Auto focus problems

I have a rebel XT and have stopped using the the auto focus feature on my lenses because the photos all seem too be slightly out of focus. I've noticed it's worse when I'm in the higher ISOs and on macro shots. Even when I manually focus if I go by the beep it will sometimes be slightly off but less often than with auto.....is there something that I could be doing that's causing this. I'm new to DSLRs. I have an old Minolta film SLR and have had very few photos come out blurry........ this is really bothering me.

Thanks in advance,
Blair

Hi Blair, it would really help if you could post a few examples of images along with the exif data.

Hi Bob, Thanks for responding. Here are some examples:

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They do look soft to me...

#1 - 1/30 shutter is a bit too slow for a 300mm lens. You should be around 1/300. But the grass looks to be in better focus than the bird.

#2 - looks pretty good but you are still way too low on the shutter speed.

#3 - not too bad.

The rest are very low shutter speed except that last one and that is thru glass which can't be optimal.

I would suggest you work on getting the shutter speed up or get a tripod. You are probably introducing camera shake. And I'm not sure about the sharpness of the 75-300 you have.

The first shot is front-focused, probably the AF system focused on the grass in front of the bird because there are more areas of sharp contrast there than on the bird itself. The AF sensor is actually larger than the box in the viewfinder, and it looks for contrast.

The shot of the monkey seems about right, though I would need to see the original image to tell for sure. The DOF is pretty shallow in this shot. The eyes appear to be focused but the nose is not.

As for the lizard and the fish, keep in mind that the depth-of-field is extremely shallow in close-up shots. The slightest movement (of either subject or camera) between the moment you lock focus and the moment you take the shot will throw the focus off. Fish shots are incredibly difficult for this reason.

Slow shutter speed likely contributed to a lack of sharpness in the first two images. I'm still trying to figure out how you got that close to the lizard with the 75-300 lens.

If I could make a suggestion, which *may* seem ludicrous, but please bear with me......

Get a couple of sticks of dark colour, and some old rags or socks, try to get them as reasonably close to the colours on the Monkey, bird. Go outside and stcik it in the grass/ground, and shoot off a load of test shots. (Bear in mind these won't be moving). It will let you see straight away if the methods you are using are contributing to the softness.
And it will also reassure you that it isn't any fault with the equipment, and give you an idea of the ideal settings.
Overexposure (slower shutter) can be useful (if not overdone) to get the results you are after - you can easily correct slight flare etc in RAW - but for anything that moves, I would personally err to underexpose very slightly (higher shutter speed) I notice the bird picture is at f11 - a bit high an f number for that type of shot, especially as it's (relatively) dark.
(and of course the shutter speed is low).

Ideally, I would not use AF for anything moving - although it is sometimes desirable where (especially with wildlife) you may not always have the time needed to adjust focus manually.
Another couple of questions: is the camera set to AI servo on AF mode?

IHow much have you used AF so far? (It does need a but of experimentation to see what works and what doesn;t)

The shutter speed seems very low for these shots.

A good guide for shutter speed is shoot at a speed equal or faster than your focal length. So for your bird shot it should have been 1/300th or higher. Of course if its on a tripod it then depends on how fast your subject is moving but I think a similar speed would be need.

Also I would suggest using center spot AF if you don't have it selected already.

Test your lenses for front/back focus problems. A quick search will reveal plent of info how to do this.

Good luck,

My suspicion is camera shake, due a combination of slow shutter speeds and just possibly moving whilst taking the shot. 1/30 with a 300mm is just plain asking for trouble, unless you've got it tripod mounted. Even with a 300mm f2.8 IS and a monopod, you'd still have some sharpness issues I'd feel with 1/30 sec. The monkey shot doesn't look too bad, possibly some slight backfocusing, but it could be from movement (of you or the monkey). The fish images - my suspicion is that it's movement blur from the fish. Fish can move quite fash, fast enough to create some slight blur. You really need to shoot at higher shutter speeds to freeze any potential movement.

Best to test this - take a shot of a fixed, non-moving object with the 300mm at 1/30, handheld. Then take the same shot, framing etc with the camera tripod mounted. Compare the sharpness.

I'll also add that the Rebel XT etc AF is what I consider sub standard and I've had this argument before on these forums. If you want decent, accurate AF there is no choice other than to go and buy a 1 series body imho. I swear my 20 year old eos630 focuses better than the 20D etc. Canon deliberately dumbs down the AF on the non pro bodies to tempt people to buy the 1 series bodies, hence a higher profit margin. This is a commonly used marketing/sales trick of most manufacturers and one I consider as being immoral. Sadly money (and profit) goes before morality in this day and age.

Dave

I don't think that I would classify the XT's focusing as sub-standard. From what I have seen - it focuses faster and more accurately than most of its similarly priced competition. It's all about perspective......

You do not need a 1-series body for quick and accurate AF - that's just not the case. That is way too general of a statement.

I believe - with everyone else - that the lens being used is not a tack sharp lens and then add to that extremely low shutter speeds and you have the results shown above. Completelty correctable with just a change in workflow - not camera.

Obviously the more advanced AF systems of the 1 series bodies are useful for demanding situations, fast-moving subjects and the like. But these images pose no such challenges.

There are thousands... thousands of tack-sharp images posted on this forum and elsewhere from the 10D, 300D, 350D & 20D. To suggest that their AF systems are substandard requires ignoring a mountain of evidence to the contrary.

Just a dumb question, but it´s the first one:

Do you use Center Point AF only or just leave the machine choose the AF points? If this is the second case, please use and selct Center Point AF only, it´s more reliable.

But I am with Curtis N at 100% about your photos

back to the store, I´m trying the 4th 350d body , all with focus defaults.cheers

Each to their own. I consider them sub standard and deliberatey dumbed down. When you have a 20 year old body that's just as good with AF as a new body, somethings wrong. That's my take and I stand by it. If you don't like it, tough, it's my opinion, and I own it. Just as you own your opinion.

Cheers,

Dave

Good luck!

Dave

I find it very, very difficult to believe that someone has to go through 4 bodies before they find one that auto focuses correctly. That just defies logic. Perhaps the problem is user error.