I’ve been experimenting with “high speed flash“, using speedlites to ‘freeze’ action such as water
splashes, breaking glasses and the like. During the realization of one of the images of an hourglass running, I noticed that it was *impossible* to freeze the running sand with any flash power setting I could use. I know that’s not possible given the short nature of flash bursts.
The EOS 5D was set in Manual mode at 1/200 f/5.6 ISO 100, using a ST-E2 and a 580 in slave E-TTL mode. I proceeded to check every possible point of problem and I noticed that the H(bolt) switch on the ST-E2 was active. Normally FP flash mode only gets active when the shutter speed of the camera exceeds the X-Sync speed. To eliminate FP as the source of the problem, I switched the H(Bolt) switch off and repeated the experiment with the hourglass. I was surprised to see that the flash was working now as expected and I could obtain an image of the grains of sand in free fall.
I could relate this problem with issues I’ve been having in recent shoots involving wireless speedlites where flash power becomes seriously limited resulting in severely underexposed images, recycle times extremely long and increase battery consumption.
I don’t know whether it’s my camera or a general problem, but if you can try to reproduce it, drop me a line with your findings. (just add a comment here)
| frozen image with FP Flash mode off1/200 f/5.6 / wireless flash 580EX controlled with an ST-E2 | frozen image with FP Flash mode on1/200 f/5.6 (identical conditions than for the previous image, except for the high speed sync button activated on the ST-E2) |
AFAIK high speed sync works by emitting several flash pulses during the exposure, to get even light while the shutter ‘slit’ travels across the film/ccd.
W/o high speed sync, the flash pulse is 1/1000 or less, freezing the action (assuming no ambient light, as in the above pics). With high speed sync, the camera shutter speed is the decisive factor.
So if I’m reading your problem correctly, it’s ‘perfectly normal’, not a problem with your particular setup.
Hello! Good Site! Thanks you! iegwaicmijomt
Hi, this is no problem neither a bug, a “high speed sync flash” is slower than when not using high speed sync. Some numbers: without high speed sync flash the flash duration is aprox 1/10 000 sec, so when you use high speed sync the flash duration slowes to your shutter speed with a max of 1/8 000, so this will always be slower. as shown by your results.
Wim.
Hi Wim,
Many thanks for your message. What you say is correct. FP flash is much slower than normal flash due to it’s ‘long burning’ nature.
But the problem I found on the 5D is that FP flash is active at 1/200 that is normally the x-sync speed. At that shutter speed, FP flash is not needed because the curtains will be fully open. Nevertheless, the 5D fails to deactivate the FP mode at that shutter speed.
It’s a bug. Canon confirmed that they could reproduce it (I’ve been able to reproduce it on each 5D I’ve put my hands on) and told me that they would come back with a response, but so far I’ve not heard anything from them anymore.
The most annoying part of this bug is that it’s as silent as a CO intoxication. Previous to realizing this problem, I had many issues getting enough power from my flashes during some location model shoots. Due to the bug, the flash power at 1/200 is strongly reduced causing severe unexpected underexposure.
If you have a 5D, I recommend you check it for yourself. Most probably you have it as well!
Of course, the workaround is fairly simple. Just keep FP flash off when you don’t need it.
-kr, Gerard.