-The new camera you're talking about is K70, not K700. It's a very capable camera for the money, and I've never heard of anyone having consistent autofocus issues that couldn't be resolved. It is possible you'll have aperture block issues with that camera (others might have more info on the likelihood of that happening).
None of the rest of the cameras I'll mention commonly suffer from that problem.
-K1 is significantly more expensive (if you get one for under US$1000 used right now, you've done very well, and you would pay US$1800 for a new K1ii, which replaced K1 but is largely the same camera), but it's very nice (it's full frame as you noted, as well as various other advanced features). You may well not need that much camera at this stage, although I can't say that for sure.
It too will autofocus capably, although still not as fast as more modern cameras from some competitors or the (US$2000) K3iii, Pentax's newest camera and it's best autofocus performer ever (a significant upgrade for the brand, which has long lagged somewhat behind the competition if AF speed)
-Another possibility would be to get a used high end crop camera from several years back, like maybe the K3ii (probably comprable price to a new K70). It will absolutely be more capable at AF than your K100D Super, and probably be more durable than a new K70 (definitely the aperture block) since it was top of the line when released. Remember, your K100D Super is 15 years old at this point, any newer Pentax will have improved autofocus over that model.
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Here's a comparison of some of these models. There are links there to the staff and user reviews for each camera as well, which should have plenty of info.
-All that said, it's possible that your current camera can focus okay with different techniques or settings, or possibly if there is an issue with the camera or lens that we can help diagnose and maybe correct. As pepperberry farm mentioned, posting some shots with exif data would be the next step to getting to the bottom of that.
A few more notes:
-Newer cameras will allow zooming in for manual focusing with the back screen in live view (your K50 might, I'm not sure), and have focus peaking which is very nice in most situations. They also can use the AF system to give focus confirmation (a little square lights up where the picture is in focus) through the viewfinder. So a split prism isn't necessary to use MF lenses, although you still may end up wanting to go that way (I have) if you continue using MF lenses.
Live view (not using the viewfinder) is always the most accurate way to manual focus, even with a split prism installed.
-Some modern lenses do have electronic aperture, but not all. There are still lenses from Pentax with the aperture ring.
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I hope any of that might be helpful to you, there was a lot to respond to in your post and I didn't get to all of it.
Last edited by wadge22; 01-19-2022 at 03:25 AM.