When hi-speed synch fails

mercphoto

I found something interesting at a night motocross this weekend and I think I know what is going on but wanted some feedback. While chimping as the light faded I found something interesting with the flash exposure metering. In daylight I usually go 1/640 with the flash in hi-speed synch mode. As the light slowly fades I keep bumping up the ISO. At some point, however, I started getting too much flash power and the bikes were getting over-exposed by the flash. I decided to slow the shutter down to 1/250 and get off hi-speed synch and voila, I got a much better, more natural looking flash exposure. I was even able to drop the ISO some. All the time I'm in E-TTL mode and at the same FEC value, but the flash metering radically changed.

I'm not entirely sure if the change is the result of not being in hi-speed synch mode, or if the change is due to the slower shutter letting in more ambient light. Even still the ambient exposure was typically about -2.

Blurmore

I've noticed some interesting anomolies with HSS myself, but I'm using it for a different reason. I shoot weddings, so the HSS doesn't serve to stop the action but to allow wide open shooting with fill flash. I'm honestly shocked that you can hit something as far away as I'd guess a motorcylce in a race would be with HSS. With a stofen omnibounce attached to the flash I find the maximum range to be about 15 feet, at up to 1/500th of a sec, after that it is just pushing too hard through the curtains to be much use.
According to the 580ex manual, setting a shutter speed 1/250th or lower automatically disengages HSS even if it is selected on the flash. Typically I find myself using +1/3 - +2/3 FEC to get nice exposures. I was playing around with full auto porgram mode at my last outdoor wedding and found that this gave better HSS exposures than AV or M and evaluative metering. In camera metering with wedding subjects is always kind of a mixed bag, you have a choice of black or white (unless the groom is wearing a gray tux, these rock) I usually meter from the dress, which is why I end up needing + FEC, but the alternative runs the risk of dangerous over exposure. It seems like adding HSS to the equation just multiplies the exposure dillema for the poor camera brain. From my experience I only use HSS when absolutely neccesary, if 1/250 is fast enough to stop the action it makes sense to use it, as you are taking it easier on your flash. Just my 2 cents, the fastest thing I have to stop is a bouquet or garter in the air so my comments don't really apply to action.

mercphoto

I've been surprised at how far a 580EX will reach at night as well. I use it pointed straight ahead with no light modifiers, and that definitely helps.

Blurmore wrote:

From my experience I only use HSS when absolutely neccesary, if 1/250 is fast enough to stop the action it makes sense to use it, as you are taking it easier on your flash. Just my 2 cents, the fastest thing I have to stop is a bouquet or garter in the air so my comments don't really apply to action.

One thing I learned is that when the flash is your primary light source because ambient is 2 stops or more under-exposed then that very brief burst of light can stop action even if the shutter is open a bit long for the circumstance. And I wonder if that was the entire problem with the flash over-exposure. In Tv mode at 1/640 the ambient exposure was so low that the camera simply fired too hard and over-exposed the subject as a result.