Why does my autofocus on portraits drift from point I focused on?

[Chuck S.]

Colin, are you saying the focus point moves or merely the square that indicates what the autofocus preference point is? Using that square to focus lock on the eyes with a half-press of a shutter should keep the eyes in focus regardless of the re-framing - correct?

[Codebreaker]

The focus point should stay the same - its just the square in the viewfinder or lcd that moves relative to the focus point as you re-frame the shot.

Of course if you use the Ai Servo Focus then the focus point itself may shift if the camera thinks there has been movement of the subject. (which is the same as movement of the camera)

I had similar problems way back with my 300D. Quite often the sharpest point of the subject was not what I focused on.... without ree-framing. There was alot of discussion about back focusing and I ran some tests on my camera but there didn't appear to be any appreciable amount of this.

Instead for some subjects I switched to AI Servo focus and found this improved things, most likely due to slight camera movement being taken out of the equation. But no good for re-framing

Now I have the 7D I find the focusing much better but I will switch from One shot to AI Servo as needs require.

Colin

[DonnaA]

My only regret, wish I'd done this sooner. Since first starting this thread 4 months ago, I continued to be very disappointed in all images taken with my Canon 17-85mm. I tried EVERY possible remedy and talked to Canon a few times. After some important events in May-June (graduation, birthdays) ending up in disappointing, soft, blurry photos, I decided to buy a new lens. This is a big investment for me (I've been saving for it) and so I did a lot of research to find just the right fit for my needs and budget. I chose the Canon 15-85mm f/3.5 IS USM (I've alwyas liked the range of the 17-85 on my 1.6x crop sensor = 24-135mm approx.)

When I got home, first thing I did was identical comparison shots between the old lens and the new. What a difference!!! CLARITY, at last!! The color is better too. And I'm very relieved to find out that just the lens was defective and the camera body is NOT. It was a big, but very worthwhile investment and I'm very pleased. Thanks so much for all the help and support you've given me. I'm a happy shooter once again.

[Ivor]

Donna, just one more thing to try - Have you got a filter screwed onto the end of your old 17-85mm lens? I made the mistake once of buying a cheap CPL filter and it adversely affected focus. Try unscrewing it and taking a few test shots.

Ivor

[DonnaA]

Originally Posted by Ivor

Donna, just one more thing to try - Have you got a filter screwed onto the end of your old 17-85mm lens? I made the mistake once of buying a cheap CPL filter and it adversely affected focus. Try unscrewing it and taking a few test shots.
Ivor

Ivor, thanks for that suggestion. I took the filter off the old 17-85mm a few months ago and did test shots with and without. The images were slightly better without, but still not what they should be. On comparison shots I did a few days ago, between the new lens and the old, neither lens had a filter.

[JulieM]

Hi Donna,

I'm glad to hear that you've now got a lens that you're happy with. I wonder if it would be worth sending the 17-85 in to Canon to have them check it for defects.

[DonnaA]

Originally Posted by JulieM

Hi Donna,

I'm glad to hear that you've now got a lens that you're happy with. I wonder if it would be worth sending the 17-85 in to Canon to have them check it for defects.

Hi Julie, yes, I considered that. But decided that if it needed repair, I would be paying the repair cost plus shipping & insurance. Decided I would rather invest in a new lens. And it's possible that someone may be interested in buying the old one "as is", defects and all. Thanks again for your suggestion!

[Canonut]

Another thing to check.....have you got the autofocus set to 'One-Shot',
(see AF button on the top plate).

If it's set to AI focus, or AI Servo, then it will refocus when you recompose !

Worth checking !!!

[DonnaA]

Originally Posted by Canonut

Another thing to check.....have you got the autofocus set to 'One-Shot',
(see AF button on the top plate).

If it's set to AI focus, or AI Servo, then it will refocus when you recompose ! Worth checking !!!

Believe me, I've tried it all. I keep it on One-Shot, but I've tried Al Servo too. I know this lens has a defect, I suspect it's a front-focusing issue.

[Ivor]

Is it still under guarantee?

If not, you can probably send it back to Canon for an estimate cost of repair. If you are not going to use the lens and the cost is less than buying one second hand, get it fixed and sell it on Ebay!

Someone with no scruples would trade it in or get some cash for it from a pawn brokers, but that would be unfair on the shop/next buyer.

You could also dismantle it to see how it works and then turn it into a still life subject.

Just out of interest, is it focusing to near or too far?

Ivor