40D autofocus problem

OK, here's the story, wondered if anyone had any suggestions.

My uncle was complaining his 40D was hunting a lot and wondered if there was something up with his lens, a 100-400. I took my 40 over, and swapped bodies, lenses and settings around. The result is;

On his 40D, with all focus points selected the camera frequently hunts, and on subjects that don't fill the frame, often fails to lock focus.

Used individually, each focus point works fine. Orientation had no effect, settings and CFs I made identical to mine, and focus mode isn't causing it.

The lens was perfect on my camera, and my lenses hunt once mounted on his. His camera is unmodified, clean, and hasn't had any major knocks.

Any clues or suggestions? If not it'll be on a trip to a service center.

As far as I know, on 40D, even with all focus points active, it will start tracking with the center focus point. So unless you have the object on the center point, the AF system will hunt...

But in One Shot AF, it ought to lock on to something, especially when all points are active. On a 40D, that implies 18 focusing elements metering at the same time, so it's not too often none of them can find anything.

Apart from that, up to the 7D, all EOS cameras started Servo AF with multiple points active from the center point. It's only the 7D and the 1D Mark IV that can start from any point.

It's definitely a fault with the camera, as mine (also a 40D) behaved perfectly focusing on the same spots. I was more wondering if anyone had heard of something similar and what the problem was.

What I find most odd is that any one focus point works fine, only when all are selected does it hunt and fail. I didn't check focus accuracy on the different points, just that they locked. I wonder if the mirror or sensor is out of alignment?

This seems to be an old thread but describing quite accurately what is happening with my 40D at the moment (after 4.5 years of use). Again, the camera was not used extensively nor had any knocks. The focusing system recently has started to hunt without success when all points are enabled. It locks OK when something is in the middle though. Each individual point seems to be focusing OK. Has there been any resolution to this problem?

I am experiencing a similar problem and posted my question here: https://photography-on-the.net …/showthread.php​?t=1165867

On mine, it will hunt and either not lock or get a false lock (not even close to being focused) when only the center focal point is enabled. If I change focal points and to the far left point and then aim it at something it still hunts. It is very intermittent on my 40D. I also have some issues with shutter lag which has occurred with auto focus disabled, so that should not be an issue with focus locking.

Interesting. A bad run of chips perhaps?

One thing you can do is make sure the lens-to-camera contacts are clean. Over time they can get finger oils or other contaminants that effect the low voltages used to comminucate between the camera's AF system and the lens' AF drive motor.

I recommend using a few drops of isopropyl alcohol (cheap and widely available 70% "rubbing" alcohol is fine) on a clean cloth to carefully wipe the contacts on the back of the lens, as well as the corresponding contacts just inside the bayonet mount on the camera body. Of course, be very careful to keep off the optics of the lens, the mirror in the camera, etc.

While doing the cleaning, also check that the spring loaded contacts inside the bayonet mount aren't sticking.

I virtually never use "all points" (IMO, it just leaves way too much up to chance or the camera making decisions I feel I should be making), so I really can't comment about the behavior using that mode of focus.

There are a lot of variables effecting AF. Having two identical cameras set up exactly the same way and trying them alongside each other with the same lenses shooting the same subject is the best way to test it yourself. But that will only flush out if there is a problem and does nothing to correct it. So it usually ends up being necessary to send the camera in to Canon Service be checked anyway. Or, since it's an older model that's out of warranty anyway, you might be able to find a local repair technician who can take a look at it for you, perhaps at least saving some time and the costs of shipping.

EDIT: I don't know that I'd call it "a run of bad chips", cameras that have worked well for up to five years to take an unknown number of shots and are now starting to show some glitches. Especially if the cameras have never been serviced.

Incidentally, it should not make any difference with respect to AF, but if the little silver "memory" battery has never been replaced, it might be about due now. Those are inexpensive, widely available and easily changed on many models.

I googled a bit but couldn't find a conclusive cause. Some was caused by a UV filter or CPL mounted on the lens, some was caused by a particular lens but worked fine with another lens.... I know, not much help here.

Just remembered something that might be behind this... though I think it a bit of a stretch.

A common problem with 40D and some other xxD models is the shutter release button getting dirty (probably mostly finger oils, over time). There is a DIY cleaning process, you can search for more info here.... It may or may not be related to the AF system issues mentioned.

