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I think the landscape shooters out there will be of more help, but from looking at the shots you sent on Tara, it would appear to me to definitely be a diffraction issue. At f22, there should definitely be some part of the shot in sharp focus but as you pointed out, from front to back the image is soft (not dramatically so, and could be improved with some sharpening in Photoshop). From looking at the shot too, it doesn’t appear to be softness from camera shake, so I’m sticking with Pete’s and my own guesses of diffraction.
The thumbnail is enclosed here: to keep things as realistic as possible, no sharpening was applied after resizing, but algorithm used was “bicubic sharpen”
For those of you that are interested in seeing the full res.. click here: http://www.thewonderoflight.com/misc/IMG_6796.jpg
I’ve done a bit of research on this specific lens (only a little, though). It seems that it suffers the perennial zoom lens problem i.e. trying to be too many things at once. I think you just need to bear in mind what you’ve discovered and open the aperture a bit more. This applies to most lenses, as Ciaran’s explanations show.
The diffraction effect is generally well known. Lenses typically are sharpest in the middle of their aperture range because of this effect.
However, the practical application gets a little confusing, especially as we are moving into the digital age and are starting to demand big prints.
For landscapes I would always have said, shoot at the widest aperture you can that gets everything sharp from front to back. f22 is not always desirable due to the diffraction effect. Now the problem. My depth of field scales may tell me that for a particular scene I only need f11 for front to back sharpness. But these depth of field scales are historically based on small print sizes and many experts on “luminous landscape” have demonstrated that they are no longer valid. The general concensus seems to be that infinity focusing is the way to go.
Andy
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