Front Focusing/Back Focusing Question

Is it normal for the focus point to change slightly on a zoom when zoom to the extremes?

I have a new 24-70mm F2.8L that I am very happy with. Just playing around with a focusing chart and I noticed this. When at 24mm, the lens seems to just barely back focus. Not much, just a tad. I would never return it for this. When I zoom in to 70mm, the lens seems to front focus slightly. Again, a very small amount. Neither is really seen in normal pictures. Just curious if this is normal in a zoom lens.

Parafocal. Not all zooms are parafocal. Means they focus at the same point in any part of the zooms range (mm wise.) You can focus at 24mm and zoom out to 70 and it will still be in focus. Given the time, I'd try to focus using the 70mm setting and then zoom back to the framing you want for each shot.

I would always re-focus when zooming in or out, particularly if going from wide to tele.

My thought on the subject is it's the AF software on the body and if you have a 1 series you can apply slight bias for your likings while prosumer bodies need to be sent in to canon for critical focus calibration.

Not recommending you call up canon yelling QC issues but instead you want your body/lens to be critically sharp and for the $$ spent that is what is expected.

I think the Canon L zooms are supposed to be parafocal but I always re-focus.

I didn't word the question right. When the camera focuses at 24mm, it seems to be right or slightly back focusing. When I zoom to 70mm and the camera refocuses, it seems to front focus.

It would seem that a zoom lens that front focused at 70mm would front focus at 24mm too. This doesn't seem to be the case. My Sigma did the same thing. The question is, does the focal point and range of acceptable focus move as the camera zooms in and out, refocusing at both ends?

20D_Newbie wrote in post #4743321
I didn't word the question right. When the camera focuses at 24mm, it seems to be right or slightly back focusing. When I zoom to 70mm and the camera refocuses, it seems to front focus.

It would seem that a zoom lens that front focused at 70mm would front focus at 24mm too. This doesn't seem to be the case. My Sigma did the same thing. The question is, does the focal point and range of acceptable focus move as the camera zooms in and out, refocusing at both ends?

I must not have phrased my response correctly. If you expect it to be anything other than what it is now, you'll have to involve canon for any body other than a 1d series.

I am just trying to understand if it is normal for the lens to be different at 24mm and 70mm. I am not necessarily trying to correct or fix anything. Just seeing if it is normal for the acceptable focus range to change when at 24mm or zoomed to 70mm.

20D_Newbie wrote in post #4743772
I am just trying to understand if it is normal for the lens to be different at 24mm and 70mm. I am not necessarily trying to correct or fix anything. Just seeing if it is normal for the acceptable focus range to change when at 24mm or zoomed to 70mm.

What do you mean by "acceptable focus range." The focus should be on target at both the near and far end of the zoom range, however, the depth of field (range of acceptable sharpness) will be different (more range at the wider focal length).

So, be careful with your terminology. It seems that you're asking if a lens can back focus at one end of the focal range and front focus at the other. To be honest, I don't know, but it could be possible, I guess.

Thanks Mark. You said it better than I did. "Range of acceptable sharpness" is what I meant to say. My question was "is it normal for a lens to front focus slightly at one end of the zoom and back focus slightly at the other end of the zoom range.

I also just bought a new 24-70 and have been wondering about exactly the same thing. What concerns me in particular with my copy is that the front-focus at 70mm is fairly severe. From 24 to 60 focus is quite accurate, but that last 10mm on the long end isn't so good.

Has left me a little disappointed as real world shots at 70mm look softer then I would like. I am in Germany (but sadly only speak English) and feel lucky that the store I bought it from has told me they will let me exchange it if I would like. Alternatively they will send it to Canon on my behalf and take care of the communication. Nice store for sure.

The thing is... it is so good from 24 to 60 that I am hesitant to do anything without thinking it over first. Would like to know if I am expecting too much of it at 70mm. Who knows, but it was a lot of money so I expected better of it. My 70-200 f/4 IS focuses tack sharp throughout it's range.

Here are examples from my new Canon 24-70mm at 70mm:

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

Gravy, you are seeing similiar results to me. That was my main question. Is it normal for the range of acceptable focus to move as you zoom in or out.

I would expect a zoom lens that is focusing properly to achieve accurate focus at all focal lengths.