What happens is oils/dirt get into the micro-switches of the shutter release button and prevents solid contact. Usually people notice delayed shutter release first... but who knows, since the AF system (and metering, and IS systems) is activated by this switch too... maybe.

A quick way to test would be to temporarily switch to using Back Button Focus... reassign AF to one of the back buttons on the camera. If AF then works properly, then suspect the shutter release button needs a good cleaning (either have it done or, if willing to do so, try the DIY fix).

Again, this is a bit of a long shot, but who knows. Might be worth a try.

If the shutter lag/inresponsiveness got worse, perhaps can give this method a shot? I have not tried it but i can imagine if the problem gets worse, it makes the camera pretty much unusable (missing critical photo moments, bad focus on important subject...etc)
http://www.youtube.com​/watch?v=tB_gSqyidI0

cfcRebel wrote in post #14162830
If the shutter lag/inresponsiveness got worse, perhaps can give this method a shot? I have not tried it but i can imagine if the problem gets worse, it makes the camera pretty much unusable (missing critical photo moments, bad focus on important subject...etc)
http://www.youtube.com​/watch?v=tB_gSqyidI0

Yeah, I pretty much shoot for fun and shoot macro florals, so my subjects are not going anywhere. If I were shooting weddings, etc... I would have bought a replacement camera last year. I can live with it as is for a bit longer. I've not had a good run of luck with the 40D. The first one went toe up (error 99) after just 2 months and Calumet swapped it out for me.

Is that you tube the one where the guy pours lens cleaner into the battery cavity? That one made me laugh...what could possibly go wrong?

amfoto1 wrote in post #14162777
A quick way to test would be to temporarily switch to using Back Button Focus... reassign AF to one of the back buttons on the camera. If AF then works properly, then suspect the shutter release button needs a good cleaning (either have it done or, if willing to do so, try the DIY fix).

Again, this is a bit of a long shot, but who knows. Might be worth a try.

I'll try it this weekend. Thanks for the tip!

kristin6 wrote in post #14163255
Is that you tube the one where the guy pours lens cleaner into the battery cavity? That one made me laugh...what could possibly go wrong?

It's the Isopropyl alcohol that you can find in most pharmacy stores such as Walgreen or grocery stores. I used it once to work on my car's power window. The window switch became inresponsive. Pouring some alcohol and worked the switch a bit, got rid of the problem. Who would've thought power window could become a alcoholic.

Sometimes, AF problems can be caused by dust on the AF sensor. The sensor is located in the bottom of the mirror box.

Remove the lens and activate Mirror Lock Up. Turn the camera upside down and blow off the AF sensor with a rocket blower. Replace lens and deactivate MLU.

It's a long shot but it has been known to work.

Good luck!

Yogi Bear wrote in post #14163419
Sometimes, AF problems can be caused by dust on the AF sensor. The sensor is located in the bottom of the mirror box.

Remove the lens and activate Mirror Lock Up. Turn the camera upside down and blow off the AF sensor with a rocket blower. Replace lens and deactivate MLU.

It's a long shot but it has been known to work.

Good luck!

I posted this in the other thread already, so sorry for the dupe, but I hope this helps...
Kinda related to what Yogi Bear just said:

A few months ago, I was having a lot of issues with intermittent focus hunting on my 50D. There was no discernible pattern I could identify, and it happened with all three of my primes (28, 85, and 200). On the forum, someone suggested to look for debris on the focus screen; stating that the AF system somehow uses the focus points in the viewfinder to compute focal distance. (I still don't know if this is true or not) That said, I took a look through my viewfinder, and there was a big 'ole stinking hair cutting directly across the center focus point. Normally, I have a fair amount of debris inside the body because I change lenses quite a bit outdoors and I usually blow the body out every couple of weeks. So, having a hair in the viewfinder is something I usually disregard to the point where I don't even notice it anymore.

Anyways, I pulled the focus screen out. Blew on it, re-installed it, powered up, and camera was working properly again.

To this day, I'm not sure if the hair on the focus screen was actually the cause of the issue, or if it was just one of those "oddball" things that corrected itself or just my general ignorance.

Anyway, i hope my post helps.