I disagree I find the images sharper using second curtain as the sharp image is exposed over the top of any movement blur not the other way around
Well you might disagree, but that doesn't make you right. You have two exposures, ambient and flash, the two exposures are exactly the same, the burn in is identical with first or second curtain sync for the same ambient exposure, one is not "on top" of the other as the pixel information relays a two dimensional image, each pixels brightness is the result of the combined two exposures. Further, any movement blur is outside the sharper flash image, the pixels that record the blur are pixels that didn't get any flash exposure, so how can it matter when the flash fired?
There are a couple of good reasons for second curtain sync, but imo your examples are not one.
ok it makes a difference WHEN the flash fires because of this
the pixels are like buckets filling up with photons, as you know, since its been done to death on these forums and then some
so the model will get some exposure by ambient light even when standing in the shadow but this light doesn't fill the buckets very much and all of this exposure is blurry especially at 5 seconds exposure!
so in second curtain, the second exposure by the flash fills the buckets up more and all the light on the model covers all the previous blurred exposure with nice razor sharp exposure from the shadows to highlights
now with first curtain sync the flash fires at the front filling up the buckets with sharp flash exposure and any movement of the model will add smudging movement blur from the ambient continuing to expose for those 5 seconds
this has a lesser effect in the highlight areas (since the extra ambient light is not likely to be brighter than the flashed exposure) than it does in the shadow areas (where if a brighter part of the subject moves across a darker area say an arm across a dress the ambient exposure will fill up the buckets with more photons and bam some extra ghosting blur) but the net result is some movement blur smudging what was a previously sharp flash exposure.
I know i'm right on this, In summary second curtain sync WILL give you a sharper subject than first curtain for long exposures. If the model stays very still then the difference might not be too noticeable and also for shorter exposures first curtain will still be fine like in the 1/4 second exposures i have there i agree first or second probably won't see a difference. But for 5 seconds there is going to always be a degree of involuntary movement which is enough to blur the fringes of detail.
Just because your say I'm wrong doesn't make it so