Originally posted by christinelandon I'll second Erik on this point; I've shot family activities with both primes and zooms over the years, and find the quick reframing of a zoom to be a major benefit when things are happening all over! BTW, I find the DA* 50-135mm to have a very versatile short-to-medium tele range and remarkable IQ for a zoom. Fairly quick, too!
I'll third you both. Just sold my 31 and 77 for the same reason. The 31 was for me an weird focal length on my K10d and I couldn't really get into it, and the 77 proved to be too slow and too long for family pics that include a 2 year old.
For posed portraits, I think you can hardly do better than the 77, but for chasing a 2 year old or toddler, the 77 is slow, and I think slower than the 31 by far. The 70, which I sold to get the 77 was also too slow, and I had about as tough a time keeping up with it as I did with the 77. I will second the fact that the 70 is a better "all around" lens than the 77 if you will be using for other than just portrait.
That said though, I don't know if I would consider the 50 to 135 either. You will probably only have about a 20mm useful focal range for chasing a toddler indoors, unless you have a 10,000 sq ft home or something. I am unsure what I'm going to do now as well, and have been looking for more traditional portrait zoom options like the tamron 28-75, but I don't know if it will focus fast enough either. This is definately where I miss my nikon cameras and afs lenses, but they didn't have built in image stabilization which is worth it's weight in gold to me.
As someone else previously posted, you may want to consider waiting for the 55 DA* as it's supposed to have SDM which most likely will make it a little faster, and it will have a wide aperature to help the camera a little with focusing. I kept my 43 limited as it is by far my most used lens for toddler shots indoors or out, as I can sort of keep up with him and the focal length is a wide portrait which helps indoors. I'm also considering the new 55 as an option to either replace or compliment the 43 if it turns out to be a quality lens.
To me, the 16-50, quality control problems aside, is just not long enough at 50mm, and the 50-135 is not wide enough at 50mm. I find the 18-70 style lenses on crop to be a better fit as it covers more of the traditional portrait range without lens changes, but in pentax we don't have any real options on that front yet.
So, after all my rambling, I think if I were you, I'd just keep using my 43 (50 f1.4 in your case) and wait to see what the future brings with the 55 DA* SDM, but what the heck do I know.