Pentax is exonerated. The fault lies with Sigma.
In a post entitled "Nikon Disgrace," Mike Johnston noted:
Nikon evidently produced a bunch of lenses that leaked gummy oil on to the aperture blades, causing the aperture blades to fail to close down during exposure--with nothing to alert the photographer that anything was wrong. So there were lots of outcries on the internet from people who had returned home from a trip or whatever to find that all their film was completely overexposed and ruined.
"Completely overexposed and ruined" really struck a chord with me, and so I took a look at the Sigma 70-200 f2.8. The aperture blades were bone dry and crisp, they snapped shut in the wink of an eye. So far so good, so I put it in the deep freeze (temp -5F) for an hour to see how it stood up.
Pulling it out, the aperture was still crisp and fast, so I assumed that this was a dead end. But as I turned the aperture ring I noticed something funny - the blades did not stop down beyond f8.
Apparently, some component in the lens distorts in extreme cold, and prevents the aperture blades from closing down. This is not the classic "sticky aperture" syndrome where the blades are gummed up, operate slowly, but eventually close down. Rather, this is a situation where, below a certain temperature (yet to be determined) the aperture blades cannot close down beyond a certain point, but nonetheless are fast and crisp below that point.
I've used this lens successfully on many occasions where the temperatures were in the teens Fahrenheit, so it seems that this problem pertains to extreme cold. So in the future I'll use other lenses in extreme temperatures.
My thanks to the list for helping to solve this mystery!
- MCC
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Mark Cassino
Kalamazoo, MI
[EMAIL PROTECTED]
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Photos:
http://www.markcassino.com
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