Back & Front Focusing

[Spatch]

I think I may have an issue with my two main lenses (17-40L and 70-200 F2.8L). The 17-40 seems to be front focusing and the 70-200 seems to be rear focusing. I have done a quick search and I cannot find the info I am after on the forum.

With the above in mind, I assume the focusing ability must be due to the lens (if it were the camera why would it focus infront with one and behind with the other?). I seem to remember reading that it is possible to recalibrate lenses - is this what is required to correct any focusing problems?

I first really noticed it on safari using the 70-200 with my 1.4TC. I always seems to have to adjust the focus slightly - I was using wide apertures to keep the shutter speed high, therefore, the focus issue was not masked by DOF. I have done a test here at home using a ruler etc and the 70-200 (at about 100mm, F2.8 ) seems to back focus by about 3cm (I was about 3m from the subject). The 17-40 (at 17mm, F4) focused about 2cm in front (I was approx 0.5m from the subject).

Should/can the lenses be sent away, and if anyone has had this done was it expensive?

Cheers.

[ed2day]

I assume you know that if the lenses are still under warranty (1 yr.) Canon will fix this problem(if they see it) free of charge.

[Spatch]

Unfortunately, they are both older than a year.

[lostdoggy]

Sorry to hear that!!!

[Spatch]

Yes, but am I on the right track of having to get the lenses looked at? Can they be recalibrated to correct for front and rear focusing? Or should I be looking at the camera?

[lostdoggy]

Just a thought. Take some pics from tripod and post them.

[Spatch]

Yes, might be a good idea, I'll have a go tomorrow.

[Spatch]

I am not sure there is much of a probelm afterall. Having taken more photos I think I can see what is happening. I generally use the centre square only for focusing. On many of the safri photos I took, and also the previous test shots, the subject focus point did not entirely fill the centre square. What I think was happening was that it was averaging the focus beween the subject and the background/forground. When I filled the focusing square with the subject focus point it was almost spot on.

Thats a relief.

Cheers,

[CyberPet]

A friend of mine have a nice 20-35/2.8 L lens and it was focusing perfectly with his 10D, but then he sold the body and bought an used 1D instead. Now the lens is front focusing (or was it back - never mind) with his new body. I think it means that the lens worked fine with the 1.6x upsample, but now that he has another body it gives other results, since the 1D uses 1.3x upsample. He's thinking of selling the lens (to me) so he won't calibrate it, but until he's found a lens he likes to replace it with (he's all into primes now) he has to manually focus and gets great results doing so.