Alternatively to changing the opacity by hand, you can also do this by selecting all your layers, turning them into a smart object and then setting the blend mode to average. Processing into a smart object usually takes quite a while though. But is does give some other useful blend modes like median or max which allow creating sort of synthetic long exposures.Another is to simply take multiple, successive exposures and combine them in Photoshop (overlay them and adjust the opacity to equally weight them (i.e. opacity = 100/image number, so the bottom image is 100% opacity, the second is 50% opacity, the 3rd is 33%, 4th is 25%, etc.).