Within the limitations of the monitor image, it looks like a classic pattern of definition to me. The lens would appear to be sharp at all apertures, but it improves to a best of around f11 or f16 after which it drops in performance as diffraction effects become significant.

I would think that if you apply Unsharp Mask to the images then you will have sharp pictures.

The red square shows the position being focused on and it does flash, but once focused it disappears. The green hexagon and indeed the beep confirmation confirm focus is achieved. If the green hexagon continues flashing, then the systemm has not focused properly.

There has always been a limited amount of this. You will find Pentax lenses made to fit Alpa and Leica at least, admittedly mosdtly from a long time ago, but they do exist.

At one point it was mooted as a major line, but the idea never seemed to get off the ground.

This is a common misunderstanding. The red square only shows where the camera is focusing, not that it's in focus. Focus confirmation is indicated by the green hexagon at the bottom of the viewfinder. If the green hexagon is steady then focus has been found.

That's Quite Interesting - do you think people change systems because there isn't a new camera due with their existing system? What about all the lenses and accessories they already have?

The idea that we need to constantly upgrade and not get left behind seems to have grown upon us, and caused a bit of a problem for the camera makers as well. As soon as we stop automatically upgrading then they will become deperate to continue expanding...and where will they expand to?

Is the market approaching saturation? Will the supply problem with credit cause the bubble to burst? Is there any dramatic and essential improvement there to be had?

:

In general it seems to take up to 4 weeks, sometimes less, sometimes longer.

You need the genuine Pentax Adapter K, which sells for just under £20.00

There is an AP test comparing the Sigma to the Nikkor and although they conclude that the Sigma is "better" if you look at the detail it is more prone to flare and has lower contrast.

I suspect I would prefer the Nikkor to the Sigma, and that probably I would also prefer the Pentax for the same reasons.

We are now so used to lenses that automatically stop down and then reopen again when a picture is taken that preset lenses are long forgotten.

It means a lens where the diaphragm has to be opened and closed manually, but the preset ring means you can set an aperture and with one twist of the ring have open aperture for focusing and then stop down to a preset aperture to take the picture.

I used to have a preset Soligor 105mm f2.8 that was actually a brilliant lens.

New one on me too.