Second guessing David's underlying understandings, but he did say "when testing the auto settings on my Metz with transparency film but accurate when using the Metz on full power."
Originally Posted by BrianShaw
Studio flashes tend to be somewhat longer duration light output, while speedlights tend to be shorter duration light output. In the days before today's higher output flash units, most speedlights might be 1/1000-1/10000 or thereabouts, while studio flashes might be about 1/1000. As my experiment showed, when the flash duration gets very short (in my example, 1/10000) my flashmeter is totally fooled and its suggested exposure to totally wrong. That fits David's experience.
The Hassy guy apparently made comments about speedlight output being 'different' than studio flash. Paul Buff's charts might help explain that: http://www.paulcbuff.com/sfe-flashduration.php
You can see that reduced power of (some) studio flash lowers the max voltage, resulting in a lower peak, but fairly uniform duration of light output. OTOH, the IGBT output reduces duration, not the peak amplitude...but that is how speedlights work. Modern studio flash use an approach like IGBT, for example my Dynalite flash duration drops as power is lowered.
My tests showed that the flashmeter could reasonably meter even the abbreviated flash duration of speedlights -- to a point. As I said earlier in this post, when the flash duration gets very short (in my example, 1/10000) my flashmeter is totally fooled and its suggested exposure to totally wrong.
What I can't explain is why my flashmeter's suggested exposure when reading my Metz on Manual power results in a brighter result than one really would want.