Hey folks. I'm trying to get a better understanding of what could cause a lens and camera to focus at the wrong distance.

I've noticed that -occasionally-, and only very specifically when shooting portraits in a tight room where the subjects are very close to the background (we're talking 3 feet or less here), and where there are tungsten accent lights (like little spotliights) shining down from the ceiling, illuminating the background (high contrast points), my 1D3 and 24-70 will grab focus on that back wall instead of faces.

I know what is happening - the contrast in light on the background wall are much greater in dynamic range than that of my subjects. But why would a lens or sensor interpret it this way, when I clearly put my focus point on a face, and not the wall?

Theoretically, if I used a lens that was even more contrasty, would this help alleviate these problems?

And yes, my 24-70 has been calibrated and works *beautifully* in every other aspects. It's just those...contrast points that are throwing it off once in a while.

Any techies in here feel free to chime in.

If the focus point in use cannot achieve focus, it should really just flash that green circle tellling you so, but not jump to something else entirely for focus. This seems to be the sort of problem the 1D III was infamous for early on. Is you camera one that should be fine, or do you have an older one with no mirror fix and latest firmware?

Sometime, the highest contrast point you saw in the VF is not the same highest contrast point that AF sensors have seen. In fact, your AF sensors might have seen mutiple contrast points that's been deflected by your lens, reflect-mirror(s) and AF-mirror(s). Best way is to custom your AF point to cut down this optical illusion. Of-course this can be eliminated by adding more errors handler into AF database F/W. Note that, not all F/W fixed can really cure the real mechanical / electrical / optical complex problem. DOF button can be used in conjunction with your AF to work around this issue.

I have an old one, with mirror fix and firmware. Mine does this in a easily repeatable way, it is clearly being influenced by adjacent AF points, even when those points are disabled.

Well, if you are talking 3 feet or less, why not use manual focus

Because its a $4000+ camera that is touted as having the best AF on the planet. Its not the point, the OP is wondering why this is hapening.

Imagemaster wrote:
Well, if you are talking 3 feet or less, why not use manual focus

Jimmy Ho wrote:
Hey folks. I'm trying to get a better understanding of what could cause a lens and camera to focus at the wrong distance.

I've noticed that -occasionally-, and only very specifically when shooting portraits in a tight room where the subjects are very close to the background (we're talking 3 feet or less here), and where there are tungsten accent lights (like little spotliights) shining down from the ceiling, illuminating the background (high contrast points), my 1D3 and 24-70 will grab focus on that back wall instead of faces.

I know what is happening - the contrast in light on the background wall are much greater in dynamic range than that of my subjects. But why would a lens or sensor interpret it this way, when I clearly put my focus point on a face, and not the wall?

Theoretically, if I used a lens that was even more contrasty, would this help alleviate these problems?

And yes, my 24-70 has been calibrated and works *beautifully* in every other aspects. It's just those...contrast points that are throwing it off once in a while.

Any techies in here feel free to chime in.



the lens should focus, hunt then focus or hunt then give up. I could understand if focusing on something in front of the subject while hunting, but not behind the subject.

I don't have 1D3, I use 30d but I notice the same problem.

In low light and when the background is more contrasty, my 30d and 24-70 (wide open) would sometimes focus behind the subject. For instance, I selected outer focus point, I focus on the face, the AF confirmation light on, then I took the picture. I would only realized this mis focus when I look at the pics after I downloaded them to the PC.

It doesn't happen every time, only occasionally, but it's still annoying.

I have this problem a lot with my 24-70 (never with any of my other lenses) and know a lot of others I've talked to who say that the 24-70 often focuses on the background.

24-70 is a problem child and it does need better coating for AF system.