Check out the in the bit you quoted from me, Don... I knew you weren't really moaning (and I know you've praised the lens elsewhere). But I do think you're setting the bar rather high.

Even in this specific case that we're talking about, of bright, overexposed sky with a lot of white in it, behind underexposed branches and twigs?

With the lens wide open?

These conditions are the Achilles heel of practically every lens I've used since switching to digital (never saw purple fringing with film) - I'd be interested in some unedited samples taken with lenses that don't give rise to this effect.

I'm pretty sure I'm not.

For there to be no fringing or other CA at all would be unbelievable. And it is there, quite clearly green along one edge of many of the twigs and branches, purple along the other. But not much.

Your examples won't show up the barrel distortion particularly, since you have very few long straight lines near, and parallel to, the edges of the frame, which is where they're most obvious. There's a picture in the photozone.de review that makes it very obvious indeed. But of course, unless you're out photographing architecture all the time, it often won't be noticeable, and for the occasional shot, correction on the computer will be perfectly adequate.

I think the bottom line is, considering the zoom range, it's probably very good indeed, and shades of things to come as lens technology moves forward.

Don, if you think that's 'lots' then you should donate some of your clearly colossal lens budget to a more needy cause - namely me!!

That's not lots. Not at the extreme wide end of a monster zoom like that - it's a tiny, tiny amount.

Anyway, looking closely, it appears that it's not purple fringing. There are purple fringes, but on the other side of the dark bit wherever they appear, there's a green bit. So I'm guessing lateral CA rather than normal PF, which is purple all the way round.

But either way, it's extremely minor, in my view - especially since the lens was wide open for that shot.

To moan thus about a Pentax lens would incur George's ire, for pixel peeping, so be glad it's a mere Tamron you're dissing there

PS... thanks for the enlargement, RG.

I'm only just starting to get familiar with these figures, but assuming the figures for the Tamron 18-250 are measured in the same way (about 4% at 18mm, at PhotoZone.de), I see what you mean !!!

How good is it at the extremes, John?

Thing is, I wouldn't buy a 12-24 because I wanted zero distortion at 18mm. I'd buy it because I wanted an extreme wide angle - so it would be the 12-16 range that I'd use most.

The 12-24, 16-50, and 18-250 are all lenses I'm considering at the moment, all for different reasons, but the one thing they have in common is that there's little point having any of them if they're not very good at the extremes (of either aperture, or zoom range).

One thing that strikes me about RG's first one is that the purple fringing that you might expect to get in the area above the roof-line on the left is fairly inoffensive - visible but not awful, even though the lens is wide open.

Hard to tell without an enlarged section (any chance of a full-size crop, RG, of the bit above the roof line where the branches in shadow have the overexposed cloud as background?), but if the PF is consistently as absent as this in places where you'd expect to get it, then it's certainly a point in the lens' favour.

Compared with what?

I really wouldn't worry about all this.

A lot of people (not here so much) talk a lot of rubbish about "sharp". They show you an 800 pixel photo that's been reduced down from the original for a web page and would have you believe you can look at it and judge how sharply the lens can reproduce detail.

Well you can't. If the lack of sharpness is less than a screen pixel, you can't see it on a computer screen.

Most lack of sharpness on the sort of pictures you and I see every day (on the web, or on smallish prints) is to do with camera shake, motion blur, lack of depth of field, or simply not focusing properly. To guarantee to not get any of these can be quite difficult.

People also tend to confuse lack of contrast with lack of sharpness. It's not surprising, because black against white will look sharper than grey against a similar grey. But this confusion also leads to more rubbish being talked.

If you're relatively new to all this, concentrate on taking good pictures, not on whether at some fraction of a micro-pixel there's a lack of sharpness.

If you're making large prints and examining them from a distance of three inches, then sure, the lens makes a difference to the achievable sharpness. But then eliminating the factors I listed above becomes even more important.

I suggest you take some pictures, and experiment with them in Photoshop Elements - or even Picasa, which is free. You'll soon find what you like, and what you don't.

I think it's entirely a personal preference thing.

But which way you go will have consequences.

An example of this is that it isn't generally considered to be a good idea to apply sharpening to a picture more than once, at the end of the editing process.

Now, for unsharp mask, for instance, the bigger the display size, the higher the USM settings need to be, and even then it's quite subjective. So how you sharpen will depend a lot on the target size, and the look you want to achieve.

If you let the camera do it, you are relying on whatever its assumptions are about about the target display size and this may not be the best for what you want.

So if you're likely to want to do much post processing on the computer, it makes sense to have the camera do as little as possible.

If, on the other hand, you want to use the results straight out of the camera, with very little computer work if any at all, there's no reason not to set the camera to whatever you like the look of.

When I use RAW+, I quite often have the JPEG sharpness up one notch, or sometimes even two. Then I get the best of both worlds - a crisper JPEG and a RAW file that I can attend to properly should the shot be any good.

56.5 x 47, including the bit at the bottom that says "Pentax".