There is a difference in characteristics between different lenses like these. I know for sure I wasn't very fond of the SMC Pentax-M 50mm f1.4 which gave very high contrast (presumably for reportage) at the expense of pictorial gradation. I much preferred the balance of properties of the f1.7 version.

I know you have asked about different lenses, but I just mention the above as an example. I have tried both the lenses you actually mention and I liked them both, so it may well be that the design objectives were similar in this case.

All you can do is try them and see which you prefer.

I understand that some of these lenses can be slightly radioactive, and this causes the yellowish effect.

No lens with this sort of zoom range is going to be as good as a more modest specification zoom or a prime. However, the lens is not at all bad. If you are only going to be making 6x4 prints it will be more than satisfactory - sharp and with good colour reproduction. Drawing is likely to suffer somewhat, so straight lines may not always reproduce as straight lines.

If you were going to make 20" wide exhibition prints you might choose a different lens, but this only means that different lenses are designed for different purposes.

Hopefully you will be pleased with your pictures - please let us know!

The worst scenario is that you buy the cheaper lens, you're not happy and then end up having to buy the Pentax lens anyway. We got wise to this after a while, and made a decision to stick with Pentax. It's cheaper in the end, and much more satisfactory.

I agree the SMC Pentax 24mm f2.8 is a bit pricey just now, but they can be found in very nice condition from time to time. I've just picked up a mint one for £125, more than I wanted to pay but it is in exceptional condition. (The asking price was £150)

You might find the SMC Pentax-A 24-50mm f4 for less than that, but not much less, and that is a fine lens as well.

It's worth watching eBay very carefully, eventually the high prices may fall a little as the mood switches to some other desireable lens!

There is indeed distortion with the SMC Pentax-M 20mm f4, but it is terrifically compact and convenient, and with landscape photography the distortion doesn't really matter too much. I wouldn't use it for architectural record photography though....

I would certainly stick with Pentax lenses.

The SMC Pentax 24mm f2.8 is superb and highly sought after.

The SMC Pentax-M 24-35mm f3.5 is also an excellent lens, really the first of the wide zooms to be viable in terms of quality.

The SMC Pentax-A 24-50mm is as good, with a slightly wider zoom range.

The more recent 24-90mm autofocus lens is superb quality but has a variable aperture. It also takes 67mm filters, whereas the older manual focus zooms take a more modest 58mm.

Finally, if you like ultra-wide photography how about the autofocus 20-35mm? I don't have one of these, but it's an interesting lens with a good reputation.

The carl Zeiss lenses that you are likely to find in M42 mount will almost certainly be East European in origin, and IMHO they are no great shakes either mechanically or optically.

West German Carl Zeiss lenses are a different proposition and you will usually find them on the Hasselblad!

There are real dangers in using non-Pentax lenses and that's why any damage caused by them is excluded from guarantees.

Personally, there are so many good pentax lenses around that I would avoid any others. Apart from the Ricoh problem there is also a potential tolerance problem. Lenses that fit too tightly on the mount could cause excessive wear, and inappropriate materials could do the same.

In any event there are very few independent lenses out there that really shine.

You do indeed need to activate the appropriate custom function, and the manual is delightfully obscure as it describes how things work...

In summary, you can use Aperture Priority and the camera will change the shutter speed as the light varies, but the lens will always be at open aperture. Or, you can use any aperture by utilising Manual Exposure and operating the stop-down lever to stop the lens down.

Some of us think it's superbly simple to do this and no more inconvenient than any manual exposure system. Others (including me) think it's all a bit of a bother and prefer to use "A" lenses and later, which work with full functionality (within the limitations of the lens of course!)

There is a thread all about this somewhere....

You can also crop slides if you want to. The pentax MZ5 and MZ3 have Panoramic options, where the film gate is cropped to a panoramic format. Ideal for slides.

Alternatively, cropping can be achieved with a variety of thin metal pre-made masks, or even using Kitchen Foil.