Originally posted by xGene I always found the 15 "quite soft" at any aperture (on both the K7 and K2) in the corners and edges, and "softer than expected" in the center.
First, if you really find it softer than expected in the center, I'd be thinking about how you are testing with respect to subject size. Obviously, a shot of a person from 10 feet away won't show as much detail with the 15 as with a longer lens. That's why I decided to standardize on a a single subject at a single size, and portraits - while far from ideal - were the most practical choice for me. So the first I'd do is do that comparison, to see if your DA15 really is underperforming relative to other lenses you might have. For reference, here's a quick self-portrait I took the other day, showing the whole frame to give you an idea of the subject size I am using:
And here's a 100% crop from a conversion (using ACDSee Pro 3, which honors my +1 in-camera sharpness setting by applying a modest amount of sharpening, which it wouldn't have by default otherwise):
This should at least give you an idea whether we simply have different expectations, or whether yours is actually underperforming.
Quote: If you have a chance, please tell me what sort of subject you would use for this purpose. as you indicated, portrat would not give you a fair idea of edge sharpness.
Judging border/corner sharpness on a wide angle lens, especially at large apertures, is indeed tough, and I don't have a great method of evaluating it. That's pretty much in keeping with the low importance I place on wide open border sharpness anyhow. But when I've successfully found sharpness on borders when shooting wide open, it's generally been shooting scenes that are basically two dimensional (nothing *way* in front of or behind the focus point but have do some depth to them - like trees with many branches, or rough groupings of objects that are lined up more or less like a wall but not totally flat. The idea being that I want objects of (somewhat, but too extreme) varying distances from me along the borders, in hopes that *something* along that border will be in focus.
For instance, here's a shot of a cliff face. It's like a brick wall but with things sticking out in front of and behind the focus plane. This is shot wide open at f/4, resized to show you the whole scene:
Here's a 100% crop from the extreme border at 3:00:
It would have been sharper stopped down of course, but somehow, I just don't find much to complain about in that crop. It's certainly sharper looking to me than most of the samples I see posted to illustrate how soft the borders supposedly are. Yet if I scroll around and check out other spots along the border, most of them are *not* in focus; I kind of got lucky with this particular outcropping.
So if I really wanted to test border/corner sharpness, I'd be looking at shooting similar subjects - essentially flat, but with just enough variation in subject distance along the border to give me a good chance that something is in focus.