Originally posted by troika BigMacCam, can I ask a stupid question? I'm not an expert photographer, so I've been afraid to ask this question for while. When you say "...at the wide end, wide open, only the centre sharpness will be good..." do you mean center of the frame or do you mean focal point? In other words, if I use non-central focal point, like in the examples I uploaded, that would be the effective "center", right?
Sorry, I warned you it would be a stupid question.
Not a stupid question at all. Never be afraid to ask - you might get a small number of people who respond negatively (pay them no heed
), but on PentaxForums there are usually many who will try to be as helpful as possible. I've asked many stupid questions, and got some *great* advice back. I'm no expert either, but my knowledge and experience is growing very quickly - partly as a result of asking questions
To
your question (which is actually a very sensible and valid one)...
The 20-40 (at least, my copy) does appear to suffer from a degree of field curvature - so, if you focus on a point in the centre of the frame, elements of the image at the same distance but in the corners can be less than sharp (a good example here would be a flat test chart, or a brick wall) - but, if you focus on an area and re-compose so that the area in question is in one of the borders, it seems pretty sharp - even wide open, actually. Personally, I would still hesitate to use it wide open (though it is certainly usable that way) as sharpness generally isn't at it's best here, but stopping down from f/2.8 to f/4 (or smaller) yields a noticeable improvement.
EDIT: I know this isn't the example you need, but I just had a quick look through my library to show an example shot on the 20-40 at a wide aperture... see below. The first shot is reduced, obviously, while the second is a 100% crop of one area of the frame (the part I was focusing on). This was taken at 23mm, f/3.5. Detail is good at the point of focus, also note that the bokeh is OK too, if not stunning... Bear in mind that there is a drop in quality of the image when uploading to this site, and the native capture looks a little better.
EDIT #2: I should add, the chicken was delicious and the scallops wrapped in bacon were absolutely divine! The mushrooms and peppers were good too... the butternut squash, however, had to be microwaved and finished off on the BBQ later... Out of picture, the cold beer was just fine!