The G1's flash system is very versatile, so it is always the subject of much discussion. The hotshoe lets you connect to a wide variety of external flash. The problems I've had with the built-in flash are bad color balance, and pre-flash eye blink.
Some users have reported problems with the G1 and non-Canon external flash. I've put together a Flash Timing Problem page that discusses the issue.
Preflash Eye Blink. Wake Up!
Canon's flash system fires two flashes instead of one. The first flash is a "preflash" that measures scene reflectance in preparation for the real flash. After this the real flash fires at the power computed from the preflash. Unfortunately, the amount of time that passes between the preflash and the real flash is just enough to catch people blinking from the preflash.
The best workaround is to use FE Lock. It isn't perfect, but it works. With FE lock you can increase the amount of time between the preflash and the real flash, and eliminate the possibility of catching your subject blinking. The drawback is having to flash twice for each picture. Your subject might think the first flash was the picture, and walk away.
Another possibility is to use full manual mode. This will give you a single full power flash, so you'll have to adjust your aperture to compensate based on distance to subject. Unfortunately in this mode the LCD will go dark since you've stopped down to accommodate the flash. In Continuous focus mode, you can get a brief look at the LCD during the half-press auto-focus. But Single focus mode is much faster, so you'll likely just want to switch to the viewfinder.
Using an external flash will solve the preflash eye blink problem. Canon's own flashes do the preflash, but reportedly, it is too quick to cause eye blink. Non-Canon flash won't do the preflash at all, so no eye blinking trouble there either.
The final alternative is to not use flash at all. The camera's light sensitivity is very good, and you may even be able to get away with this at an ISO setting of 200 or 400 if the noise doesn't bother you.
When doing macro work, the flash on the G1 will overexpose the subject. You can throttle back the flash by using the "f/8 flash trick". Set the camera to Tv mode. Set the shutter speed to 1/640 or higher. The camera will now fire at 1/250 and f/8 which will allow the flash to throttle back enough for a good range of macro distances. Sometimes I just throw a folded tissue over the flash to reduce and diffuse the output.
I did a series of test shots using various white balance settings (daylight, cloudy, flash) with the built-in flash vs. a Sunpak 383s. The Sunpak 383s always gave significantly better color than the built-in flash. Oddly, the white balance setting appeared to have no effect on the Sunpak series, while the built-in flash showed noticeable variation from one setting to the next. My guess is that the hotshoe switch on the G1 forces the camera into daylight color balance mode.
As mentioned above, external flash is one solution to the eye blink problem. But should you go for one of Canon's dedicated flash units or a non-Canon flash that will have a lot of the same features, but cost a whole lot less?
One drawback to using non-Canon flash is the lack of E-TTL (Evaluative-Through The Lens) flash metering. E-TTL should be better than non-TTL metering, but still it isn't perfect. Like all automatic exposure systems, there are subjects that can trick even E-TTL flash metering into under or over-exposing a scene. I consider the lack of E-TTL on non-Canon flash to be a non-issue given that you can instantly review your results in the LCD and adjust exposure as necessary. Most non-Canon flash offer non-TTL flash metering that works just fine. E-TTL would be helpful in a case where you don't have time to review the shot, or if you are doing a lot of zooming in crowded surroundings. In these situations, E-TTL might give you an edge over non-TTL.
Another thing the Canon flashes offer that the non-Canon flashes don't is automation. Certain Canon flash heads will automatically zoom with the camera. You can also select any of a range of f-stops (in Av mode) and the flash will accommodate your selection (although maximum distance will change as expected). It will also deal with fill-flash ratios automatically, and you can further tweak the ratio by specifying a flash exposure compensation like -2/3 stops. I haven't tried any of this, only relaying information from folks who've used the Canon flash. While you can still do all of this with a non-Canon flash, you'll find yourself fiddling with a lot of settings to get things set up properly.
But beware, the world isn't perfect here. With either the G1's built-in flash or a Canon external flash it is not possible to have the camera in full manual mode ("M") and the flash in E-TTL mode (instead the flash always fires at full power). This means you can't shoot at f/8 without going to Av mode which is really a fill-flash mode. It is only with non-Canon flash that something similar to this is possible. You can set the camera to M mode and have the Flash do non-TTL metering. Check out Samu's page for more.
Note that Canon specifies that external flash voltages through the G1 should not exceed 6 volts. I measured two of my flash units and found that my Vivitar 2600 presented 148 volts to the camera, and my Vivitar 550FD presented only 8 volts. My bet is that anything under 15 volts probably won't hurt the camera. I've been using my 550FD and Sunpak 383s with no trigger trouble so far. The 550FD is a cheap flash and the color balance isn't as good as the 383s. Kevin Bjorke's G1 strobe page covers the flash voltage issue in detail.
Users on rec.digital.photo have reported success with Wein's HS to PC adapter (with voltage regulator) and Vivitar's 285HV. With this HS to PC adapter you don't have to worry about high flash voltages frying your G1. It reduces the voltage to something the G1 can handle.
Sunpak's 383s flash is a really affordable and flexible non-Canon flash that works very well with the G1 assuming you don't have any external flash problems. It offers a manual mode, three f/stop settings in auto mode (at 100ASA, f/2 f/4 f/8), and power reduction of 1/2, 1/4, 1/8, 1/16. Only Canon's top-of-the-line 550EX has manual power settings. The 383s can also swivel 360 degrees, and tilt 90 degrees for complete control of bounce flash. I've been using the 383s since July 2001 and I am very happy with it. It is one of the few photo gadgets I own that I have no complaints about.
As mentioned before, the color balance on the 383s is really great. It is much better than my Vivitar 550FD flash which has all sorts of color problems.
I ordered the Sunpak FK-1 filter kit (651-738 #1738) for the 383s from Adorama but was disappointed to find out that it doesn't have the required filter holder (#1847). Hopefully Sunpak and Adorama will update their websites to help us consumers out when purchasing these accessories. On the plus side, I got an opportunity to check out Adorama's customer service by doing an exchange. The exchange was a bit slow, but very smooth.
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