My whole life is research.

That's easy to do. I'll just think back to that K1000 in 1981. When I picked it up I couldn't find anything wrong with it at all, except that it seemed a bit heavy. I didn't feel limited by having only a 50mm lens, needing to manually change ISO at the end of every roll, not getting feedback on my shot immediately, having no video, being restricted to manual focus, having only one exposure mode, dealing with scratched negatives, having to wait to develop... or anything else for that matter.

Of course, in hindsight...

Likewise the K-5 is not limiting to anyone who actually learns photography. For everyone else there are various toys they can use, first and foremost their iPhone.

I kept the FA43 but traded the DA35. Nothing wrong with the lens except the inability to manually focus out beyond the macro range. But, as many threads will illustrate, the FA43 has a special something only two other lenses possess.

If you have the money there is absolutely no beating the FA 77 Limited as Robbie suggests. It is an update on the optics of the venerable A* 85mm and is closer to 80mm in true focal length. I stretched to buying this lens for a portrait session, little knowing it would become my single favourite lens. It is my default walk-around.

However, without knowing what type of portraits you prefer (head shot? head and shoulders? environmental portrait?) it is impossible to recommend one focal length or another. To be honest if it is just for a toddler then almost anything will do.

In that case you might follow the advice and get a fast 50, almost all of which are good if Pentax/Takumar branded. I would particularly recommend the A50/1.7 if you are on a budget and don't mind manual focus. Or the FA 43 Limited if you want the absolute classiest rendering and don't mind paying for it.

This question comes up so often I wrote an article on it: Perspective Distortion, Sensor Size And Portraiture

Exactly my point. People are expressing amazement that prices rise and fall with demand, currency changes and other market forces. I am expressing my amazement at their amazement. What's not to get?

A useful summary! Few notes follow.

Longer focal lengths will always have an intrinsic advantage! The trick is to compare the FA31 to other lenses in its focal range. And then it is noticeably better.

But less distortion even than the DA*16-50 and sharper too.

Distinctly better than the alternatives, completely blowing away the off-brand zooms.

That point can be argued indefinitely. I wonder what it means for one lens to be "as good as" another? People have different levels of quality they are satisfied with. What is good enough for 98% of people or shots or situations will still not work for the other 2%... but you may pay dearly for the extra little bit of quality. And no lens is perfect, so it is always a matter of compromise.

For me, a lens is never complete without a hood, and with a hood the zooms are monsters (even without they are too large for comfort). Why is this an issue? I carry an SLR every single time I go anywhere. And I rarely travel by car so I must haul everything. For those who use lenses only on designated shoots where photography is the reason they are present, this factor of weight and bulk won't matter so much.

The fastest zoom you are going to get is an f/2.8 and often their IQ benefits significantly from being stopped down one. (On the other hand, the f/4 zooms seem to be very good wide open.) But almost every prime I shoot with is f/2 or faster and works just fine at that aperture (except the wider ones). Getting one or two more stops out of a lens is a big deal for many.

The best zooms now have a nice bokeh, but primes still give a lot more control over DOF and OOF areas. That's a reason to have three or more lenses in the same focal length -- for different renderings. You get a lot more personality with primes. So I can afford to take better shots.

Zooms are functional and relatively boring. So they suit situations where you just need to get a job done. My shots would not have the look clients like if I shot zooms. Then again I am not a professional (eg I don't make a living this way).

Finally, there is no doubt that Canikon have much more capable camera systems overall than Pentax. Except when it comes to primes. My love for primes is why I shoot Pentax and vice versa.

Oh dear, not... capitalism!