Tuesday, January 8, 2008

G9: Flash Sync Speed

How did Elmo get stuck in that tree at night? Well, actually it was mid-day. This photo is an example of overpowering the sun with flash. The day was slightly cloudy but the G9 was set for ISO 80, 1/2500 second, f5.6 in manual exposure mode. Therefore Elmo, the tree and even the sky were significantly underexposed. A Nikon SB-28 was then mounted in the G9 hotshoe and also placed in manual mode at full power to provide “fill” flash. A bit of cloud in the background is still visible.

The G9 is officially described as limited to 1/250 second with external flash; however, it turns out that, if the G9 does not know about the external flash then it will work at higher sync speeds. In fact, the EXIF data for this shot indicates that flash is “off”!

The G9 recognizes Canon flashes but apparently gets confused if non-Canon equipment is attached to the hot shoe. With a Nikon SB-24 or 28 (and probably others, see the compatibility posting) in the hotshoe, the flash is fired regardless of the sync speed. This posting at Strobist explains the details in general.

Prior to photographing Elmo, I did a few experiments with a Nikon SB-28, SB-24 and Vivitar 285HV by shooting in a nearly dark room with the flash in the hot shoe and the G9 in manual mode. The G9 will sync with all these at 1/2500; however, it seems that the SB-28 and SB-24 “lose” the equivalent of about 1-1/2 stops at 1/2500. The Vivitar loses 2 stops. It also seems that this loss of flash efficiency is roughly proportional to shutter speed; that is, the Vivitar loses about 1 stop at 1/1000 second. The light from the flashes seemed evenly distributed even at the high shutter speeds – there was no mechanical obstruction of the light. So these flash units can be used at very high sync speeds but they will appear to be somewhat less powerful.

Note that the G9 does not permit shutter speeds faster than 1/1600 if the aperture is larger than f4.

With a Canon flash (580EX, 420EX or 380EX were tested) mounted in the hotshoe, the G9 resets from 1/500 to 1/250 when in manual exposure mode. The shutter speed is changed whether the flash is in ETTL or manual mode (580EX) unless the Canon flash itself is set to high speed sync mode. (Canon high speed sync is accomplished by pulsing the flash.)

… and I did get Elmo down from the tree after snapping his picture.


1 comment:

John Mathieu said...

Very cool post. I just tried my SB-26 (a gift from David Hobby, no less! :) and was able to fire off a shot at 1/2500...nice!