Originally posted by Marc Sabatella Well, it's been a while since I tested, but what makes you say think it's *supposed* to be f/2? One bit of circumstantial evidence that it isn't: at least two other Pentax lenses I own have an white dot marking the unlabeled stop, and someone told me this to to indicate that the unlabeled stop really was a "full" stop. Since the 50's lack a white dot, I might conclude it was not a full stop.
It's kind of like interpreting the Constitution or a religious text: what did the original author really mean? Just like those interpretations, we have inconsistent evidence.
- K300/4 - dot marking f5.6
- K135/2.5 - unmarked stop between f2.5 and f4, f2.8 on mine
- M50/1.7 - unmarked stop between f1.7 and f2.8, f2.0 on mine
- M135/3.5 - unmarked stop between f3.5 and f5.6, f4.0 on mine
- K55/1.8 - unmarked stop between f1.8 and f2.8, f2.0 on mine
All we need is a charismatic leader for each side and a fragment of Japanese text from the Asahi Opt. factory and we could have a war. "As you can see, Captain, Lokai is black on his
left side."
Quote: My M50/1.7 was a beater, that's for sure, and it seemed pretty clear someone at some point had taken it apart- or tried to, anyhow. So that could well be.
I took some photos to show my theory about this. First, the M50/1.7 with the name and filter ring removed:
The red arrows mark three screws holding in an assembly of the aperture blades and all the glass.
The assembly is removed, and the arrow marks a fork that connects to the aperture ring and lever mechanism.
This is the assembly and the arrow marks the chrome bar that goes into the fork above. The bar moves the aperture blades. (With a little work, Pentax could have put weather seals on this lens.)
If the three screws in the first photo are loosened, the assembly can be rotated in place. Then the chrome bar moves while the fork doesn't, so the blades move too. The assembly can shift enough to make the aperture ring settings wrong without causing any other assembly problems. If I do this just right, I can get that unmarked stop to be closer to f2.4 than f2.0. The other stops are not quite right either, but close enough so that you'd only notice if you did back-to-back controlled tests with another lens. And why would anyone ever own more than one of these?
(The big one is the F version. Aperture rings all set to the unmarked stop.)
I have no opinion on how this affects dynamic range.
BTW, I just noticed my last post said "I don't think I've come across any lenses with aperture rings in third-stop increments." A few lenses
start on third-stops, like the K135/3.5 or the K55/1.8.