First question if you want a Pentax lens is: older manual focus lens or newer(but still not new) autofocus lens?
There are no current 28mm prime lens offered for sale by Pentax. The only new for sale 28mm Pentax mount autofocus lens I am aware of that covers the full 35mm frame for film is from Sigma.
If you're happy with a manual lens, I like the Pentax M series 28mm F3.5. Not quite as common as the M 28mm F2.8 but both can be found for something in the order of US60-80 dollars. The M 28mm F2.8 came in two versions. The later version is the same optically as the A series 28mm F2.8. I have the later version of the M 28 F2.8 so my comments probably apply to the A 28mm F2.8 as well. Of the two, I prefer the M28/3.5 colours (really good blues in particular) and it is sharper in my opinion. But I have some nice sunsets and autumn scenes with the M 28 F2.8 as well so i can't really complain too much about this lens either. The A version will allow you to use aperture priority on the SF10 while you will need to use manual exposure metering on the M series. The older K series F3.5 is highly regarded and like the M series, you will need to use manual exposure metering. Any of the 28mm F2.0 (K, M or A series) lenses are scarce and expensive, so you can probably disregard those from a practical perspective.
While you don't stated you don't want a zoom, I have been using my father's M series 28-50mm zoom which is in pristine condition. Stunning colour saturation and contrast (must have lashings of SMC coating applied??), and at F5.6-8 darn close in sharpness as the equivalent M primes. More pronounced barrel distortion at the 28mm end than the 28mm primes is the trade-off. Not overly common but can occasionally be found for sale here and on Ebay. I know of a couple for sale in a camera shop here in Adelaide.
I'm not familiar with the SF10's autofocus compatibilities and whether the F or FA series of lenses would work well. Of the autofocus lenses I believe the FA28 F2.8 is quite good but there's not a lot of them about and are quite pricey. The F 28mm F2.8 I believe is optically similar to the M (version 2) and A series F2.8 lenses. I umm'd and err'd about buying a FA28mm F2.8 (or an FA 20mm or FA 24mm) but ended up with a secondhand FA 20-35 mm F4 zoom which I rather like (and resolved the which focal length question - have them all), and has become my current walkabout lens. Great Pentax colours and sharpness at F5.6 to F8 is really quite good.
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