You have another PM

I've recently bought a Sigma 30mm f/1.4 and in reading up on it beforehand, it appears that these lenses are prone to front/back focus issues. Fortunately mine doesn't seem to be affected, but I'm a bit confused as to why any lens should be.

From what I understand about the AF system, it adjusts the focus to make the image at the AF focus sensors in focus.

This being the case, I can't quite get my head around why, for some lenses, the AF sensors register correct focus but the image projected on the sensor is consistently slightly front or back focussed. Does anybody know why this happens?

I'd second this, I've tried a range of lenses in the couple of years I've used a DSLR and have found that some are much harder to get good results with than others.

One I have found particularly challenging is the Jupiter 9, of which my copy is a pain to focus. When I get it right, it is capable of producing very sharp images, but I get more misses than hits.

The only 'bad' lenses I've come across have been inherently poor designs (an early Sigma 80-200 zoom lens is probably the worst I have owned), I don't think I've ever come across a bad copy of a well regarded lens.

Whether anyone has ever had a really bad copy of a Pentax Limited lens is an interesting question, I'm not sure I've ever heard/read about it.

I'd second this - I recently got one and find it makes an excellent indoor portrait lens (shooting with my 2 year old daughter, anyway).

The alternative I'd suggest isn't a prime, but I find the Sigma 24-70 EX f/2.8 works really well for shooting portraits of my kids as it is just about wide enough indoors, has AF (for action shots), goes to 70mm which is long enough not to be in their 'space' and has excellent image quality.

The downside is that it's quite big (doesn't really bother me) and the out of focus rendering isn't quite as good as a prime, but it is pretty good for a zoom.

Your PK/A mount may not be working properly - some of them appear to suffer from an issue where threadlock has seeped through from a screw near the electrical contacts on the mount face. This threadlock isn't easily visible but effectively insulates part of the metal mount from one of the contacts on the camera which causes spurious behaviour with the aperture readings.

If you clean the area of the metal Adaptall mount flange with a solvent based cleaner (such as nail varnish remover) about 15mm each side of the electrical contacts, it may help. I have 'fixed' one of my PK/A mounts with a similar problem this way. Some people advocate cleaning the mount face near the electrical contacts with fine emery paper, but I wouldn't recommend this because it damages the surface of the mount and also creates dust that could get into the lens or camera body.

I don't think so, certainly not an a crop sensor DSLR anyway - I checked the Meyer above by measuring how far the rear of the Exakta mount protruded from the flange and it is similar to the protrusion of the rear element of my Chinon 55mm f/1.4 at infinity.

You're right about the Zeiss glass - it might be worth a look, as the Exakta mount versions seem cheaper than M42. I don't know whether it would be possible to adjust the focus enough on Zeiss lenses, or how you would do it. I've taken apart an M42 Tessar before and that had a Helix that could be started in a range of positions, each one leaving the optical block at a different distance from the mount flange, so it may be possible to accommodate the difference that way.

Alternatively I think that the focus calibration is done by loosening a ring that can be unscrewed using a lens spanner, but I was so glad to get the helix back in the correct orientation, I didn't touch this!

M50/1.4 arrived today and is in excellent shape - Looking forward to trying it out. Thanks for a smooth and pleasant transaction.

Ah, this means I've been doing it wrong. Is there any disadvantage to mounting the adapter on the camera first and just screwing the lens in to it? The only one I can think of is additional wear to the electrical contacts on the body.

Yes, you can get such an adapter and I believe that they are still in production. It is basically a machined insert for the bayonet mount on your camera which is slotted in and then allows the M42 lens to be mounted to the camera. You can then use it in either Av or M mode. I find metering with the green button in M mode produces the most consistent results for me.

There are a range of types and I would strongly advise against anything except the original Pentax one, as others are not made to such good tolerances and there is a risk that they may get stuck on your camera.

The original Pentax models can be found in the classified section here occasionally or on eBay for £20-30.

Hope you have fun with the 1.8/55 - I have one and it is a very nice lens.

I've got one of those too, badged Carl Zeiss Jena. I haven't got around to taking mine apart yet, but I believe it is optically identical to the Pentacon 50 1.8 which is itself a rebadged Meyer Oreston so it's probably worth a go with the grinder. I'd be interested to hear how you get on. I bought mine thinking it was a rebadged CZJ Pancolar and couldn't understand why it was so cheap.

My next project is a Komine 90mm 1:1 macro lens in C/Y mount. I've had one previously in M42 mount and was pleased with it and recently came across a really good deal for one in C/Y mount. I thought it would be worth a go since the register distance is similar to the M42/PK mount, but the bayonet is integral to the rear part of the lens body, so it will involve some metalwork and possibly a donor lens for an M42 mount.