I own a K-70 as an upgrade from a prior K-50. In terms of AF, there are only a few select areas of improvement in focus, otherwise as explained above it's pretty close to the K-50 and so likely to the K-S2.
- Liveview AF has improved, not sure if it's the SAFOX X or if it's the "much overhyped" Hybrid AF. I find that I'm using it a lot more than I ever had previously. CDAF isn't the only function working as it's not hunting like Liveview did with the K-50.
- Pentax FAQ does show Liveview has the Hybrid AF (Type Hybrid AF *1 = TTL Phase-matching AF + Contrast-detecting AF)
- AF-C is possible in video now with my 18-135 DC lens (there's a longer list of lenses
PENTAX K-70 | FAQ | Support | RICOH IMAGING.
List as of now, I recommend checking the FAQ periodically as new lenses are released.
*1 Compatible lens
smc PENTAX-DA 18-135mmF3.5-5.6ED AL[IF] DC WR
HD PENTAX-DA 55-300mmF4.5-6.3ED PLM WR RE
HD PENTAX-DA 16-85mmF3.5-5.6ED DC WR
smc PENTAX-DA L 18-50mm F4-5.6 DC WR RE
HD PENTAX-DA 18-50mmF4-5.6 DC WR RE
HD PENTAX-DA 20-40mmF2.8-4ED Limited DC WR
HD PENTAX-D FA 28-105mmF3.5-5.6ED DC WR
HD PENTAX-D FA* 70-200mmF2.8ED DC AW
HD PENTAX-D FA 150-450mmF4.5-5.6ED DC AW
HD PENTAX-DA 560mmF5.6ED AW
- AF-C and Electronic AE works in video with my 55-300mm PLM lens.
Use of the AF with the Viewfinder appears to be identical, which is PDAF with 11 points, nine of which are cross-type. Lame.. as this seems to be 10 year old AF tech.
There are a number of significant upgrades in terms of ISO limits with very low noise due to the "accelerator" chip that goes with the Prime MII (same exact CPU as your K-S2 and the prior K-S1, the accelerator is the only new function). More pixels resolution and the Pixel Shift feature. But that really is only significant over the older K-30 and K-50 cameras. If you have a K-S2, either wait another generation or as recommended by the other posters go for a K3 II. I'd do that over a K3 as the KAF4 mount is supported on the K3 II with a firmware upgrade. Regardless if the hardware can support it or not, Pentax doesn't appear to have any intention to add it to the K3 regardless of survey attempts to call it to their attention.