An old thread, but I found it because I had a similar problem - or most likely, the very same problem. My Sekonic L-358 with the 5 degree NP Finder attached would give readings sometimes that are around 2 stops underexposed. I.e. it would give a reading of let's say 1/15 @ f/11 if the correct exposure would actually be around 1/4 to 1/3 @ f/11.
@ic-racer above is right about the switches; those appeared to have been the problem at least in my case. I've written about my experience with tracking down the problem and (hopefully) resolving it here:
https://tinker.koraks.nl/photography...rong-readings/
While I specifically use the 5 degree finder, the story also applies to the 1 degree and 10 degree models. It's just a matter of which combination of switches is depressed by the fitted metering accessory.
The problem of wildly inaccurate readings is highly unlikely to occur if no spot finder accessory is fitted, since the white dome for incident metering is 'coded' by neither of the switches being depressed, so it's the most fail-safe mode.
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