Sharpness problem in high key studio 17-85mm IS lens

[dales_daisy]

<p>I'm using a 50D with the Canon 17-85mm IS lens in a high-key studio and I can't work out why some shots are so much sharper than others. I've gone back and checked focusing points, measured DOF, all is as it should be. I'm shooting family groups and small kids running around and I need some short focal lengths. Is this a typical problem with this lens or do I have a fault? Is the lens struggling with so much white background maybe? Is this lens usually soft in the middle?<br>

Examples:<br>

125th at f8 with 28mm focal length, about 8ft from the subject: Sharp as anything.<br>

125th at f8 with 35mm focal length, also 8ft from the subject: Very, very soft.<br>

125th at f8 with 70mm focal length, about 9ft from the subject: Sharp.<br>

All seated subjects.<br>

Any advice or thoughts would be welcome.</p>

[robert lee]

<p>Focus manually in LiveView at the 35mm focal length. Is it as sharp now as the other focal lengths?</p>

<p>Shoot again at the 35mm focal length but with the standard AF. Is it soft now?</p>

[steve_dunn2]

<p>What happens if you use the same lens with the same (or very similar) settings in a different environment? What I'm wondering, since you specifically mentioned high-key studio, is whether there's a flare issue that only happens around 35mm. (You're using the lens hood, right? As it's designed for a 17mm lens, it's of limited effectiveness at longer focal lengths, but it's still better than nothing, and as a general principle, you should always use a lens hood unless there's a specific reason not to.)</p>

 

<p>That's just a guess, and certainly only one of a number of possible explanations; the first response is also a good one to try, to identify whether it's an AF issue. Another focus-related test would be to arrange a scene with things at a variety of distances, have the camera focus on something in the middle, and then see if <em>anything</em> in the scene is sharp, to determine whether there may be a front- or back-focus issue at play.</p>

 

<p>If nothing you see in responses to your question seems to do the trick, post two samples from the same image: one showing the entire frame (but, of course, downsized to a reasonable size for posting), and one 100% crop showing an area that should be sharp but isn't.</p>

[dales_daisy]

<p>I will test the AF over next weekend I think. Yes I am wondering whether it can be confused by all the glare. I can't run another test in the studio until I get another hire slot which may be a few weeks, but I'll set up similar contrast scenarios using sky (if the weather permits) and see what happens. I wasn't using the lens hood, I've never been taught that for studio but I'll have a go.<br>

The fact that I haven't been inundated with responses saying "this happens all the time" tells me something. Thanks.</p>

[peter_rowe]

<p>The 17-85mm IS may have its problems around the edges but it's centre sharpness is not one of them, mine is razor sharp at centre at all focal lengths. So this should not the problem unless the lens is faulty. I think if you post an example photo showing the problem that has not been "focus and recomposed" so we can all see what AF points are used then you will get much more valuable responses. The key to focus is a contrast edge in an orientation the sensor is sensitive too. Only the centre AF point can see both horizontal and vertical contrast edges. Perhaps the high-key is washing out the contrast needed to focus. You should be manually selecting the centre focus point or the camera can jump to any AF point which may not be what you expect. The actual AF points are also large than the viewfinder indicates so anything closer that is just outside the orientation of the AF point can cause the camera to lock AF there. But an example shot will answer a lot of these questions.</p>