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I'm about to buy a Pentax K-x camera with a Pentax 18-55 lens for 50€.

I also found a few lenses, quite cheap, but I don't know if they are compatible. What should I look for to see if they are compatible?

The lenses:

  • Vivitar 70-150mm f/3.8 for 8€

  • Sigma 70-210mm for 10€

  • Sigma zoom 35-70mm f/2.8-4 and Sigma zoom 80-200 f/4.5 multi-coated for 30€

  • Makinon 200mm f/3.3 for 20€

Are they compatible with the K-x?

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    What mounts do those lenses have? Third party lenses are made with a variety of mounts. – Eric S Mar 28 at 21:22
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    Do not just go buying random lenses expecting them to be compatible. Lenses need to have the correct mount in order to be compatible with a given camera body. I don't know much about Pentax, but I see online that "the K-x offers full compatibility with a large variety of K mount lenses." So look for K-mount lenses. – osullic Mar 29 at 0:07
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    Also: film-era, variable aperture consumer zooms tend to yield very disappointing results on digital cameras. The makinon telephoto prime might be fun - or horrible, makinons tend to be all across the board :) – rackandboneman Mar 31 at 4:13
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The Pentax K-x has a K-mount and you need a lens with the same mount to be able to attach them together directly. Most third-party lenses are available in multiple mounts and so you have to check which version is being offered. Pentax has been making lenses for a very long time and so you should be able to find numerous K-mount lenses on the used market too.

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The first task you have to figure out lens compatibility is to find out what lens mount your camera body uses. Pentax 35mm film SLRs and dSLRs use the Pentax K mount.

The second task is to find out what lens mounts the lenses are using, and whether or not it's Pentax K. That may be harder than it sounds :). But all of the lenses on your list are from 3rd party lens manufacturers (i.e., not someone who makes the same brand of camera), so you have to be extra careful because 3rd party lens manufacturers like Sigma often make the same lens in multiple mounts. And a brand like Vivitar is often used to rebrand many different generic lens manufacturers. You'll probably have to google each lens model specifically, or look very closely at the sale listings to see if they're Pentax K. And you may also want to consult a visual lens mount guide like this or this.

The third and final task is to find out how much function the lens will give you. There are often reasons 3rd-party lenses can be found super-cheap on the used market, since OEM camera manufacturers have no vested interest in maintaining compatibility with 3rd-party lenses for functions like autofocus. And a mount like Pentax K or Nikon F, which have been around for decades often can encompass a myriad of quirks and incompatibilities as the mount and its electronic communication has been modified through the years. Googling "Pentax Lens compatibility" resulted in this chart, which charts out Pentax lens compatibility by specific Pentax bodies. And Ricoh themselves have a chart on the functional differences between versions of the K-mount.

It may come down to individually googling each lens with "K-x" to see if someone has mentioned the lens works or doesn't work (or how much), but it may also be you're just out of luck and will have to buy the lens to see if it works.

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