11 February 2014

I’ve looked at the flash and sync speed of the Canon Powershot G15 with the built-in and with external flashes. My principal conclusions are:

I reached these conclusions by measuring the guide numbers of the built-in flash of the G15 and an external Canon Speedlite 270EX II flash in different modes and at different shutter speeds.

Specifically, I used the G15 to take exposures of a clean, white, internal wall at a distance of about 2.5 meters. The exposures were taken at night to minimise the correction for ambient light. The camera was mounted on a tripod. I used manual mode, ISO 80, an equivalent focal length of 50 mm, and f/8. At 50 mm the pattern of illumination of both flashes is fairly even, so I don’t expect these results to depend significantly on the precise focal length. I took four sets of exposures at shutter speeds stepped by 1/3 of a stop from 1/100 of a second up to the fastest speed the camera would allow. The sets were:

The 270EX II was extended to the “tele” position and aimed directly at the wall in front of the camera (i.e., not bounced).

In the exposure with the 270EX II, I had to use the nominal 3-stop ND filter. I calibrated this by comparing images at 1/16 setting with and without the ND filter, and determined that it was equivalent to 2.87 stops.

I measured the mean signal level in the green channels in each image, subtracted the mean signal level in the green channel in an ambient light exposure, and for the images taken with the 270EX II adjusted the signal level to compensate for the ND filter.

The 270EX II is specified to have a GN of 27 meters when used in normal mode at 50 mm effective focal length. I used this to scale the signal level in each case to a GN. If you prefer to think in stops rather than GN, remember that a factor of two in GN corresponds to two stops.

The GN as a function of flash, sync mode, and shutter speed is shown in the following plot.

GN plot.

GN for the G15 built-in flash and 270EX II as a function of flash, sync mode, and shutter speed

I interpret these results as follows.

If you’re interested in confirming or extending these results, and don’t want to have to determine the mean signal levels, you can simply see what sort of aperture or ISO adjustment you need to give similarly exposed images under different conditions. Each change in GN by a factor of two corresponds to two stops. So, for example, a normal sync exposure with the external flash at f/8 and a high-speed sync exposure at f/2 and 1/1000 of a second should give similar exposures, provided the flash setting and ISO are equal.


Thanks to Rodney Bartlett for helping me understand how to get the G15 to work in high-speed sync mode.


© 2014 Alan Watson Forster. All rights reserved.