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BF only in low(ish) light--anybody got this problem fixed??

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Joe Bloggs View Drop Down
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Post Options Post Options   Quote Joe Bloggs Quote  Post ReplyReply Direct Link To This Post Topic: BF only in low(ish) light--anybody got this problem fixed??
    Posted: 12 January 2007 at 03:33
I admit that my camera is a 5D, but I heard people having the same problem with the 7D too:

1. The problem is back focus BUT
2. only the five sensors in the centre have this problem
3. but only in low(ish) light--e.g. normal indoor incandescent lighting
4. and only at certain distances--over 1m and somewhere before infinity--e.g. it happens most when taking pictures of people. (which is bad)
5. and (perhaps) only with wide(ish) angle lenses--it happens with my kit lens set to wide angle, and the Sigma 30mm.

Over at dpreview there is someone in Taiwan with a 5D with such symptoms, who sent it in 3 times and still couldn't get it fixed--the CS people said there's nothing they can do!

http://forums.dpreview.com/forums/read.asp?forum=1035&message=21620117

I am in Hong Kong with a 5D with this kind of problem, and have sent it in once but they said the AF is fine. Then yesterday I told the CS rep on the phone that it only happens in low light, and they said they have come across this before and can fix it--but if so why couldn't they find the problem last time? Will I be wasting another $100 if I send it in again?

Has anyone had better experience with fixing this kind of problem?
 



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Post Options Post Options   Quote Joe Bloggs Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 15:47
...apparently not? :/
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nigelbrooks View Drop Down
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Post Options Post Options   Quote nigelbrooks Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 16:26
Sorry, mised your thread.

AFAIK the back focus issue is due to a mechanical misalignment which is why a test lens and pattern are, I believe, used to physically adjust the camera. The amount of lighting should not therefore have an effect.

Is it back focus or simply out of focus with the camera failing to lock on your intended subject? Do you actually get focus lock? Just a thought.

Would suggest that you do a back focus test in normal light to determine if you actually have a back focus problem or a wider auto-focus problem.

Edited by nigelbrooks - 17 January 2007 at 16:30
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Post Options Post Options   Quote gm4jjj Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 17:10
There was a discussion on DPR about BF and the effects of artificial light.

I paste a comment below:

This was discussed a few months ago, I think on the News forum.
It was about Pentax whose AF seemed to be colour dependent.

Well, my understanding of that discussion was, that the phase detection
system focuses the light on monochromatic sensors. If the focusing
lens/group isn't achromatic (and for cost reasons, I assume it typically
isn't), different colours will be focused on different spots, just like
chromatic abberrations we are familiar with.

Incandescent is red dominated and has a very weak blue component.
Flourescent tend to have a strong green component.
Daylight is typically more balanced.

The dominating colour of the ambient light will also give the strongest
response on the AF sensors when shooting something roughly white/grey
and will decide where the AF will settle.
-- David
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Post Options Post Options   Quote Owanneke Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 19:02
Actually, I tend to agree with the topic starter.

I've done some concerts lately and ALL had terrible backfocus problems.

When I got home, I started testing and couldn't reproduce until I tried it in the semi dark.

I think the low light condition increases the problem...
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nigelbrooks View Drop Down
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Post Options Post Options   Quote nigelbrooks Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 19:17
It's NOT back focus then, it's an auto-focus problem. Sorry to appear pedantic but back-focus in the context of the 7D was (is) a mechanical problem and is easily fixed. Poor focus in low light is another issue.

Edited by nigelbrooks - 17 January 2007 at 19:17
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sooten View Drop Down
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Post Options Post Options   Quote sooten Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 19:57
I agree with Nigel. My 5D is very picky about focus in low light. If the focus point is off the mark just a little, it will focus on something else. It is especially problematic when your subject is moving and there is a cluttered background. You might get better results if the background is less cluttered.

It will also be a factor of the max aperature for you lens. As the max aperature gets smaller, then there is less light getting to the focus points. Though you say that your problem seems to be at the wide end, I would expect to see even more problems at the tele end of the kit lens as the max aperature drops to 6.3.

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Post Options Post Options   Quote Owanneke Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 19:57
I link it to the backfocus problem because the bad focus is always... behind the subject. It never misses in front, always behind! And it's very subtle, I can't see a misfocus through the finder, not even on the pic on the screen, I've really got to zoom to the max level to see it...
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Post Options Post Options   Quote sooten Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 20:27
Owanneke, that is exactly what I see to. I think the camera gets confused and locks on to something in the background. You don't see it until you download the pics to your computer screen.

I shoot a lot of gymnastics pics in very low light - think ISO 1600, 1/125, f4 still about 2/3 of a stop dark. It is hit or miss with the focue. If the suject is moving and the background is stationary, then it may slip off to the stationary background. This is VERY annoying, but I am not sure that there is a 'fix' for this, especially a 5D. Now maybe the Alpha is better at this, but I don't know.

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Post Options Post Options   Quote nigelbrooks Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2007 at 23:19
Try to beg or borrow a faster lens or open yours up (diificult I know when you try to capture action) and see what the results are then. I'm not a 5D owner. Could you focus 'lock' (like the 7D) on a point where you will expect the action. DOF shouldn't be a problem if you are shooting 'long' from the seats.
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Post Options Post Options   Quote Joe Bloggs Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2007 at 18:03
Firstly, I would like to ask if you guys had mapped the actual detection area of your focus sensors? I found that mine are actually a bit down from where they are supposed to be. But even after accounting for that in my focusing I still get the backfocus in low(ish) light.

Secondly, the focus problem isn't really dependent on having a small subject as foreground. You can try printing a big piece of paper, with the top half black and the bottom half dark red, and paste that onto a cereal box. Then see if your camera can focus on it... (the dark red colour sets off two problem conditions--focusing in the 'dark' and focusing on low colour temperature contrast.)

I'd like to try this myself, but my 5D is in for repairs at the moment. Hope they really manage to fix it too.
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