Weird Focus Problem with L series

[Tyke Tiler]

Hi,

Yesterday I had a brief chance to sample a Canon EF 70 - 200mm f/2.8L IS USM, I'll be owning one soon but I had a weird problem whilst sampling.
The auto focus was very fast in deed, but seemed to avoid focus on the center or foreground, unless the subject was filling the frame this continued to happen.

My other lenses worked fine and the 70 - 200 worked fine on my friends own set up too.

Any ideas?

Thanks

Tomas

 

[RobertP]

Not sure why you would let the camera decide which AF point to use. I have mine set to use the middle one only unless I manually select one of the other points.

 

[Tyke Tiler]

Not sure why you would let the camera decide which AF point to use. I have mine set to use the middle one only unless I manually select one of the other points.

Hi,

Thats exactly how the camera was configured. I really don't know what to make of it, I took 40 shots, all of them using the center AF point and nearly all of them had a sharp back ground instead of a sharp subject.

Tomas

 

[RobertP]

Then the logical step is to try a focus test. there are charts you can print out. You focus on the middle with the chart laying at an angle and then see where the focus in the picture is.

Try different lenses. If the body needs calibrating all the pictures will be out. If it is the lens then just that one will be wrong.

 

[pxl8]

Can we see an example shot?

 

[Tyke Tiler]

Can we see an example shot?

Certainly:
The first one! The center AF point was right ontop of the chap, it lit up and beeped signalling it's AF success but it's the tree's that came out in focus.

Number 2: Same thing, the cars have more edge instead of the subject, despite the subject as being the point for successful AF.

I've found a few scraps of info on the net regarding some copies of camera bodies or lenses suffering from 'back focussing'. Maybe that's what it is? Or could it be me?
As soon as I looked at the sample pics at home on computer, I tested my other lenses, no problems. I suppose the good or bad thing (which ever way you look at it) is, the 70 - 200 f/2.8 IS lens used for the above. doesn't belong to me, it's my mates.
I am in the proceeds of buying one from Kerso's.


Tomas

 

[Exodus]

I was going to say that it sounds like back focusing to me. Apparently its more likely to occur when all focus points are active and the camera auto selects what it should be focusing on. Ive heard that selecting a focus point manually prevents it from occuring, other than that I would reccomend calibration in the first instance.

Have you tried selecting your focus point manually (i.e. fix the centre focus point as the only active one)?

 

[pxl8]

It's hard to say for certain what's happened. What AF mode were you using? One Shot? Servo? Was the IS turned on? The second shot it looks like the AF locked onto the the car before the guy walked into the centre. The first shot looks a little soft overall, maybe a 100% crop around the guy's head would help to see how OOF it is. Front or back focus issues are normally less severe than this and if it was the lens I think your friend would have noticed the problem anyway.

The only other straw I could clutch at is to ask if you weren't accidentally adjusting the focus ring whilst holding the lens?

 

[CT]

In normal use for shots like this you want to be selecting just one focus point so you decide where the camera focuses on. However, I can't really see any evidence here on which to doubt the AF system of your lens at all.

Looking at the first pic, you've chosen a totally inadequate spot on which to attempt to focus. I've marked the centre AF point pretty accurately:-

That sleeve doesn't contain anywhere near enough contrast or pattern detail to focus on other than the contrast between the shadow and none shadow areas. Were it not for the shadows there, the AF system would hunt back and forth all day trying to focus. I'd have gone for his face, his hat or even his T shirt as being better AF points. The other thing is this shot will be softened just by being reduced to web viewing size. A bit of sharpening makes the shot far more presentable?

In the second shot, the AF point could be better placed, it looks as though you could well have got the gap between his chin and his hand - in other words the system will struggle if it has two planes in the AF area - his chin and the foliage visible through the gap.

Just lifting the AF point to cover his face more would have been better, but again, just normal sharpening makes the shot a lot better.

 

[Tyke Tiler]

Hi and thanks for all your input, earlier this morning, a canon service center in helsinki diagnosed my friends lens with a back focussing problem and now it is going to be adjusted.

I pretty much tried all of the different AF functions, servo and one shot, tracking the subject along etc and every one came out the same.

All of my other lens are smack on target and never struggle with the AF unless your trying to focus on something silly like a piece of white toilet paper ontop of snow (hmmm, what an anology, scuse the bad double pun).

Considering it was the first L series I have ever tried, I expected so much and was quite dissapointed, I'm very glad there is a fault to be fair.

Anyways, thanks very much for the help. This forum and the people within are absolutely great, smashing super!

muchas gracias

Tomas