I'd be inclined to check out "The Lens Book" by Roger Hicks and Frances Shultz.

This will be an interesting and (fairly) painless introduction to lenses in general.

If there are larger, more widely spaced pixels there will be less light spillage from one to the other, less electronic "crosstalk" and less noise as they will each produce a larger signal. The signal-to-noise ratio will improve.

If you recall, when the 10MP Pentax K10D was introduced Pentax themselves recommended the 6MP DSLRs if low noise was your priority.

A lens projects an image onto the sensor/film and, uncorrected, this would be dish shaped, just as the lens elements are curved, so projecting onto a flat sirface the centre might be in focus and the edges not, or vice-versa. This is called curvature of field.

Macro lenses are designed to focus very close and to have no curvature of field. This is so we can copy flat subjects such as documents. The downside of macro lenses is the smaller maximum aperture.

Standard lenses (as 50mm lenses were once called) are a compromise and the faster they are the more manufacturers let the curvature corrections slip. There's no free lunch in lens design and fast lenses, it is reasoned, will be used for sports, reportage, stage, concerts, and other low light applications. Not so much for close up work. Hence in general f1.4 and f1.2 designs don't behave very well close up.

The classic "double-gauss" 50mm design is a 50mm f2, although only Nikon seemed to be able to market that as a quality option, and most manufacturers would "stretch" that design to f1.8 and then f1.7. It just sounds better on the spec sheet....

In theory, the Pentax 50mm f2 lenses would be best if you don't have a macro lens, but in practice the 50mm f1.7 is more reliable. It also has better resolution than the faster versions, but lower contrast.

So for the highest optical quality and the ability to abuse a 50mm lens go for the f1.7 and if low light is your thing go for the high contrast, grittier f1.4, being aware that the resolution is lower.

Hope that helps, if anything needs clarifying then just ask!

I wonder if there's a bit of a misunderstanding here. Unlike film, where bigger format=more detail, this isn't true of digital unless the pixel pitch remains the same and the bigger format has, therefore, more pixels.

A 14MP APS-C sensor will resolve the same as a 14MP FF sensor, providing the lens can also resolve an appropriate amount of detail. In this case we would however get an improvement in noise and possible less abberation due to the pixels being too close together or too small.

If the pixel pitch was kept the same however, then the FF frame sensor would have more pixels, just as you get more film grains from a larger format, and that's a different issue. Noise might be the same, but resolution would then increase.

The main difference will be that the f1.7 version has a flatter field, so will be better with close up devices such as bellows or extension tubes.

The optical formula is the same now as it was for the M lenses but, as pointed out, the A series or later give you better functionality with DSLR bodies.

The link from the homepage is being done very soon, thanks for the comments!

I've added a new Coppermine gallery to my website forum. You can get to this via my website www.johnrileyphotography.co.uk and by following the link to the forum and thence to the Gallery.

You can rate and comment on the images if you wish, in fact you are welcome to do so!

Sigma "reverse engineer" their lenses as they avoid paying Pentax for licensing, so existing Sigma lenses may show anomalies in their performance on new Pentax bodies. If it can be upgraded then Sigma may be able to re-chip your lens, but next time you get a new body there might be different problems.

I've settled long ago on just buying Pentax lenses, which is fairly well known around here!

It's very important to set the dioptre adjustment on the camera eye piece using the correct technique. Point the camera at a smooth surface or the sky, don'y focus, but adjust the slider until the etched lines in the viewfinder are sharpest.

Your eye is then at rest focusing on the camera screen and manual focusing will be easier and more accurate.

Hope that helps!