Originally posted by normhead I honestly would use any of these interchangeably
I fully agree.
The more you know your lenses, the more you get the subtleties and understand which is best for what.
Of course when OOF rendering is important, longer focals allow to blur the background much more than shorter ones.
I didn't check the link, so I didn't know which optics you used. I was unsure about the 90mm, could have been the new Tamron, the old one, or one of the various versions of the Komine manual focus.
I never tried most of them, but I know that the new Tamron is a hell of a lens.
I have two version of the old one (f/2.5), MF and AF. Very good lenses, and quite cheap. The main problem is the dreaded light spot due to inter-reflection between the back (perfectly flat) glass and the sensor cover. Fortunately I never experienced it with the K-1
I'm not surprised to see you own the 70mm. It's the best of Sigma's macro family, IMHO. Sharpness wise, I'm sure it plays in the same league of the new D FA 50mm (and some leitaxed"blue blood" glasses). Though we can't expect that its magic would work past physical limits (that is: optic laws), so when bokeh comes into play longer focals will blur the background a little more, and give a stronger separation of the planes. Which confirms what you wrote: they are not interchangeable
We know that the difference becomes more evident the closer we focus. Your comparison is interesting just because of that: macro lenses, with their short MFD, allow to highlight the differences due to the particular "character", and specific focal length, of each lens.
I understand your love for the Pentax-D FA 100mm. I acquired the old FA version quite recently. It is an amazing optic! Such sharp, contrasty, crispy pictures! It has a few cons, and one plus, vs the D FA, bu nothing that could really affect the final picture, I guess.
Which means I believe to have a fair idea of how your D FA performs (and why you keep it for yourself
).
Very recently I found a Pentax-A Macro 4/100mm, which has been for a while in my wish list. From the pictures it looked quite battered, and I even spotted a kind of cobweb inside!
Fortunately it cleaned up nicely, even the filth on the flocking paper deep inside, and the damage was limited to the filter.
I used it extensively since then. Despite the recessed from element it suffers from flare (low contrast) in extreme lighting conditions, and is a little inferior to the FA sharpness-wise, however most of the times it gives crisp, very sharp pictures, and I'm starting to believe that bokeh is actually better.
I should take the time to compare it with the FA in the same conditions, and see if it's true.
Recently I'm shooting more and more with bokeh in mind, framing the picture in a way that leaves part of the image completely out of focus, and using mid apertures only when required.
Doing that I am completely rewriting the hierarchies of my favorite optics, and found that best sharpness and best bokeh often don't go together.
My gut feeling, yet not proved by any proper test, is that in general the less the lens groups, the best the bokeh. In fact the f/4 100mm has only 3 glass to air surfaces (Heliar design).
I'm curious to find if others had the same impression