Fastish AF lens for portrait

I've got an F35-105mm lens for longer portrait shots, and it's great BUT - A) it's not fast, so difficult to use in low light or to isolate subject and B) the AF is noisy and could be a little snappier.

It looks like there aren't any cheap options (with AF) so I think I am looking at the DA50-123/f2.8, the old non-OS Sigma 50-150/f2.8, or perhaps even the DA70/f2.4 which is cheaper but obviously less versatile. (FA77 is sadly a bit out of my price league!)

Cut to the chase, which one is best? Leaning towards the DA50-135/f2.8, but have slight concerns about the AF...

Just sold my 50-135. It's a great lens and the SDM never gave any trouble.
Not the fastest focuser though but great for static portraits

The 50-135 makes for a very nice portrait lens. I do like to use a tripod though at shallow depth of field to ensure the plane of focus stays just so and I do find the size of the lens makes it rather cumbersome on a tripod.

I've been looking at similar lenses for the same purpose, but I think I wouldn't use the upper end of that range so much.

I'm thinking about buying the new Sigma C 17-70mm f2.8-4

It seems to get good reviews, is quick and silent focusing, and is a pretty fast lens.

I've been after a 50-135, a very nice lens by almost all accounts. I have been bothered by the (poor) reputation of its SDM -- not all the tales can be urban myth -- but somewhat reassured by discovering that its firmware can be altered in a rather nail-biting procedure to use the screw drive. I have a DA70 and it is a joy of a lens and perfect for most portraiture. In practice, it may not prove that much less versatile, either, if there's space to take a step forward or backward.

DA* 50-135mm f2.8

Best portraits I got from Pentax gear were taken with the DA70Ltd and FA77Ltd. Both exceptional and hard to choose between them, although the DA arguably makes more sense on an APSC sensor.

Why a zoom?

Pentax F 55mm f1.7

Cheers... It's the range at the longer end I'm more interested in, to complement the Tammy 17-50. Looks like the DA50-135 probably wins.

Well in that case I'd personally also seriously consider the Sigma 50-150. Superb lens which I think performs at least as well if not better than the Pentax, has more reach at the long end, hasn't had the same issues with SDM, and may be cheaper on the odd occasion you can find one.

IMO the 50-135mm has better IQ and is slightly sharper than the 50-150mm (and I have had multiple copies of each). The Sigma will be cheaper (if you are going for a used lens) and has (obviously) longer reach.

SDM is a concern on the 50-135mm, I recently had to have one done (although I bought it as faulty) and with the PU discount it cost approx £140 to fix. Saying that it is now much much better than any previous copy I have had and actually focuses as fast (if not faster) than the Sigma.

The sigma is also getting rare to find. The 50-135 will complement your tamron well, only trouble is once you get one you will start wanting the 16-50 ��

Hi, why not think about the Sigma 85mm f1.4. Perfect for portraits, I find the Pentax 55mm f1.4 a bit short. Regards Ian

Another prime to consider would be the Samyang 85mm f1.4. Cheaper than the Sigma or Pentax versions, but MF only of course. I've got one, and am very impressed with it, it's very nerly as sharp at f1.4 as at any other aperture, by f2 it is as aharp as at f8.

With only the slightest of vested interests - it doesn't get much better than the FA* 85mm f1.4 - I just happen to have one for sale at the moment https://www.pentaxuser.com/forum/topic/fs--smc-pentax-fa--85mm-f1-4-42503