Originally posted by Javaslinger There was a SMC 100-300 FA there for $89... Is this a good deal?
If they're ok for my camera, I guess they'll just fit??? How do I know if they are a 'K' mount? I didn't see a designation for this even on the Pentax branded lens....
The FA 100-300 was available new from B&H for $99 a few years ago, so I never have been able to convince myself that I should pay a lot for a used one.
A lens with the Pentax brand on it will almost certainly be a K-mount lens. A small number will say "645" or "6x7" on them, and they are for the 645 and 6x7 cameras. These lenses will also appear to be kind of big, kind of slow, kind of long and really expensive. (If they're cheap, buy them and resell on eBay.) Takumar, Super-Takumar and Super-Multi-coated Takumar are screw-mount lenses. Takumar Bayonet and Takumar-A are K-mount. Again, watch out for "6x7" in the name. Refer to this site's reviews for quality and price information.
Lenses from other big camera manufacturers won't fit, so rule out Minoltas, Canons, Nikons, etc. Don't rely on lens caps to identify brands. That leaves you with third-party lenses to identify, such as Sigma, Tamron, Tokina, Vivitar, Sears or others. The screw mount is easy to recognize, and you can carry an M42 adapter or screw-on cap with you to double-check the threads. Sometimes these mounts are marked U for Universal. That takes care of a host of brands you might not have heard of (yet), and leaves you with mystery bayonet lenses.
Some third-party lens makers put a big PK, PK-R, PK/A or similar on the lens or mount, but lack of identifiers is not unusual. Avoid lenses with small rectangular notches in the bayonet tabs. These are for Minolta or Konica. Remember the Ricoh pin but it's easy to fix, so don't let it keep you from a good deal.
There are a few lenses that use a system of interchangeable mounts. The common ones are Vivitar (TX and T4), Soligor (T4), and Tamron (Adaptall or Adapt-a-matic). If you find the correct K-mount adapter, you can buy lenses with the "wrong" mounts and swap them. The TX and T4 lenses have a rectangular protrusion below the aperture ring with O and L on it and a small tab sticking out of that. Also the aperture ring has aperture stops listed twice in both directions and different colors, like 16-11-8-5.6-4-2.8-2.8-4-5.6-8-11-16. Once you've seen one, they stick out a mile. Adaptall adapters are much harder to identify at a great distance.
These days, everyone knows to look stuff up on eBay before putting the price sticker on, but they don't always do that as carefully as they should. That's about all I can think of. Good luck shopping.