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Need some help explaining variation in exposures


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3 replies to this topic

#1
DocPit

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I'm attaching a .pdf showing 4 shots of a rattlesnake in an exhibit.  Not great shots, but no matter.   Each was taken at f/8, 1/160, manual with auto ISO, 8mm center weighted.  The 4 shots appear nearly identical in subject and background. But ISO bumps up from 3600 to 4000, then 8000 , then 12800 (a range of two stops).  It seems like the initial exposure should have been sufficient (ISO 3600).  Naturally, I don't want to be jacking ISO higher than I need to.  I'm thinking of either going to a 12mm CW or matrix with exposure compensation.  Does anyone see in these photos what is causing ISO to rise?  Am I just getting the same instability that often occurs with spot metering?

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#2
Merco_61

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I suspect that you might be right. The heavily centerweighted metering is more like a semi-spot and must be used carefully.

Another factor is how the terrarium was lit. Under incandescent lighting, exactly where in the on-off cycle the exposure is locked can make a big difference. If you switch Auto-ISO off in such conditions, the meter will even things out. 1/160 can be high enough to get erratic exposure under some light sources anyway as the light cycles so you need at least 1/60 if not longer to get a full cycle. 



#3
TBonz

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I agree with Merco on the light cycle, but I also see that from the lowest to the highest ISO, the center area in the image gets progressively darker so it is quite possible that a wider center-weighted area might have helped.  You should also be able to limit the highest ISO if need be.  



#4
DocPit

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Thanks for the replies.  I would never have thought about the light cycling.  This forum is a great help.