As for the choice of Pentax lens, please help me!

Let’s talk about the da50 135 first: I think this lens has basically no shortcomings. No matter which lens evaluation agency, the resolution of the evaluation results is very high. It is basically the best zoom lens currently produced by Pentax; built-in SDM motor, focusing quickly and accurately; well-made, bright appearance, coupled with the internal zoom, it is almost perfect. The only nitpick might be that the SDM motor is prone to breakage. Also, you said you like candid photography. Maybe the 135mm focal length is not long enough for candid photography. You can look at the da60 250, which is also a great lens. The focal length of 50 135 and the constant aperture of 2.8 can basically be defined as a portrait lens, the best choice for portrait zoom.

Let’s talk about da16 50: This star has mixed reviews and is quite controversial. Many people think that it is sorry for the star (I think so too). The purple fringing is serious at large apertures, and the out-of-focus linearity is serious. At 16mm, the distortion control is not even as good as that of the da16 45, and the SDM motor of this lens does not seem to be very reliable. Compared with the spectrum and imaging of the Kotou and da16 45, I think the price increase is far less obvious. The advantage, I think, is that the colors are more intense. In short, I think da16 50 is not worth it.

Finally, da12 24. The DA12-24 mm F4 ED AL and DA12-24 MM F4 you mentioned are the same lens. The previous ED (low dispersion lens) and AL (aspherical lens) just describe this lens in more detail. If the da16 50 is still controversial, I think the da12 24 is completely worthless. The price is twice that of the Sigma 10 20, and the imaging quality is almost equal to or even slightly lower than that of the Sigma 10 20. I don’t know if you choose wide angle to take landscapes or portraits? If you are shooting landscapes, I recommend you take a look at Voigtlander’s 12mm prime lens. It is a very good wide-angle prime lens, but it’s a pity that it has to be manual.

If you can’t stand the manual one, I recommend Sigma 10 20/f3.5 (second generation) for zoom. For fixed focus, the original da14 da15 is a good choice.