Originally posted by séamuis seems a bit high for a 55 but I say if you have the money go ahead. the 55 is completely different from the 50. the only thing they share is a similar FL.
I understand what you are saying here, and I agree. Totally different.
But still, a point of order is... well... in order...
The F and Fa 50mm F/1.7 seems to be a direct descendant of the 55mm f/1.8.
The 55mm f/1.8 has Six elements in five groups. So does the F/Fa 50mm.
The 55 has a minimum focusing distance of .45 meters. So does the F/FA 50.
55mm aperture blades 6, 50mm aperture blades 6
55mm filter diameter 49mm, 50mm filter diameter 49mm
55mm angle of view is 43 degrees, the 50mm 47 degrees (difference expected)
There are perhaps more comparisons but one of the most convincing is the formula in graphic format.
Please compare - on top the optical design for the F/FA 50mm f/1.7:
You'll see that they are actually very, very similar. This is actually not all that surprising because they have been doing a production model of this formula since the Zebra lens starting in '58. This was the first of the famous Gaussian design 6 elements in 5 groups that became a Pentax standard for the next 40 years for the 50-55mm focal length of that speed.
It is tempting to think that the F/FA 50mm f/1.7 would have more in common with the F/FA 50mm f/1.4 or perhaps going back to the Super Takumar 50mm f/1.4. The latter was a 7 elements in 6 groups as was the former. I believe that those lenses each have 8 aperture blades. I know the F/FA does.
Compare - again, on top, F/FA 50mm f/1.4:
So in the end it seems that the f/1.4s have a common lineage and that the 1.7/1.8s share a different common lineage. As i said... just a point of order. so... Séamuis... does that mean that to have a complete collection of 55's, you actually have to collect an F 50mm f/1.7 ???
Respectfully,
woof!