Positive differences with the Sigma 17-70 2.8-4.5

I dont have the DA 16-85, I but have both DA 17-70 f4 and DA 18-135. Use them on a 16 MP K30.

I think the DA 17-70 and DA 16-85 are very close in sharpness and IQ: same size and weight, close looking optical formulas and both sharp from corner to corner at all apertures and focal lengths, same ratings in most reviews.

IMO, the DA 18-135 is a travel lens, optimized for compactness and focal range, whereas the 17-70 and 16-85 give priority to IQ over size and weight: the 18-135 is really smaller and lighter than the 17-70 or the 16-85, and, after a few hours, it makes quite a difference when hiking, traveling by plane or sightseeing in a foreign country.

Both the DA 17-70 and DA 18-135 have very good contrast and nice Pentax colours and a good handling of flare.

The DA 18-135 is sharp in the center at all focal lengths, very sharp in the center from 18 to 70.

It is sharp across the frame when stopped down between 20 and 70, it has soft corners between 70 and 135, the longer the weaker, even stopped down. But it is very convenient to have this longer reach available at a twist of the hand, which neither DA 16-85 nor DA 17-70 can offer.

At F8-11 between 20 and 70, the 17-70 and 18-135 output are very close. The 17-70 is already sharp from corner to corner wide open at f4 at all focal lengths, whereas the 18-135 needs to be stopped 1 or 2 stops to get equivalent IQ.

The DA 17-70 has less distortion and corner falloff, and the DA 18-135 has less CA, but these aberrations are easily corrected either by enabling the in-camera correction when shooting jpeg, or in pp when shooting raw.

I use the 18-135 for travel and hiking, most often I dont take any other lens, so I dont bother about changing lenses and feel protected by the WR.

When size and weight is not a problem, for instance at home or traveling by car, I use the DA 17-70 for general purpose high IQ shooting, with useful zooming range. I then sometimes carry my old DA 50-200 for occasional longer reach: acceptable IQ and very light and small lens.

When IQ is my priority, I shoot with primes (DA 21 limited, DA 35 f2.4, FA 50 f1.8, DA 70 limited): IMO, these lens are more resistant to flare and much sharper than any zoom, and the two limited deliver unrivaled contrast and colour transmission. But I must admit I am mostly a zoom shooter, as for me convenience and versatility is more important than ultimate IQ (the best lens is the one mounted on your camera, not the one in your bag or in a drawer....).