Originally posted by Hexism Are they capable of replacing 50mm primes?
No. 20-40 is f2.8 at 20mm going up to f4 at 40mm. 60-250 is constant f4. For subject isolation at around 40-60, you need a faster aperture. (At longer focal lengths, e.g. 100mm+, you don't need apertures as fast to get good subject isolation.)
And the various fast primes lenses in the 40-60mm region are great value. Even the cheap ones almost all good - eg. Pentax M50/A50 f1.7, DA50 f1.8, just to mention a few. The best of them (e.g. FA 43, DA*55) are amongst the best of all Pentax lenses.