[dcpropilot]

Just curious if someone can help me out. I have a SIGMA 70-200mm F2.8 II EX DG MACRO HSM on my K-1 and it appears that the aperature is not fully opening (looks like it's opening only about halfway) when set to fully open @ 2.8. Sigma firmware is the latest....I'm a little perplexed.

[Adam]

Is it fully open at 200mm?

On some fixed-aperture lenses, it's normal for the diaphragm to close down a bit at the wide end.

[dcpropilot]

It also behaves like it's on a different exposure scale as opposed to my Pentax lenses.

[rawr]

Does it behave that way when you shoot, or only when 'idle'?

If it shoots properly (ie shoots f2.8 when it's told to), not much to worry about.

[dcpropilot]

In outdoor conditions, it says 2.8 or 4 at 1/100 but ISO is way too much, like 1600.

[FS1]

I ran into a similar issue a few years ago with a copy of the 70-200mm APO DG EX. It was one of my best and most-used lenses, but even with the generous warranty, it was well beyond that time frame. The issues were intermittent but could be replicated on different camera bodies. At the same (fully manual) f-stop and shutter speed, some exposures would be perfect but some would be very underexposed. (It was easiest to notice at wider apertures.) It appeared the aperture lever was not fully engaging consistently, and I believe that ended up being the issue - the repair wound up being "replaced lens bayonet mount unit". The repair was well worth it to me - a little over $100 at the time, and the lens operated perfectly again! Good luck!

[dcpropilot]

Very interesting. I wonder if something happened when they changed the mount to the the one that doesn't scratch the K-1.

[FS1]

When the issue mentioned above began occurring on my lens, it was somewhat noticeable if I gently physically pulled the aperture lever that protrudes beyond the metal lens mount. The lever didn't always 'snap back into place' with much conviction, and sometimes it didn't seem to fully complete that return trip. Overall, the spring action back to the starting position seemed very dampened. So the aperture lever probably didn't always make it back to the proper position before each exposure... some exposures would be accurate, but then a bunch would be underexposed. In the case of my lens, the issue probably just occurred as a result of actuations and wear over the years.

FYI, although the brief repair description above sounds like the recent mounting plate updates (and is in the same general area), I'm not even sure if the metal lens mount was replaced as part of my lens repair. But the structure just beneath it certainly was updated, because the aperture lever (that sticks out beyond the metal mount ring) worked perfect again after that, and then exposures begain working as expected again.