Small haul of Pentax gear, what have I got here?
You've got an old film body, two fairly mediocre older zoom lenses, and a pretty good 70-210mm f/4-5.6. The two telephoto zooms have quite a bit of overlap. You've got one of several different 50mm prime lenses offered by Pentax over the years. You've also got an old 1.7X AF adapter that is worth a bit on the used market. It wouldn't be very useful with a modern Pentax digital body or with your three 'slow' (narrow maximum aperture) zoom lenses, but it is a coveted item by those who use early Pentax screw-AF cameras with old manual prime lenses or fast (lenses with a maximum aperture of f/2.8 or wider) zooms.
Is that enough to choose a Pentax digital body over other systems for the sole reason of being able to use those old lenses? Probably not.
That's not to say Pentax wouldn't be a good choice for some people. But if Pentax is a good choice for you, it isn't because you've got a headstart with those old zoom lenses. The same would be true for someone who has a few old consumer grade zoom lenses in Nikon or Canon or Minolta/Sony mount from 30 years ago. If Pentax, or any other brand for that matter, is the right choice for someone with a collection of older zoom lenses it is because of the merits of the newer camera bodies and newer zoom lenses.
As the additional information added to the question since this answer was originally written indicates, you've also got something that has a lot of personal sentimental value to you. We'll get back to that a bit later in the answer. We first need to address this from a point of view of someone else that might read this in the future who has a similar collection of older camera gear and does not have any sentimental attachment to that gear.
Prime lenses (lenses with a single focal length that don't zoom) from that era are, for the most part, very close in optical quality to more recent consumer grade prime lenses offered by most camera manufacturers. Prime lens optical design has been very mature for several decades. Because they can often be had very cheaply, older manual focus prime lenses can be some of the best values around for lenses if you don't need AF lenses.
The same is not true of most cheap zoom lenses from several decades ago compared to the current crop of kit zoom lenses from most camera makers. Today's 'kit' zoom lenses are better than their counterparts from 30 years ago. The increased expectations from using high megapixel digital cameras and 'pixel peeping' the results at 100% on large monitors have forced lens makers to improve the quality of their zoom lenses. There are a few dogs still around. (EF 75-300mm f/4-5.6, anyone? But Canon does also offer the newer EF-S 55-250mm f/4-5.6 that is a much better lens for not much more money.) But for the most part today's 'kit' zooms and other consumer level zooms are much better than those of the past.
From a standpoint based solely on the photographic merits of the gear listed in the question, my advice would be to leverage the value of the 1.7X AF adapter and apply it to the cost of a digital body regardless of whether you stay with Pentax or choose to go with another digital system. You can then look at the rest of what you have as either: 1) Additional sources of revenue on the used market or 2) Additional time to build a Pentax digital system from the ground up. If you wind up buying a digital Pentax body and stick with learning how to use it properly you'll eventually want to upgrade at least some of your existing lenses.
Keep in mind that other than the new Pentax K-1, all Pentax DSLRs are APS-C sensor format cameras with a crop factor of 1.5X. This means that the field of view when using a Pentax APS-C camera will be the same as using a lens on a 35mm film camera (or the FF K-1) with 1.5X the focal length of the lens you are using. Your widest lens at 28mm will give you a field of view that looks like what you are used to at 42mm on your film SLR, so you'll probably want to also buy an 18-55mm or similar kit lens to give you the same wide angle capability with a digital body that you have now with your SFXn film body.
Since this answer was written quite a bit has been added to the question that places it in a different light that as originally asked. The two most significant revelations are:
- The gear holds significant sentimental value to the person who asked the question that far exceeds the market value of the items.
- The person who asked the question indicates they would only use any DSLR they might purchase occasionally and aren't necessarily interested in spending much money on photography or looking at another system than Pentax.
