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Old Jan 31, 2006, 9:08 AM   #1
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Hi!

I'm having problem with "low light" focusing on my Canon 350D / Digital Rebel XT also. It seems to front focus in normal indoor lighting... Even when I don't consider it very dim, e.g. normal size bedroom lit by two 100watt bulbs - minimal light shade it misfocuses. Or indoors daytime with the curtains open and a fair bit of light coming inside (but not direct sunlight on the subject).

(I've done this in real life situations, e.g. objects / stationary people as well as tests with a tripod, mirror lock up, timerand so on). And it almost consistently (95% of the time) front focuses indoors. However outdoors in brightconditions, or if the light source is brought right close indoors (e.g. putting thesubjectvery closeto indoor lighting)then if focusses ok / perfectly.

So when I'm trying to get a sharp indoor portrait photo using ambient lighting it is annoyingly just out of focus enough to make the photo bad! (obviously especially at wide open aperatures).

When people say "low light" do you mean, almost completely dark room? Because my XT won't use the built in flash to assist in autofocus unless it's really quite dark, that is, it will only pulse-beam and focus when I call it "very very low light" (and even then it will still misfocus). But when it's just "low light" (normal indoors / office type of lighting) the camera thinks it get's focus right without the flash assist - but in the end it's not in focus. The "front focus" is the same (noticeable) when using flash and when not using the flash in "low light".

All my 3 lenses (50mm f1.8, 18-55mm and 28-135mm) do the same - bad focus in low light, but spot on outdoors on a sunny or mostly sunny day (even when I use wide aperatures then).So it's not an aperature or "lens softness" thing. It's very clear that it's front focus, because I can see it focusses on objects a few centimetres before the point in focus.

And yes I have selected and do only use the central AF point (which is the most accurate AF point ). And yes, just to be sure I've tried with other AF points too. It is always the same -slight but definitely noticeable front focus. It's very annoying for indoor work. My point and shoot camera focuses MUCH better indoors, it gets things right 99% of the time in the same amount of light when my DSLR can't focus accurately!

Can anyone give me any advice?

I also want to know if I bought a Speedlite (e.g. I'm thinking of a 430EX) - will the "near infrared" focus assist beams only work in REALLY dark situations (the same as when my built in flash is used by the camera to "pulse")? Or can you "force the Speedlite beam to come on" by enabling the flash (but by turning the actual flash away from the subject to still get ambient light photos when needed...)

Because as I said my Canon 350D / digital rebel XT camera thinks it can focus indoor when it's not really that dark.. but it nearly always misfocusses indoors. And it is NOT camera shake, because I have tried repeatedly on a tripod, on still subjects etc. So would I be able to solve this problem using a Speedlite focus assist beam or not? Or what other suggestions do you have? do I have a bad copy camera and should I try to get it serviced? I guess it is NOT the lenses because they work fine outdoors and all 3 lenses exhibit the same pattern of "misfocus" under normal household lighting.... I live in Romania but bought my camera in UK!!! So it's quite annoying to try getting it serviced!)

Looking forward to your answers. Thanks.

Paul
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Old Jan 31, 2006, 10:03 AM   #2
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How are you shooting (aperture priority)?

If not, in low light, a camera's autoexposure is going to select a larger aperture (smaller f/stop numbers). So, your depth of field is shallower.

In better light, a camera can use smaller apertures (higher f/stop numbers), so any focus errors are not going to be as obvious (you have greater depth of field at smaller apertures).

So, if you're letting the camera control the aperture, that would explain the symptoms in different lighting (if AF is out of alignment).



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Old Jan 31, 2006, 10:26 AM   #3
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P.S.

You'll find a lot of focus check charts and instructions on the internet. But, a "quick and dirty" (and IMO, and eaiser and more realistic way) to check for AF error is setting up some books, CD/DVD Cases (or something similar), spaced about 3/4 inch apart. Shoot at the center one and watch for FF or BF using a bright prime at larger apetures (smaller f/stop numbers).

