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11-11-2009, 09:57 PM   #1
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Interesting observation with Sigma 10-20mm f4-5.6

I was just playing around with my new Sigma 10-20mm f4-5.6 on my K-7 using the built-in flash, and noticed that although you can still set the shutter speed to 1/180 (the normal sync speed for the K-7), when you press the shutter release button, this automatically drops to 1/160. Really weird. And this seems to be specific to this lens, as it did not happen with my Pentax 18-55mm lens, or even my Sigma 50mm f2.8 macro.

CHANGE OF SITUATION: For the fun of it, while writing this post, I decided to give the 10-20mm another try, and the problem is no longer occurring; it's not jumping to 1/160 anymore. Strange stuff. Maybe the lens wasn't properly attached to the camera body that one time. In any case, hopefully that won't happen again!

11-12-2009, 04:53 AM   #2
Ash
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Were you in Av mode?
If so, what was probably happening was that insufficient ambient light was available to adequately expose the landscape at 1/180 sec. This changes automatically if the camera meters the scene to require a longer exposure, according to the ISO setting given. To keep it at 1/180sec, you should switch to X mode.
11-12-2009, 08:07 PM   #3
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No - the weird thing is, it was happening in M mode, but NOT in Av or Tv.

However, it turns out it was not a freak incident. It started happening again today, but this time with every lens I have (including Pentax lenses). So I have no idea what's going. The only thing I can say is that I never noticed this before updating to firmware 1.02. That's not to say that I just never noticed it before, though.
11-13-2009, 03:02 PM   #4
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Hmm, I remember this happening with either my sigma or another lens... can't remember when or why and with what body though.

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