EF 28-135 IS USM focusing

[Persio]

Gentlemen,

I was not happy with the focus on my D30 with the 28-135 IS lens, so I ran the following simple test:

Fixed distance between camera and object, test pictures shot a different focal lenghts, distance from camera read from the EXIF data in the pictures.

These are the results I obtained:
28mm - 11.0m
35mm - 11.0m
50mm - 3.7m
75mm - 2.75m
100mm - 2.75m
135mm - 2.75m

The correct distance is really 2.75m.
Is this a problem of the lens or the camera?

[Gomez Photography]

Persio ,
I have this lens too, forgive me, but I 'm not sure what you mean,about the correct distance being 2.75m .Could you please elaborate on this further so dummies like me will learn something. I thought that if the image looks in focus in the viewfinder using auto or manual ,the image should be sharp if the subject is still or the shutter speed is enough to stop motion.
Thanks Michael

[gandini]

My interpretation of Persio's data is that he set a fixed target at 2.75m. Then made photos of it at various focal length setting with his zoom. The EXIF information in the picture tells you what distance the lens focussed at. He reports these and they differ from 2.75m. Especially the short focal lengths.

Very interesting. did the images all look equally "in focus" Persio?

cheers,

[Persio]

Michael,

I used a fixed subject (motionless) positioned at 2.75m from the camera. The test pictures were taken with auto-focus using the center focusing point only. The problem is: why can't the system (camera + lens) objectively focus the subject consistently at different focal lengths?
When I say "objectively" I mean the distance recorded in the EXIF data fields as opposed to someone's judgement of focus looking through the viewfinder. As a matter of fact, when I ran the test I could even see the difference in focusing by reading the distance on the lens barrel!
I am puzzled!

[Persio]

Philip,

It is difficult to judge objectively because if I blow up the 28mm test picture to appear as large as the 135mm picture, I introduce other variables and the image gets distorted.

When I look at the 28mm picture at 100% magnification I realize it is not as sharp as the 135mm picture.

[Gomez Photography]

I understand now. I know what you mean when it comes to unsharp images toward to outside when using center focus. If I shoot a subject using a wider setting ,I use the left or right side auto focus settings. Have you used a film camera to do the same test? I wonder what the result will be? This broblem should not show up as much with a 1:1 ratio or a 1:3 ratio like the 1D has.
It is good to have the Canon Digital Photography Forum and people like you around. I have learned so much about my camera and other software. Thanks Michael

[benamen]

Whatever!!!

[yikes]

What aperature were you using, at the wide end? If you were using something like f/11 or above, would the lens not focus @ 11m/infinity? on this lens?

Just a thought.

Nick

[KennyG]

You must realise that the distance information shown in the Exif is only stored there for reference and not used by the camera. It will be effected by the object's contrast and where the extents of the focus point are. The only device that interprets distance information is the new 580EX flash.

Too many things effect focus test results and without knowing exactly what the setup is any suggestions as to what may be the problem are pure guesswork. For example, at 28mm there is more chance of the focus point extending outside the target than it would at 135mm (assuming the camera and target remaining in the same place) with a risk of it picking up something behind or in front to focus on.

Not enough information and why not post a sample?