I'd use your 50mm f/1.8 out of the lenses you've got, since it will be sharper and have a shallower depth of field (larger available apertures). Shoot wide open (or stopped down just a little bit) to check AF alignment.

This is what I'm talking about (but, I'd find something with less reflective covers to use). If your focus point (center book) is sharpest, and you have equal blurriness in the text on the books in front of and behind the one you're focused on, all is OK. If not, you may have a problem requiring service (manufacturers can calibrate/align for AF errors).

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Old Jan 31, 2006, 12:11 PM   #4
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JimC wrote:
Quote:
If your focus point (center book) is sharpest, and you have equal blurriness in the text on the books in front of and behind the one you're focused on, all is OK. If not, you may have a problem requiring service (manufacturers can calibrate/align for AF errors).


Jim, I can't remember where I read this, but I thought optics will drive one or the other to be more in-focus. That is, the center should always be perfectly focused but you have better focus either in front or behind the focus point - I don't recall which. I could be half-baked on this - wouldn't be the first time :-)


Edit - I knew I'd seen it. It was on Luminous landscape. Here's the quote and the whole article:

[align=left]
Quote:
If you look at a DOF table, the DOF scale on a lens, or use a computer program to make the calculation, you'll see that one of the laws of optics is that the DOF extends from 1/3rd in front of the point focused on, to 2/3rds behind it. In other words, you have twice as much DOF behind your point of focus than in front of it.
[/align]
[align=left]Whole article:[/align]
[align=left]http://www.luminous-landscape.com/tu...ries/dof.shtml[/align]
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Old Jan 31, 2006, 12:27 PM   #5
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JimC/JohnG

I don't think he's talking about 'normal' AF - He has problem with low-light, which is different!

As I recalled, some of the earlier procedures for testing the D60 from Canon required a certain minimum illumination, below which the test is bogus: i.e. the camera gives up the search algorithm and assumes it has achieved the best sharpness possible under that lighting condition

-> which is why he's asking if the AF beam from the flash (built-in or external) helps or not :idea:
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Old Jan 31, 2006, 12:33 PM   #6
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Quote:
Jim, I can't remember where I read this, but I thought optics will drive one or the other to be more in-focus. That is, the center should always be perfectly focused but you have better focus either in front or behind the focus point
At closer ranges (where you'd want to check for BF or FF), it's almost right on 50/50.

The closer you get to infinity, the more it follows the 1/3 in front, 2/3 in back rule.

For example, plug in 48 inches (4 feet) into this DOF calculator, using the Canon Rebel XT with a 50mm lens at f/2:

http://www.dofmaster.com/dofjs.html

The split is only 49/51 (0.84 inches in front, 0.89 inches behind).



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Old Jan 31, 2006, 12:39 PM   #7
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NHL wrote:
Quote:
JimC/JohnG

I don't think he's talking about 'normal' AF - He has problem with low-light, which is different!

As I recalled, some of the earlier procedures for testing the D60 from Canon required a certain minimum illumination, below which the test is bogus: i.e. the camera gives up the search algorithm and assumes it has achieved the best sharpness possible under that lighting condition

-> which is why he's asking if the AF beam from the flash (built-in or external) helps or not :idea:
Perhaps, but if he's using Autofocus versus Aperture Priority, the exposure algorithms are going to select a larger aperture indoors and a smaller aperture outdoors. This is one mistake some users make when checking focus (thinking that it works fine in good light, when the reason they don't see a problem is because of smaller apertures).

If he's testing that way (Autoexposure), that would explain the more obvious AF errors in poor light versus good light.

It wouldn't hurt to check it both ways (better light, lower light), making sure you're shooting at wider apertures so any AF alighnment problem would be more obviious if one exists.

Edit:

NHL:

I missed the part where he said even at wider apertures outdoors. So, you may be right.
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Old Jan 31, 2006, 4:53 PM   #8
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Hi Jim, John and NHL!

