are big pictures -- I run at 1600x1200, and I had to scroll to find the reply button! I don't think they needed to be quite so large; even crops of the area of interest would have sufficed.

The auto-focus seems to be ahead and behind alternatively, so it doesn't seem to indicate a simple mechanical calibration problem (which I think would tend to one side or the other). Canon also says that focus in only guaranteed to be within the depth of field, not "exactly right".

The first few shots seem to be borderline/slightly out of spec from my viewing, and latter ones seem close to right on, but I'm not an authority, and you'd be better off showing the samples to a Canon tech if they'll look at them and getting comments from them.

I've heard some people say that using such a test target isn't exactly right because the focus target is too simple -- this concern could be mitigated by using a real object at the point of the test pattern, or by making that focusing target more complex.I did the former in my own tests (I used a CF card to start, and something else afterwards; be careful about the really small distances involved here). Here a reference to this point of view from the article from which I think you got the test target.

What I can say though, is that the strictly diagonal charts do present a challenge to the AF system and that results, as a consequence can be inconsistent. Rather than me repost my modified target, I'll describe an alternative, more consistent test that can be used with the one published here. Take the test chart as provided here, lay it flat on a table, with the grid lines running away from you, and place a vertical object, of similar size say, to a lens filter box, cigarette packet, matchbox etc on the centerline.
Place camera on tripod, and shoot the target from as low an angle as possible, whilst not being so low that you can't see the grid lines. (remember, the chart is lying on its back on the table, with target standing up at centerline). This gives a much more definite and real world type target at the focal point than the thin line used originally. One can still ascertain the focus accuracy, as you still have the grid lines to review the point of focus.
Whilst my 10D gives variable results, depending on ambient light etc with the unmodified test, it's bang on using my modified version, regardless of available light levels. I suspect the variances we are seeing reported are down to differences in test conditions. A simple change can eliminate those inconsistencies, and hopefully many who thought they might have a problem, won't send their camera's back unnecessarily.
For what it's worth, I've seen people reporting front focus using the unmodified test, sending their cameras back to Canon, and getting them back rear focussing. A theory I have, is that Canon, putting the camera on their test rig, find nothing significantly wrong. An assumption may be being made that cumulative tolerances of the lens and body are the cause of the incorrect focus, and to satisfy the owner, the camera is adjusted to focus rearward slightly. Probably still within specs, but they might just be altering the bias to the other end of the tolerance, thinking that the front focussing lens, will mate with the rear focussing camera, and lead to a satisfied customer.
However, what really happened was that the customer used a slightly flawed test chart/lighting, got an anomalous front focus result, and Canon gave them exactly what they asked for, even if that isn't exactly what they wanted.
My advice then, is be careful folks - if you use the chart as it stands, it may give you a false result. Test using a real world object at the centre line of the chart to get the focus point well away from those sloping grid lines. Then if your results indicate problem, send it back to Canon, but don't send it back on the basis of the unmodified test alone, or you could be wasting your time.

-- Simon Watkins, May 21, 2003


I have two other suggestions:

1. I don't know if this is relevant, but in case... : The depth of field seems to be quite narrow; increase the f-stop to increase your chances of getting the subject in focus.

2. For when you have a really shallow DoF and need fine focusing -- override auto focus using your eyes and the focus indicator light, and perhaps even add an Angle Finder C to magnify your subject further in the viewfinder.

Very precise manual focusing can be done with such test targets, because you can view the area of focusing move even if you can't see the center clearly -- you can do this to (a) check that the viewfinder / etc. is itself not out of focus, and that manual focusing is correct (b) check that the focus indicator lights up in this case, showing that that sensor is not incorrect. If such tests pass but auto focusing tests don't, then either (c) you're just hitting the limitations of the auto-focusing system and need to take some manual measures to compensate or (d) your auto-focusing mechanism isn't working properly.
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Old Sep 5, 2005, 9:27 PM   #3
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Does this have anything to do with the higher precision AF sensor in camera not being enabled with lenses slower than f/2.8?
(this might tightened the offset a bit since the camera will more out of focus info to work with the larger aperture...)
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Old Sep 6, 2005, 1:22 AM   #4
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Something else could be going on too.

You probably think you're aiming your focus point at the centre line in the middle of a bunch of white space. The actual focus point isn't always lined up perfectly with the etched marking however.

When you're zoomed out the AF point isn't hitting the centre line, but because of the wide zoom it's missing the centre line altogether - so the AF is trying to focus on the blank white and front or back focussing unpredictably. When you zoom in the AF point (slightly misaligned) is now hitting the centre line and the AF is correct.

What camera are you using? Chances are the 20D is slightly better than the XT, but I believe that this bit of the camera is aligned by hand and not machine calibrated so no two cameras are exactly alike. If that is your problem you could probably carry out some tests to determine exactly where you AF point is in relation to the etched marking and adjust accordingly when you shoot.

Or of course you could upgrade to a 1-series camera where you're paying for them to calibrate the AF points precisely. Essentially I guess that you may be testing your camera outside the parameters it was designed for.

I suppose that giving the AF something larger to focus on, as suggested above, and then checking DOF would be an interesting test.
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Old Sep 6, 2005, 2:51 AM   #5
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Here's another focus test, with a target that the author explains and defends.


The main differences I see with the above one are that it's bigger, and he implies closer working distance -- both of which would present a larger object to the sensor (although I'm not sure that the previous one is too small; it seems more likely to be fine now that I've thought about it some more).

He also offers to advise people on their test pictures. I'd try his test,consider the findings, and then take him up on that if needed.

I also found a picture of one of my test targets (but not the target -- I ate it
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Old Sep 7, 2005, 7:23 PM   #6
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I cut down the file sizes by cropping--sorry about that...

I have an idea for a new focus sheet pattern, I'll try it out to see how it works...

Thanks for the suggestions.

I've got a 300d...
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