One thought. On the SB-800 display the flash shows an underexposure value after the exposure if the flash could not put out the requested amount of light. When I mistakenly had my lens set at f/22 my flash with 0 compensation showed a -2/3 EV under exposure. The other flash that was set at -1.3 compensation did not display any underexposure value. Make sure your flash shows that it was able to provide the requested power.
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