I'm back home and now have time to get to the post! Thanks to ALL of you for your very valuable help and consideration in the few hours while I was away.

Indeed I AM shooting ALL in Aperture Priority (AV) mode.For indoorI set my lens at MINIMUM f value (that is widest aperture) e.g. f1.8 on my 50mm prime and take the tests inside. Outside whatever I shoot is pretty much in focus unless I stuff up something esle. (note: Obviously in bright sunlight even at just 100 ISO that lens soon reaches the 1/4000 maximum shutter speed.)

But yes, if I shoot the back of a building(not direct sunlight) or a car under a tree (in the shade) with ANY of my lenses it's "tack sharp" at minimum aperture. I go inside and do a simple shot and it's OOF (out of focus). In "low light" indoor shots (e.g. normal household / office lighting) 95% of them are front focus, about 4% are IN focusand about 1% back focus (that is I had back focus just once or twice!)

Yes, I have already downloaded various "focus test sheets" (e.g. 45 degrees to the camera), ruler test, and done something similar to your "book trick" Jim - and they show consistent to what I see in real life. And I DO use the central AF point (the one with the "cross sensitive sensors" - ie. both horizontal and vertically sensitive) for the test, and I've tried using the other AF sensors, it's not different (or if anything... worse).

Indeed what NHL wrote is correct, I'm highlighting that the problem I have is with low-light! NHL, you wrote:

-----------------

"As I recalled, some of the earlier procedures for testing the D60 from Canon required a certain minimum illumination, below which the test is bogus: i.e. the camera gives up the search algorithm and assumes it has achieved the best sharpness possible under that lighting condition

-> which is why he's asking if the AF beam from the flash (built-in or external) helps or not".

-----------------

That makes me curious... what is the "certain minimum illumination" you talk about? I know according to its specifications that the Canon 350D / Rebel XT's auto focusis supposed to operate from EV +0.5to 18 (at ISO 100). I looked up on the net what +0.5represents e.g. EV +1 (one) ="distant view of lighted skyline"and EV +5 (five) is "Night home interiors, average light. School or church auditoriums. Subjects lit by campfires or bonfires. " then I think that it's safe to say I'm testing in plus 5 (five) EV when I start to have the autofocus problems I mentioned!

So also the question remains, if I got a Speedlite and used the "near infrared" AF assist beam, do you think that would help me out (even if I wanted to use the ambient light to shoot in, e.g. if I swivel / tilt or turn the flash away from the subject...?)

Below are a few TYPICAL examples of my problem. And note when I take photos of exactly the same situation with my Fuji point and shoot, it gets the focus perfect, even with low (f 2.8) aperture and background blur. That's why it's so dissapointing that my DSLR can't seem to do the same (or I would expect even better AFfrom aDSLR compared to a P&S).

Please let me know what you think.... remember there is no Canon service centre in Romania near me. I bought the camera in UK. Of course I would like to get the camera repaired if you think it's definitely a problem with my camera in low light autofocus!

Paul
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Old Jan 31, 2006, 5:00 PM   #9
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Example 1. (please read my last post just above these example shots first).

Note I have cropped all these photos to show clearest the oof (out of focus) problem I seem to be having. And crops are not all made from the centre, what I mean is the centre of the crop might NOT be where the selected central focus point was on... just so you don't get confused!

This is an orange - I focused at the green "bud" part where the orange used to connect to the stem. It obviously front focussed - look at the table cloth! Canon Digital Rebel XT / 350D with 50mm f1.8 lens at f1.8, ISO 800 and 1/100sec exposure time.
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Old Jan 31, 2006, 5:04 PM   #10
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Example 2. (please read my last post just above these example shots first).

These are a set of "desk organiser" drawers pulled out about 2cm (yes close to 3/4 inch for imperial based users!) and I focussed on the i of "Romania". Clearly the UK in the drawer below it is clearer in focus!


Canon Digital Rebel XT / 350D with 50mm f1.8 lens at f1.8, ISO 800 and 1/80sec exposure time.
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