You don't need as much sharpness as photography as the maximum definition is like 1/4 of your photos (max 1920x1080 vs 4000+), and some (short) out of focus moments are acceptable in video while it's not in photo.
If you shoot wide open (F2/F2.8) you may have difficulties to stay focused cause the DOP will be very short, and the screen is not always big enough to see if your focus is perfect.
But you'll get such beautiful videos, you'll forget about it.
If you shoot at like f8 or more, focusing becomes easier, and you could just rely on the distance printed on the lens instead of watching closely the screen.
A fully manual lens is perfect as the k-5 will adjust the exposure while you open/close aperture which brings a feature you won't have with auto lenses (you can only adjust aperture before shooting, or let it auto which is bad).
Manual focus is really easier with large adjustment range vs autofocus lenses where focusing ring is not really intended to be used
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I have a ricoh M50F2 and a smc pentax A50F2, the 2nd one is supposed to be much better than the 1st, but believe it or not, i prefer the ricoh, cause the focus ring has a different range which makes it easier/quicker to focus, and i run both in M mode so i keep a hand on the aperture while shoot. for info ricoh=20€ smc=50€ ( and they are killer primes for video)
I like the way the k-5 behaves when you turn the aperture ring while shooting, it adjusts but is overexposed during a short moment which looks like 35mm movie film camera stops (white flash), very nice...
The only lack of k-5 to me is you can't adjust iso/shutter in video for now ( and a 48/60 fps 720p for slow mo experimentations)