Searching for the sweet spot....
At full telephoto
I've started to have a play with boring test-charts and my first results relate specifically to the tele end of the lens. Starting with distortion - well, there really isn't enough for me to be bothered about, but that is what I would expect from most tele lenses (you wouldn't normally expect more than a little pincushion effect). Lens aberrations are well controlled at this longer length too... no signs of nasty fringing.
Another finding at this length is that performance in the centre is decent all the way from wide open but I'd probably avoid going beyond f8 - and diffraction definitely softens things up by f11. After having been used to other compacts that seem to have been optimised for use wide open, I'm surprised to see the lens on this little Fuji behave a little more like you would expect from an interchangeable model. That is, performance improves by stopping down to around f5.6-8, especially around the borders and corners. Having said that, the results at f2.8 are good - it's just that the edges get a little better by the time you get to f5.6. In reality, this should pose no problem in real life - shots taken wide open at telephoto are probably being done that way because you're trying to get some background blur and emphasise the subject...in which case having corners that aren't pin-sharp really shouldn't matter.
In the middle of the range
The picture is similar (but different!) in the middle of the range. At around 50mm equiv, the lens is again decent even wide open but does even better when stopped down to f4 - including in the centre. F4- f5.6 definitely seems to be the sweet-spot at this length, and things start to go downhill from f8. Once again, f11 is best avoided. And also once again, no nasty fringing and not enough distortion to worry about (nor should there be at this kind of length). I'm starting to think f5.6 may be the sweet spot for the lens, but need to check out wide angle too before making that conclusion.
By the way, I haven't mentioned corner-shading (vignetting) at either of these focal lengths so far - even wide open it's just not a big enough problem to be worried about.
At wide angle
It's hardly a surprise that things get a little more complicated at wide angle. I took a few shots of a test chart with the X10 and compared them using a Panasonic GF1 micro four thirds camera with 14mm prime Lumix lens...hardly fair, you might think, but the results are more even than you might expect. First thing with the Fuji is that even at wide angle and wide open aperture, vignetting is really not too noticeable. There is some barrel distortion - more than I'd like, to be honest, but less than you get with the micro four thirds lens. What you do get more of with the Fuji is hints of purple fringing - especially around the borders. It's not horrible and not unexpected (especially from Fuji), but Panasonic do a better job of managing this out of their images (most likely through camera firmware rather than lens design).
The X10 lens is decently sharp but not outstanding at wide angle. Those familiar with the Panasonic pancake lens will know that in the centre it is superbly sharp even wide open - and it shows: centre performance is crisper than the X10 can ever quite manage, even after stopping down. However, the Panasonic lens varies a lot between the centre and borders - whereas the lens on the X10 is more consistent - borders from the Fuji wide open are crisper than those with the Lumix lens a f4 - so perhaps a score-draw overall?
Sticking just with the Fuji lens for now, even stopping down to f2.8 makes a difference from f2, and there is a slight improvement in sharpness from wide open. Carry on towards f4 and sharpness doesn't change much, but some of the lens aberrations and fringing get a little better. And with my samples, f4 is as good as things get - f5.6 offers similar performance, diffraction is setting in by f8 (still usable though) and f11 is best avoided. However, this really shouldn't be an issue when you bear in mind the amount of depth of field you get with the 2/3" sensor. Even using the X10 at f4 and shooting a subject 6 feet away will mean everything is in focus from 3 feet to infinity - it's more like f8 on a micro four thirds camera or f11 on an APSC SLR.
So what's the bottom line from all of this?
If all the above is tedious and dull, it eventually leads to the following conclusions:
(1) the X10 lens performs decently wide-open but does improve if you are able to stop it down a little
(2) border performance is generally good, and the lens is quite consistent across the frame
(3) there is some barrel distortion at wide angle, which would be noticeable in shots requiring straight lines. This would need to be corrected in software, and is par for the course. Distortion is otherwise negligible
(4) corner shading (vignetting) is well controlled even wide open at wide angle
(5) lens aberrations and purple fringing are apparent at wide angle, especially wide open and around the borders - there may be occasions when post-processing would help here
(6) in general, the sweet-spot of the lens is around f4-f5.6, so that is probably where you should be unless you need to open the aperture for a specific reason
(7) f11 is best avoided at all focal lengths because diffraction has a significant impact
Go to testing the lens page 3