The camera flash should be fully masked. Otherwise you will have sediment bloom. It is becoming apparent to me that the YS dTTL is not particularly effective or consistent. Your YS strobe emits a preflash triggered by the camera preflash for your camera expousre setting then followed by the main flash for the actual exposure based on/mirroring the output of the camera flash. Then you have to use the strobe output knob to adjust to your liking.
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