Like Phil said, sometimes a lower ISO setting is better to get rid of motion blur from hand holding, because a higher percentage of light will come from the strobe, and you'll get a Doc Edgerton style freeze frame.
What I ment with my statement to shoturtle, was that if you cant shoot steady you need more strobe power, so that you can dial in a faster shutter setting on the camera. If the strobes are maxed out (unlikely when doing macro) you either have to buy a more powerfull one or use a higher ISO setting so that you can shoot at 100th to 180th of a second, less than 100th is pretty much a waste of time in my experience. Especially when shooting fish, I've got tons of unsharp fish photos in 1/60 second shuttertime, prior to moving to a DSLR ;-)
1/180 sec is the way to go.
But all this is a fairly unnecescary discussion as Shoturtle was talking about shoting without a strobe (Ambient light only). Sligthly confusing in a thread about housings for m4/3 cameras..
As others have mentioned a low ISO setting (100) is standard when using a flash UW.
I've seen some nice aimbient light werck photos done with a tripod and high iso settings, totaly different game thoo...
About high Apertures and studio tests, gues you'd only notice the unsharpness if you start to pixelpeep the pictures at 100% crop.
Ie in real world the 50mm Zuiko is performing so good that one can safely shoot at F22
I'm very hapy with my 50mm can't complain about the sharpness
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