All the K-mount Pentax 50mm lenses are at least very good (I've no experience with M42 ones but it probably applies to them as well) however IMHO the SMC-A 50mm F1.7 is perhaps the best cost/quality/functionality compromise of the lot if you're a K10D user.

This is my current lens list:

AF Primes
Pentax FA* 85mm F1.4 (KAF)
Pentax FA 50mm F1.7 (KAF)
Cosina 100mm F3.5 Macro (KAF)

AF Zooms
Pentax SMC-FA 28-200 F3.5-5.6 (KAF)
Pentax SMC-DA 18-55mm (KAF)
Pentax SMC-DA 16-45mm F4 (KAF)
Pentax SMC-DA 10-17mm (KAF)
Pentax SMC-FA 28-80mm F3.5-5.6 (KAF)
Sigma 10-20mm F4-5.6 (KAF)
Sigma 24-135mm F2.8-4.5 (KAF)
Sigma 70-300 F4.5-5.6 APO Macro Super II (KAF)
Tokina ATX-Pro 20-35mm F2.8 (KAF)
Tokina ATX-Pro 80-200 F2.8

MF Zooms
Tokina RMC 25-50mm F4 (K)

MF Primes
Pentax SMC-M 50mm F1.7 (x2) (K)
Pentax SMC-A 50mm F1.7 (KA)
Pentax SMC-M 50mm F1.4 (K)
Vivitar 55mm F2.8 Macro (K)
Tokina 500mm F8 Mirror (K)
Mir-1B 37mm F2.8 (M42)
Hamimex 35mm F2.8 (M42)
Super Ozeck 28mm F2.8 (KA)
Soligor 135mm F2 (x2) (M42)
Prinz Galaxy 135mm F3.5 (T)
Helios 44-2 55mm F2 (M42)
CZJ Tessar 50mm F2.8 (M42)
Sigma 24mm F2.8 (KA)
Hanimex 28mm F2.8 (K)
Vivitar 28mm F2 (K)
Tokina RMC 28mm F2.8 (KA)
Meyer Optik Trioplan 100mm F2.8 (M42)
Meyer Optik Orestegor 300mm F4 (M42)
Meyer Optik Oreston 50mm F1.8 (M42)
Meyer Optik Lydith 30mm F2.8 (M42)
Jupiter 200mm F4 (M42)
Porst 55mm F1.2 (K)

A lot of the lenses on that list don't see much use and the kit that I usually carry is:

Pentax 10-17 Fisheye
Sigma 10-20 F4.5-5.6
Pentax 16-45 F4
Tokina 80-200 F8 or Sigma 70-300 APO (depending how light I want my kit)
Cosina 100mm F3.5 Macro
Pentax FA 50mm F1.7

Along with a 2x TC I find that kit gives me an appropriate lens in the vast majority of situations. Quite often I'll go out with a kit made up of just a few manual focus primes lenses, or sometimes just a single prime lens, to force me to be a little more creative.

"Imps Motorcycle Display"



K10D, DA16-45, handheld

I've never used one but it's supposed to be soft wide open, especially towards the long end, but ok when stopped down plus decent for CA, flare & distortions. It's probably not significantly different in performance to the Pentax 50-200.

For bird photography you might have been better considering a Sigma 70-300 APO Macro (even a used one) or the Tamron 70-300 as they are sharper, longer and faster (but bulkier) the the 50-200 or 55-200.

It's a very decent lens given how cheap it is (mine was £37 used).

It is - I was playing about with an old M42 manual focus lens at the time and that, combined with having to lie on the ground to get the shot, meant I was a little rushed and slightly missed the focus point. The cat rushed out to attack the squirrel and I only had time to get two shots off before it moved - in both shots the focus point is slightly forward of the cat.

I spent a very pleasant few hours in lovely weather at the Edinburgh botanic gardens today - here are a few of the results (the rest are at http://www.pbase.com/niblue/botanics_291107)



K10D, Meyer Optik Gorlitz Oreston 50mm F1.8



K10D, Meyer Optik Gorlitz Trioplan 100mm F2.8 (the cat was stalking a squirrel and came very close to catching it)



K10D, Cosina 100mm F3.5 macro



K10D, Cosina 100mm F3.5 macro



K10D, Sigma 24mm F2.8

One of the difficulties with the fisheye is making sure you fill the frame with something more interesting than just a lot of bland sky or grass/water. The example above has a lot of interest in the sky, which helps a lot.

I finally got round to trying de-fishing software (trial version of PT-lens):




The results aren't bad at all but notice the "purple fringing" CA in the trees on the left (pt lens can fix lateral CA's but not that kind) which is much more noticeable than in the original shot:

Purchasing both was deliberate. The Sigma 10-20 and Pentax 10-17 produce very different results although their having similar focal lengths does seem to confuse some folks.

It's a fisheye all the way but more obvious at the wide end. If you shoot with a central horizon it can look un-fisheye like however anything in the foreground will still show distortion. I have the Sigma 10-20 as well and it gives very different results to the 10-17 although both are excellent.

This is a shot taken with the 10-17 but not showing much in the way of distortion: