Originally posted by kostarae keyser,
I understand this and plan to purchase a prime for the K-x. I am not sure which one to get yet. Have any suggestions?
Get the prime if you want, but for your intended subjects I recommend a better option is to get a flash that bounces to the ceiling. If you want shots of moving kids, you need faster shutter speeds to stop the action AND maintain the DOF. There's no point in getting a shallow DOF "fast" prime, soft wide open (unless you spend huge $$$), only to get a limited DOF in all your shots because you don't have enough light. And older primes like the FA50 are soft wide open, so you wind up with every shot looking creatively the same: very shallow DOF and slightly soft. It's a lot of $$ spent to get a standard looking, if technically excellent, snapshot.
For the K-x, check the WB as it is a bit on the blue side. And the EV is often a tad high (with flash too). It shows in all your samples. A prime will assist with colour (as will PP, but that's work), but it won't correct the camera pre-sets. I agree with the posters here that the prime vs. kit lens is an awkward test.
My suggestion for a prime (new) would be the DA35, largely because its a decent indoor length without going to the cost extreme of the FA31. It focuses tight and fast, which for tracking a growing kid is far more important a real world measure than other technical arguments. 50mm is too long for moving indoor work. The DA40 is another option, but I still find it too long indoors. Tracking kids you really want AF, especially to take advantage of the shot-to-shot blaze of the K-x (isn't that a reason why you bought it?), so older MF lenses again, aren't maximizing the camera and your subject matter.
A beter option may be a higher quality zoom, like the Tamron 17-50/2.8, or, for more $$$ the DA*16-50/2.8. Bigger and heavier than a prime, but far more versatile. Again, I am trying to match your intended subject matter in all conditions with the advice. A 2.8 zoom lens with a bounce flash can handle any indoor/outdoor situation for following a growing child, with excellent quality, and go wide enough to grab scenes with context, group scenes, etc. Primes are technically great, but have practical disadvantages. The zoom would also work better with video. It's a cost effective solution that should not be discounted if you want versatility.
I cannot speak for the focus issue. Not quite enough evidence to be definitive.
Good luck.