This was from the 18-270mm this morning at 270mm and f/9.0. It's not going to be the finest lens at 270mm, but still useable I think. At this distance it didn't seem to front focus.

Unless you have good light it means using a tripod if you want low ISO (this was 1/4s ISO 80). A tripod rather negates a lot of the convenience of a 'travel zoom' - you might as well carry a better lens!

That's a big factor with some of my lenses. On the screw driven 18-250 I think it makes a difference of as much as 2/3rd depth of field (wide open). It's something I commented on years ago. The SDM driven 18-270mm is a big improvement and there's hardly any difference in acquiring focus from near or infinity.

I know what I'm seeing with the refocussing with changes in aperture. I'm not imagining it and it surprised me. It looks like an attempt to compensate for focus shift, but it's acting in the wrong direction!

Yes, it's an improvement on my old 18-250mm which feels like it's about to die. It may not be the finest quality glass but competition judges don't seem to notice! Pity it's not weather sealed though.

I had vowed not to buy any more Pentax gear but it I couldn't resist.

Yes, I'm testing reasonably 'scientifically'.

I first noticed it at a distance of about 0.6m with a focal length of 270mm. Using a tripod I focussed at f/6.3 and set the camera to manual focus. I fired a shot at f/6.3.

I then set to f/16. The camera wouldn't fire because it considered it to be out of focus. When I enabled auto focus the camera refocussed slightly nearer.

I then returned to f/6.3. The camera then refocussed slightly futher back again. I am making no changes to the focus or camera position.

I have repeated this several times - always the same.

Like you I didn't realise the firmware took account of the aperture when focussing, but it does. At least with the 18-270mm lens at 270mm.

It gets weirder. I've just tried at 270mm and found that the camera focuses slightly differently as you stop down. This suggests the firmware attempts to compensate for the focus shift.

I never realised the camera took account of the aperture when focusing. It's cleverer than I thought. Only problem is, it compensates in the wrong direction! At f/16.0 it sets the focus slightly closer than at f/6.3 (wide open), yet what we want is for the focus to be set slightly further back. Ah well!

It's not good for your health to investigate these things too deeply.

I don't suppose newer bodies would be different. I suspect they still acquire focus wide open and won't perform any tweaks for different focal lengths, apertures and focus distances. Fortunately in reality I rarely do much that's focus critical anyway, as I'm sure you'd agree!

Nice try with the KP!

There's no point in making micro AF adjustments. The problem is focus shift. I notice it with the lower grade Pentax zoom lenses. Is there no way of focusing stopped down using contrast detection?

Yes, of course I used a tripod and 2 sec delay and natural light. I also tried acquiring focus from infinity and near, although that's irrelevant to the focus shift with aperture issue. In any case, I didn't re-acquire focus bewteen the f/4.5 and f/8.0 shots - the difference is entirely due to aperture related focus shift.

Here's a higher res image on flickr. I'll leave it up until tomorrow morning. The focus shift should be obvious (further back when wide open).

flickr link

I've just dusted off my tatty old adjustment chart and took these two shots. Focal length 58mm. Distance about 60cm. I'm not sure how well the image will look at forum resolution.

It's clear that f/8.0 is focussed in front of f/4.5, even though I didn't refocus betweeen shots. At this distance it's f/8.0 that looks about right and f/4.5 is back focussed (by about 12mm?). The error will probably change with focus distance but I think f/4.5 will always be behind f/8.0.

As you say, this is the sort of focus shift you might expect with a large aperture spherical lens. However, I've noticed this characteristic before with some of the Pentax zoom lenses. It means it's pretty pointless attempting to program micro focus adjustments on these lenses!

I was hoping there might be a way of focussing with live view contrast detection, but this seems to focus wide open too. Or is there an option somewhere to focus stopped down?

I recently picked up a 18-270mm to update my ancient 18-250mm 'travel zoom' for my K-5 IIs. I realise it's not going to be up to the quality of a set of primes, but sometimes convenience is more important to me.

One thing I notice, as I did with the 18-250mm, is how much the focus varies with aperture. The effect varies with focal length and probably focus distance too.

For instance, at 53mm and at a distance of 1.4m focus is about right at f/4.5 but I get front focus at f/8.0. This is presumably due to lens aberration and because phase detect focussing is always performed wide open, regardless of aperture setting.

However, I get the same with live view using contrast detect. I had hoped contrast detect focussing would be perfomed stopped down, but it appears not to be.

Is there a menu option somewhere to enable contrast detect focussing to be perfomed stopped down?

Otherwise I guess it's a case of learning the lens and where best to focus when composing.

My reply wasn't just intended for JW but also those who replied.

There seems to be a very defeatist attitude towards asserting your consumer rights in the UK. I've only encountered this sort of problem a few times but always found, as long as you're prepared to stand your ground, the seller will be prepared to remedy.

I doubt it would ever have to go as far as filing a claim with the Small Claims Court but, even if it did, I think the effort is still going to be less than taking your camera apart, trying to fix it and hoping it will still work when you put it back together!