my 20D is "hunting" for AF lock, why and how to correct?

[EricKonieczny]

I have had my 20D now for about 6 weeks. I have the 580EX and the 28-105, 3.5 - 4.5 lense.

The last few times I have shot in nightclubs and very low light, I am having major trouble focusing, and getting it to lock.

I ususally shoot in Manual or AV with the 580 as a bounce flash.

If I select one of the AF points rather than "all" it has a tough time locking down. I usually shoot ISO 800 and a wide range of shutter speeds and Apertures. My camera was really hunting in in AI Servo, so I went back to One Shot.

This is one photo, I took and tried to focus pretty close like 2 feet away. and I just couldn't get it to focus. I used the far AF point to get the pictures in focus and leave the eye and face out of focus. This is just one example.



Suggestions? faster lense, recompose? Here is the Exif and the shot in question. But this is just one example. Thanks.

http://www.ekreating.com/images/exifbad.jpghttp://www.ekreating.com/images/bad.jpg

[CyberDyneSystems]

Hunting in low light certainly is not a unique phenomenon,. so I doubt there is an actual malfunction.

A fasrer lenms than f/5.6 certainly does help focus in low light.
Therefor the best advice I could offer would be to grab/borrow an affordable fast lens,. but one with a fast focus motor as well,. like the 85mm f/1.8 USM or 50mm f/1.4 USM and see how that impacts your issue.

In the case of the image above? It looks as if the subject may simply be too close for that lens to focus on it? Is this possible?

[ScottE]

Try selecting the centre autofocus point only. That is the most sensitive.

In any case, you don't have to be slave to autofocus. Manual focus is still the best in some circumstances.

Scott

[Tom W]

the 28-105 f/3.5-4.5 is a good lens, but at a max aperture of f/3.5-4.5, it is not the best lens for very low-light applications. Think of it this way - the 20D specifications say that it can AF at 0.5-18 EV at 100 ISO (EV are basically units of available light adjusted for ISO setting - Exposure value). These specs are probably made based on the 50 mm f/1.4 lens. Anyway, for each stop slower that your max aperture is, you lose 1 EV of low-light focus capability, so for an f/2 lens, your AF working range would be 1.5-19 EV, and for an f/2.8, it would be 2.5-20 EV. In other words, the slower lens will have more trouble than the faster lens at very low-light shooting.

For reference, an exposure value of 1 (EV=1) occurs at ISO 100, f/1.4, 1 second exposure. That's pretty doggone dark. Each change of 1 unit of EV equals 1 stop. There's a cool chart HERE (http://home.pcisys.net/~rlsnpjs/minolta/srtmeter.html) that shows relative exposure values in terms of percieved brightness.

Note that changing the camera's ISO setting doesn NOT increase the sensitivity of the AF system, but only increases the amplification of the basic signal recieved from the image sensor. So, switching to ISO 800 will not improve AF performance.

Anyway, to make long story short, you probably need a faster lens such as the 50/1.8, 50/1.4, 85/1.8, or 28/1.8 depending on your focal length needs.

[jrsforums]

In dim light the 580 IR emitter will really help with AF...at least it does for me....I have shot successfully outside with almost no light.

First, make sure you have not turned off emitting in the Custom Functions....I have turned emitting off for the in-camera flash as it is annoying, but if you are not careful you could turn both off.

Second, I have not tried it, but at 2 feet I am not sure the IR will work properly. Someone else may be able to comment better on this.

John

[Tom W]

In dim light the 580 IR emitter will really help with AF...at least it does for me....I have shot successfully outside with almost no light.

First, make sure you have not turned off emitting in the Custom Functions....I have turned emitting off for the in-camera flash as it is annoying, but if you are not careful you could turn both off.

Second, I have not tried it, but at 2 feet I am not sure the IR will work properly. Someone else may be able to comment better on this.

John

John is right, and I hadn't considered the focus assist IR light on the flash. It should help you quite a bit by giving your AF scheme additional light. For some reason, I had not considered that you were using flash.

Also, make sure you're working outside of the minimum close focus distance of the lens, which is 1.6 feet for the 28-105 f/3.4-4.5 USM.

[Marshall]

You are also better setting not just any focus point use the centre focus point only. Also I agree the subject does look a little close to the lens

[tim]

To summarise:
1) Use the centre focus point.
2) Make sure the focus assist bean on the 580EX is emitting. It can be turned off with custom functions, and doesn't work in AI servo mode.
3) Manual focus if the lens won't auto focus.
4) Get a fast lens (50mm F1.8's cheap) as the wider aperture makes auto focusing easier, as more light gets into the camera. 85mm F1.8's better than the 50mm, but costs about 4 times more.

Good luck :)

[J Rabin]

As others have told you:
1. The 20D center focus sensor is the only hi-precision cross sensor, hi precision is activated on lenses f/2.8 or faster.
2. But, heck, this is why lenses have distance scales: You should look at subject and guess and prefocus lens as best you can before aiming at subject to reduce hunting.
3. In low light, use a prime (fixed focal length) lens because when a zoom lens "defocuses" it really loses focus and has to rack long distances back and forth to find focus. The best low light Canon primes have ring-type USM with rear element focus. Like the 85 f/1.8. I was never impressed with the 50 f/1.4 or f/1.8. Sharp, yes. Good autofocus, no. I sold my 50.
4. Faster the lens, the brighter the viewfinder is for manual focus in low light. Since 1.5/1.6 crop digital SLRs have dark viewfinders that suck compared to even modest manual SLRs (The Nikon D70 is worse than the Canon 20D and the 350xt is bad too) you need to use the fastest lens affordable just to see the subject.
Jack

[yenoram]

Eric, there is nothing wrong with the camera hunting in AI Servo - AI Servo is for continuous focusing so that is exactly what the camera is supposed to do. Follow the advise from the previous posters and you should notice marked improvement. I have a 20D and I'm continue to be impressed by its low-light focusing capability particularly in comparison to the D60 I owned previously.

[smudge]

I have had hunting/ no focus problems with my 20d since the day I got it. I put it down to AI or low light, now yesterday My 20d has stopped beeping in all creative modes, to confirm focus. ( NO, the beep has not been turned off) When in auto modes no hunting, and beep confirmation.
Creative modes, no beep and continual hunting. I'm started to get annoyed, BG-E2 went friday, auto focus today.

[elbirth]

I was going to point out that it will not achieve a single focus lock in AI Servo, but I see that's now been pointed out. I also think the subject is just too close.


I have had hunting/ no focus problems with my 20d since the day I got it. I put it down to AI or low light, now yesterday My 20d has stopped beeping in all creative modes, to confirm focus. ( NO, the beep has not been turned off) When in auto modes no hunting, and beep confirmation.
Creative modes, no beep and continual hunting. I'm started to get annoyed, BG-E2 went friday, auto focus today.

Take it out of AI Servo mode, it won't give the beep confirmation while focusing, since it never locks in on a single focus