No, it means the apparent focal length increases, because the sensor is smaller than a piece of 35mm film. So, if the multiplier were 1.5x then a 50mm lens would appear to act as a 75mm one would on a 35mm camera.
A 100mm lens would be equivalent to a 150mm, and so on.

This means that wide angle lenses have to be very wide indeed for digital SLRs.

The aperture relates to the amount of light that the lens lets in, and a f2 lens remains f2 because the aperture is focal length divided by the diamter of the front element. So, in our 50mm example, the focal length of the lens is still 50mm, divided by the 25mm diameter of the front glass gives us f2. The lens only behaves as though the filed of view id a 75mm because the small sensor only uses part of the field of view of the lens.

I shudder as I write this, because it is straightforward but horrible to explain!

Likewise, any that are Pentax autofocus fit will be fine. Manual focus lenses will be iseable as well, but I have suggested the straightforward approach so that you will be sure of getting something suitable.

Although I prefer pentax lenses for all sorts of reasons, lenses from Sigma, Tamron, Vivitar and others will be compatible.

Every lens Pentax have made since the early 1960s will fit your camera, but with varying levels of compatibility and functionality.

However, if you stick with Pentax autofocus lenses of any vintage you should be fine.

Make sure that the batteries are the right way round (flat side down if I remember correctly, as you look into the battery chamber.) The LEDs should light up if you are set on auto or M settings, but won't on B or flash 100X settings. A slight pressure on the shutter release should be enough to activate the meter and light up the LEDs.

You really need an instruction book, and you can find the details on other threads in this forum.

Hope the problem is simple and doesn't need a repair.

I know what you mean, but to be honest if over the years I had bought what I wanted instead of trying cheaper options, finding they were under par, buying something else.......It could be less expensive in the long run to just let the credit card creak a little in the short term and make just one purchase that actually does what you want it to.

Just a thought!

I agree with Kim regarding the distortion of the 20mm f4 M lens, and this is why it is very useful to know what a lens has been designed for, and to think of what you want to use it for. The distortion does not upset me too much because I am shooting landscapes with the lens, and it is not much of a problem. If I was shooting architecture I would want something with more accurate drawing.

Having said that, a lot of modern lenses, particularly zooms, are very, very sharp but have very poor control of ditortion. It could well be that for general purpose lenses distortion is the one parameter allowed to drift a little in exchange for the sharpness that many people expect.

The quality of any picture is only as good as the weakest link, so I think a pentax lens will make a difference. It's a matter or sharpness, accuracy of drawing (distortion), gradation, lack of flare....and of course the construction quality that ensures accurate alignment and long operational life. I tried a Sigma lens at an exhibition and the focusing and aperture were loose and sloppy and the rep said "It's been used a lot", which meant to me that it was not up to hard use over a long period of time.

The Pentax lens is expensive, but I would buy it without hesitation if I could afford it, but I can't! So I have a second hand SMC Pentax-M 20mm f4 in superb condition that does the job just as well. Cost - probablt £125-£250 depending on what you can find.

I think it's the very highest quality and although I have not used the lenses you mention, I am aware of their design and I would expect the Pentax lens to have the edge.

I'm sure it is worth the price generally asked, that, is, perhaps up to £200 or so. But hopefully less! I have seen them around £150.00

It is a more modern design than the lenses you mention, and will benefit from that. It also takes 49mm filters, a usefully inexpensive size.

The SMC Pentax-M 20mm f4 is one of my favourite lenses, and I use it a lot.

Sharpness is exemplary and A3 or larger prints will not be a problem. The lens is compact, smooth as silk to operate, outstandingly sharp and with a clean, crisp image quality.

It is useable for architecture, but there may be a hint of barrel distortion that needs watching for critical reproduction of straight lines. This is, however, nowhere near as noticeable as on most modern wide zooms!

It's a fantastic lens!

The square Cokin-style filters have the drawbacks of being easily damaged and very susceptible to flare. Multi-coated glass filters are much better IMHO - I use SMC Pentax, B&W and Heliopan filters mainly. And a lens hood, which is cumbersome with square filter systems!

It would be nice (but expensive) if modern lenses used bayonet filter fittings like the old Rollei TLRs did. That is so much faster than screwing filters on and off lenses.

You could buy filters in the largest size you need (62mm) and some step-down rings to enable them to be used on the 58mm and 52mm fit lenses.