With those two caveats, as long as one is willing to work within their limitations (manual focus for the 50mm, image quality limitations of older zoom lenses, etc.), the lenses listed will be usable on a Pentax digital camera body. The 1.7X AF Adapter will have very limited value due to the narrow maximum apertures of the three zoom lenses listed. Some things to keep in mind:
APS-C digital cameras tend to have smaller, dimmer viewfinders than their older film counterparts. They also lack the focusing aids that most film SLRs have. This makes manually focusing a lot more difficult than is the case when using older film SLRs. Some of the APS-C Pentax models have better, brighter viewfinders than others. Be prepared to be frustrated regarding the difficulty of manually focusing without resorting to magnified Live View, particularly if you wind up with one of the early entry level models with a dimmer viewfinder. I would strongly suggest getting one with a pentaprism rather than one of the few with a dimmer pentamirror. Even the models with a pentaprism, though, lack the manual focusing aids that most film SLRs have.
Manually focusing with the 50mm lens will be difficult enough. Adding the 1.7X Adapter will reduce the maximum aperture f-number of the lens by an additional 1.5 stops. That means the viewfinder will be even dimmer. On the flip side, if you focus it close enough manually, the 1.7X AF convertor will finish the job using the camera's AF system.
Your zoom lenses with the 1.7X AF adapter will become 48-136mm f/5.8-7.7, 119-357mm f/6.7-9.5, and 170-510mm f/8.2-10.5 respectively. This is in addition to the 1.5X crop factor when using an APS-C body. Given that you'll have all of these focal lengths below 300mm covered without the 1.7X converter, you probably wouldn't consider using it with anything other than the 100-300mm zoom (and, as mentioned above, possibly the 50mm prime as an 85mm in the f/2.8 to f/3.3 range depending on your exact 50mm model).
None of your lenses will AF on their own with the 1.7X AF adapter attached. Although the camera can use the screw drive AF motor to alter the focus distance using the AF adapter, the total range of the AF adapter is limited. You'll probably have to manually focus the lenses to near the correct position before letting the AF take over. That's also assuming the camera you buy will AF at all when using a lens/converter combo with a maximum aperture of over f/8. Most digital cameras won't. Most of the few that do are full frame cameras that have a wider baseline for the PDAF system. Even the ones that will AF at f/8 or narrower will be less accurate and slower than when they have a faster lens mounted. So while you'll gain AF with the manual 50mm lens, you'll most likely lose it with the AF telephoto lenses when using the 1.7X AF converter.
With the possible exception of your 50mm lens, all of the existing lenses listed in the question are K-AF mount and will be capable of stop-down metering, but older K mount lenses you might later consider will not. From Wikipedia's Pentax K-mount entry.
All digital K-mount Pentax SLR bodies lack the ability to read the
position of the aperture simulator. This means that lenses that lack
the lens information contacts introduced with the KA-mount (Pentax K-
and M-series lenses as well as some third-party products) do not
support open-aperture metering on these bodies. Instead, stop-down
metering must be carried out by pushing the “green button” on the
camera before taking a shot. This variation of the mount is commonly
referred to as the “crippled“ K-mount.
I think you'll find over time that the most usable lens of the ones you now have will be the 70-210mm f/4-5.6. In daylight it's not a bad lens at all. But the narrow maximum aperture limits it's use with a teleconverter or in low light action situations. The 50mm manual lens will give you very good optical quality (pretty much all of Pentax's various 50mm do that) if you can master focusing it with a digital DSLR and its limited manual focusing aids. On the other hand, 50mm AF lenses are dirt cheap, even when new. The 28-80mm and the Tamron 100-300mm are mediocre zoom lenses that may or may not meet your expectations and requirements.
You'll probably find the need for a wider lens to use with an APS-C camera so my advice would be to try and find a body that includes the kit lens it was probably originally sold with. That would give you an 18-55mm, a fast 50mm, and a good 70-210mm. That's a pretty decent starter kit with the right body. The K-7 or K-5 seem to be at the sweetspot regarding price/performance ratio on the used market right now. I'd probably stay away from anything introduced before 2009 unless someone practically gives it to you. The other, more capable cameras, are a lot better for not a lot of money.
Here's an excellent run-down of the evolution of the Pentax K-mount through its various iterations. Anyone considering using older K-mount lenses on a Pentax DSLR would do well to read it.