Sigma 105mm macro exposure

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simonbarker

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I've recently acquired a Sigma 105mm macro lens and don't understand the results I'm getting.

When I'm using it at f/13 it's a bit over half a stop brighter than my other lenses, the wider the aperture the less the difference though, at f/4 it's only about 2/10 of a stop brighter.

I can't recall seeing such a pronounced difference between lenses before, any ideas?
 
A sluggish diaphragm could account for what you're seeing.
 
Canon Bob said:
A sluggish diaphragm could account for what you're seeing.
Click to expand...

Sounds like it maybe, it would explain why the problem gets worse the smaller the aperture too.

It's generally caused by too much lubrication on the blades right?
 
Traditionally that would have been the case but I've no idea whether Sigma's construction has evolved to use lubrication free materials. A weak 'motor", dodgy return srping or coupling backlash could all feature.

You could peer into the lens and toggle the DoF preview button. In aperture priority you could start at 1/3 stop down from maximum and see if the blades move.....that would show whether there's any backlash in the mechanism.

Bob
 
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Thanks Bob, looks like that's the case as I can't see the blades move at all when it's the smallest step down from wide open.
 
With electronic lenses it may be motor or an elctronic fault not just the diaphragm I had a ribbon connector failure on a zoom lens which was a common fault for that lens.

Have you checked all the contacts are clean?
 
Pondering........

You might need to get someone to run a comparison test with a working lens to confirm the theory as it's possible that Sigma don't activate the aperture on 1/3 stops.
 
Canon Bob said:
You might need to get someone to run a comparison test with a working lens to confirm the theory as it's possible that Sigma don't activate the aperture on 1/3 stops.
Click to expand...

Anyone with the same lens (the OS version) that can confirm? I can't see the blades until it's set to f/4.
 
Sounds more like a calibration issue to me... I believe the aperture lever has an adjustment, or maybe it got bent a little. It's probably reasonably repairable, but I don't know that I would bother.
 
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