field curvature problem?

[Limosa]

Problem with new Canon 24-70/2.8 and wonder whether its my fault or not, or even whether it sounds familiar to anyone here on this forum. Situation: at the wide end, people half-length near the side of the image, be it right or left. Using the center point for AF, then recompose with the area supposedly in focus (the eyes) somewhere near the edge of the image, say about 3/4 or 4/5.

The more carefully I recompose, in exactly the way I'm used to with other lenses (i.e. thinking of a pivot in the imaging sensor and turning the cam around it)  the more consistently it backfocusses - I get very sharp ears. It appears this lens has a somwhat flat field of focus, not the curved field I'm used to.

So it's like focus, recompose, and "back off about 1 to 3 inches" Is this normal? I do have the manual-focussing screen installed, but in typical event situations you want the speed of the AF, also because of lack of light and this screen being dimmer etc etc.

By the way, having the focussed spot in the center of the image, AF works perfectly. I don't use the outer focussing spots, too much hunting and lost time. Canon 5d.

[Jonathan Wienke]

Lenses are supposed to have a flat field of focus; there is nothing wrong with your lens. Your technique is the problem; it is guaranteed to cause back-focus in the situations you describe. Read Why Focus-Recompose Sucks for a detailed explanation.

[Limosa]

Thanks Jonathan, I was a bit afraid that this flat plane of focus might be the intention of the lens designers (as it seems perfectly flat - or should I say _even though_ it seems perfectly flat). So now it's official, plus it appears I've always been using pieces of crap! Well, well. Even resulting in in-focus images, consistently, sheesh.

So, what's the advantage of this flat plane of focus? I mean, other than photographing paintings or walls?

In the meantime I'll have to do some serious practicing with the mf focussing screen in low-light situations.

again thanks for the help - Rik

[DiaAzul]

So, what's the advantage of this flat plane of focus? I mean, other than photographing paintings or walls?

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More a feature of camera design. Because the recording plane (film or sensor is flat) then so is the plane of focus. If we had concave sensors we would have a concave field of focus. Changing to a concave field of focus against a flat sensor would be a nightmare for abberations and ensuring consistency of operation.

This specific feature of camera operation is exploited by tilt-shift lenses and large format cameras to shift the plane of focus by shifting the orientation of the recording medium relative to the lens position. What is one persons problem becomes someone else's opportunity.

[Jonathan Wienke]

So, what's the advantage of this flat plane of focus? I mean, other than photographing paintings or walls?

When using a flat recording medium, a flat field of focus lens gives the fewest aberrations. As an extreme example, the Lensbaby has a cone-shaped, rather than flat field, which gives it its characteristic (and rather extreme) pattern of distortions. While the Lensbaby's "look" is useful for niche applications, it is not suitable for general photography, where sharpness and freedom from lens aberrations are considered positive attributes.