Although the G1 has several flash problems, this one is the most interesting. It involves the use of non-Canon flash and appears to be a timing issue.
Given a non-Canon flash, set the camera to "M" manual mode, set the aperture to 1/125, zoom out to full wide angle, set the aperture to f/2. Set your flash to expose properly for f/2. Now shoot at each f/stop from f/2 to f/8 (f/2, f/4, f/5.6, f/8 is a good set to test). At each successive stop, the image should get darker. If it doesn't, you have the flash timing problem.
For some, this problem is easily repeated, for others, it is intermittent. You may need to run through the test several times to be sure you don't have the problem. Zoom position might have an effect, so if you can't reproduce the problem, try zooming in and trying again.
I'm one of the lucky ones. My G1 doesn't exhibit this problem. Some G1's exhibit this problem more than others. The theory goes that the G1's aperture needs time to settle into its selected aperture (this seems odd). For a very brief moment, the aperture might be at f/2 even though it was set at f/8. It is at this moment that the flash could fire and cause overexposure no matter what the aperture setting. Remember that the flash has a very short duration, so it has a chance at being finished before the aperture settles.
Someone who has a G1 with this problem could try using a flash delay of some sort to test the theory and see if they can get around the problem. If so, then Canon's solution would be a firmware fix that adds a slight delay to the firing of the flash.
Canon's dedicated flash do not exhibit this problem, probably because they have a delay of some sort built-in. It is most likely due to the way the E-TTL preflash system works. Here's my guess at the E-TTL sequence:
There is probably a delay in the flash so the flash won't fire before the camera's shutter is open. This allows it to work properly given a G1 with the flash timing problem.
A workaround for this problem is to trick the camera into thinking there is no external flash attached. There is a small switch on the hotshoe that tells the camera when a flash is attached. If you modify the hotfoot on the flash to not activate this switch, the timing of the flash will be different. However, the internal flash will fire as well. You might be able to block the internal flash, although the heat build-up can damage it. This only works in "M" mode on the camera, as the other modes will try to E-TTL preflash with the external flash.
<- Go Back to my Canon G1 page.
The following are my notes as I read through the posts on dpreview.com. Everything has been summarized above. You don't have to read this section.
From Michael W http://www.dpreview.com/forums/read.asp?forum=1010&page=2&message=1084058 Problem: Flash with non-Canon flash is over exposed as if aperture is full open at 2.0, regardless what the actual aperture setting is. Only happens in Manual mode, not Tv/Av (but then you're in fill-flash mode). Flashes used: Vivitar 283, Sunpak 383. The problem is that you can set aperture to f5.6 and the camera will expose the flash as if it was set at 2.0 or other sporatic values depending on zoom / shutter speed combination. Peter T clarifies: I have done a lot of reading on this subject. Here are the conditons under which the flash will work in manual mode: using only f/8 or using only 1/10 sec or slower or using the old firmware. Obviously the first two are very limiting. Michael W clarifies: at speeds 1/10 sec or slower exposure results are just as expected. At faster speeds, e.g. 1/125 sec., f4.5 through f7.1 will actually expose MUCH BRIGHTER than f4.0 given everything else is identical for example. nahau agrees: I have read this thread with interest, and have tried and experienced the exact problem using a Minolta 360PX TTL/Manual flash. It is definitely a Canon screw up!! Peter T restates the experiment: Put your flash in manual mode. Then put the G1 in manual mode at 1/125 sec. Without making any changes to the flash take exposures from f2 to f8. Then let me know if the exposures are consistent with stopping down the lens. Thanks. Micheal W makes new post: http://www.dpreview.com/forums/read.asp?forum=1010&page=1&message=1100395 My experience: No such problem given the 550FD, or even the 550FD with the dedication contacts covered with tape. Full zoom or full wide. 1/125, f/2.0-f/8.0. No problems. Answers: The original firmware probably would have disabled the flash entirely. [This only applies to non-Canon flashes that claimed to support EOS E-TTL] You can get around the problem by shooting in raw. [Probably same as above] Original 22.214.171.124 firmware did not have this problem(?) euthon ran the following test: Equipment: - Flash: Spectrum 370T Auto flash in manual mode, bounced off the ceilling to prevent gross overexposure, plugged into the hot shoe. - Camera: Canon Powershot G1, updated firmware, zoom set at its longest, 1/125 shutter speed, manual focus, on a tripod, ISO 50, RAW mode. - Target: 8x10" gray card, about 4 feet from the camera. I shot the card at f/2.8, 4.0, 5.6, 8.0 and exported images into Photoshop. Below are averaged RGB values for each shot: f/2.8 -- R: 199, G: 190, B: 172 f/4.0 -- R: 125, G: 117, B: 103 f/5.6 -- R: 111, G: 106, B: 92 f/8.0 -- R: 23, G: 22, B: 20 White balance was set to "flash". I repeated the test several times and the results were pretty consistent. I can definitely see the problem you describe around f/4 - f/5.6, but it's not nearly as bad as in your case. Branch ends with euthon's post! Entire mess ends with Michael giving up. My post to r.p.d: Just so everyone else doesn't have to go through all 100+ messages in that thread, I can sum this up for you. One Michael W. reported trouble with the Sunpak 383 and Vivitar 283. With the G1 in "M" mode, shutter speed at 1/125, and various aperture settings, he claims that the G1 always behaves as if the Aperture is set to f/2.0. He doesn't say whether the EXIF info in the images says f/2.0 or not. He just says all his shots are, "over exposed as if aperture is full open at 2.0". Another poster Peter T. confirms this problem. Another poster, nahau has same problem with a Minolta 360PX TTL/Manual flash. However, poster euthon cannot reproduce the problem fully (he noted less of a change from f/4.5 to f/5.6, but normal behavior outside of this range), and no one seems very interested in this fact. I too am unable to reproduce the problem with my Vivitar 550FD with or without tape on the dedication contacts. As I move through the f/stops in the G1's "M" mode (1/125 shutter), the image becomes darker from f/2.0 to f/8.0, as expected. Flash is set in non-TTL auto mode, f/2.0. (Note that euthon is running 126.96.36.199 firmware, I am running 188.8.131.52 firmware). In the end, Michael W. admits on a totally different thread: http://www.dpreview.com/forums/read.asp?forum=1010&message=1108358 That the problem varies from G1 to G1. That seems consistent with my observations from reading all this. There is no correlation to firmware version. Whew! The rest of the posts to the thread are really painful to wade through. Mostly flaming Michael W. for thinking the G1 would ever work with a third party flash and lots of other useless fluff. Kevin Bjorke's page hints at the problem: http://www.botzilla.com/photo/G1strobe.html See the section titled "The Wide-Angle Mistiming Problem"