100-400 - Hunts for focus

[Double Diamond]

I know we have a lot of user of the 100-400 L here so hopefully someone will have an idea about what is going on. I took some picture with my 100-400 today and noticed something kind of strange. Several of the pictures were out of focus and I kind of thought that I heard the lense hunting for focus. I got back home and shot some pictures in the backyard, paying particular attention to the sounds that it was making. Camera was mounted on tripod, IS off, range set to 1.8m. I was shooting at an object that was maybe 8 to 10 feet away. Anyway, I noticed the same thing. It seemed that it would continue to hunt for focus (don't really know how to describe it other than that). Would get to a point and then kind of move back and forth in a very small band. Then it would take the picture. When I looked at them, some of them they were also a little out of focus, it was like it couldn't decide where the proper focus point was so it just chose one. Not sure what would cause this. Anyone else experienced anything like this? Any suggestions?

[Scottes]

I spent a few hours shooting some flying sea gulls the other day and I was quite pleased as almost all where at least mostly in focus. I had a tough time when the bird was 150' away when starting from a 50' pre-focus, but I guess that's expected.

Since AF uses contrast to do it's job, I'd ask how the contrast was. Was the focus point large enough to cover a sensor? What was the difference between pre-focus and final focus?

I've been happy with the AF speed of mine in it's limited use of a few hundred pictures.

[Double Diamond]

Well, the pictures that I took early this morning were of deer. I went to Red Top Mountain, drove and walked around and got some good shots. Problem was that some of them were a little softer then usual and I heard the lens kind of hunting for a focus. Seemed to be moving back and forth between a couple of points very close together. Didn't think much about it until I looked at the pictures. In the back yard I shot an ear of corn that is on a squirrel feed that we have and a flower in our flower garden. The camera was on a tripod, used center focus, Av (with different apertures). Paid particular attention to the focus, at times it would get to a point that it appeared to be in focus and then just kind of bounce back and forth between two area that appeared to be really close. I could just see the numbers moving in the distance scale window a little bit, kind of like a vibration. Anyway, it would lock focus and shoot. When I looked at the pictures some of them (I tried to keep track of the number when it did this) were softer then normal. Kind of out of fucus as opposed to just being soft. Don't know, kind of odd, just thought that somebody might have run across this before.

[robertwgross]

Set your camera in the same tripod position for the ear of corn shot. This time, try each of the following:

(1) Do it at a different time of day, or with brighter sun.
(2) Kick the ISO up two or more notches.

If that doesn't work better than before, then hang a high contrast target at the ear of corn and try it again.

---Bob Gross---

[defordphoto]

Your AF could have gone south. But, it will also hunt in lower light as it's not a speedy lens. If in sun you still have probs then you'll have to pack it up and send it in. Happened to my lens last summer, and took the fine folks at CFS three times to fix it!

Good luck.

[iwatkins]

AI Servo mode can cause hunting in lower light conditions.

I've only seen it a couple of times with my Sigma 50-500. Set the camera to AI Servo and shot a stationary target in low light and it will hunt until you take the shot. Only happened a couple of times.

Of course, if you are in One Shot mode, ignore the above.

Cheers

Ian

[Jim_T]

FWIW, I've never noticed excessive hunting, but I've found the 100-400 produces more out of focus images than my other lenses when in single shot mode. It's not terrible.. perhaps three or four out of every twenty - thirty shots. I attribute it to the narrow DOF at long focal lengths with the lens wide open. You have far less margin of error.

I've taken to shooting wildlife in AI servo.. I never have problems in that mode.

Don't discount other factors.. Camera shake is always a problem at long focal lengths.. The IS works great, but it can't cure everything. You may find some of your blurring is from shake..

Also, you should only be using one focus square.. I've got mine set to use the center one only.. Using all seven could result in the camera focusing on the wrong part of the frame.

[Mark Kemp]

Try the centre focus point only - in one shot focus not ai servo and put the centre spot over a sharp edge (clear high contrast target) like the edge of a building. Its should find it easily and stay put as long as it is a reasonably bright day. If not you may have a problem.

[chris.bailey]

Have you got the lense in full range focussing. Cutting it down to 6m upwards cuts down on the hunting a great deal.

[stevelew]

I had the same problem with a 10D and different lenses. The problem was the camera. I sent it back to Canon about 5 times before they could duplicate the problem and fix the problem. The lens was fine. It really bugged me because at times it would work fine then it would go out and start hunting after about 5 shots. Best of luck.

Steve

[selym]

Could any of you guys apply your knowledge to my problem listed in "lens not focussing help " thread....I think its something similiar to what you discussed.


all input appreciated

Selym

[robertwgross]

Selym, did you ever identify the lens in that thread? Or, was it the 100-400mm L, like in this thread?

Did you apply all of the suggestions made in that thread?

---Bob Gross---

[Ikinaa]

...

(2) Kick the ISO up two or more notches.

If that doesn't work better than before, then hang a high contrast target at the ear of corn and try it again.

---Bob Gross---

Question : What has the ISO to do with AF?
I always thought that AF worked with aperture wide open.
ISO has effect on the CMOS (boosting signal), but as long as you're focusing, the CMOS is hidden behind the curtain...
Or did I miss something there?

[robertwgross]

Question : What has the ISO to do with AF?
I always thought that AF worked with aperture wide open.
ISO has effect on the CMOS (boosting signal), but as long as you're focusing, the CMOS is hidden behind the curtain...
Or did I miss something there?

Let's say that there is dim light around a focus target. Depending on the exposure mode, the camera may be struggling around a wide open aperture. If you suddenly goose up the ISO by two stops, then suddenly the camera no longer has to struggle to get that exposure solution, so it is stopped down from wide open. You know how narrow of a depth of field some lenses get when wide open, and that can contribute to apparent focus error. When it is stopped down two stops, the depth of field is much deeper, so everything seems to be in better focus. Did you miss anything?

---Bob Gross---

[Ikinaa]

Let's say that there is dim light around a focus target. Depending on the exposure mode, the camera may be struggling around a wide open aperture. If you suddenly goose up the ISO by two stops, then suddenly the camera no longer has to struggle to get that exposure solution, so it is stopped down from wide open. You know how narrow of a depth of field some lenses get when wide open, and that can contribute to apparent focus error. When it is stopped down two stops, the depth of field is much deeper, so everything seems to be in better focus. Did you miss anything?

---Bob Gross---

Ok... I see what you're meaning.
I think I'll try it out and play around a bit in low-light situations...

[robertwgross]

Ok... I see what you're meaning.

None of us knows any of this with 100% certainty. All we can do is to post a theory that is consistent with the facts of the scene, as presented, and hope that meshes with the technical facts of what is going on inside the camera or inside the lens. Then it becomes the original poster's job to run that theory against his scene and see if the final outcome goes as predicted. Otherwise, it is all one big crapshoot.

---Bob Gross---