70D triggering flash too early on second curtain sync

[midluk]

I tried to use second curtain sync flash on my 70D for the first time yesterday. I noticed, that it seems to fire too early. I'm using 1/10 s shutter speed with a moving light source in the image (to test this some more I just moved the camera with a static light source).
When I'm using first curtain sync, I get an image with the light trail starting directly at the position where the light source was at the time of the flash. When I'm using second curtain sync, the position of the light source at the time of the flash is at about 90% of the total trail length. This means that the flash happens about 10ms (10% of 1/10 s=100ms) before the shutter closes and makes the image look strange. This happens with both my 600EX-RTs as well as my 430EXIII-RT and also with the internal flash of my 70D.
Is this expected, or is there something wrong with my camera?

 

[risc32]

what about with differing flash power settings, and is this manual flash mode or ettl ?

 

[Khufu]

Oooh, this is an interesting one! Apologies for not really having anything to contribute but I'm interested in following this discussion...

I wonder if manufacturers and firmware programmers play with this kind of thing in different models due to mechanical concerns and capabilities regarding rolling shutter speed etc. Do you reckon it might make a difference if you have the light source moving along the top of the frame compared to along the bottom edge? I might be a bit naive here in wondering that, the minimum sync speed is surely a fraction of the remaining exposure time after your flashes are triggering... I'm just thinking out loud now, ha - I'm looking forward to hearing people's thoughts

 

[midluk]

Happens with M (does not seem to depend on flash power) and ETTL.
Also does not seem to differ between normal shutter and silent shutter.

Minimun sync speed is 4ms, so it's much less than the estimated 10 ms. I think I have read somewhere that the flash is supposed to fire 1.5 ms before the curtain closes, but I'm definitely seeing more.

Rolling shutter is important for video (and perhaps for live view) but not for still shooting with the normal shutter.
There will likely be some slight difference between a light source at the top and at the bottom of the frame, but at least with my handheld testing it is not visible. It would take a light source moving with a perfectly constant velocity to see it. So the flash definitely fires way before the shutter closes.

 

[midluk]

OK, I did some more testing. Up until 1/25s it stays at roughly 10ms before the second curtain (then at about 75% of track length).
For 1/30s shutter speed and faster, setting second curtain sync does nothing but the normal first curtain sync is used. The change at 1/30s is documented in the manual of the 70D (it is also in the manual of the 5D3).
If they implemented this limitation, the 10ms I'm seeing might be normal, but should in any case be documented, as this drastically reduces the usefulness of the second curtain sync for any shutter speeds faster than about 1s.

First curtain sync flash seems to happen about 2ms after the curtain opens at the bottom of the frame (nearly at the middle of the 4ms track) and nearly at the start of the exposure at the top of the frame. This means that the shutter takes about 2ms to fully open (starting at the bottom of the frame) and afterwards the flash has about 2ms to fire until the shutter closes again (starting at the bottom of the frame).