How does AE work with flash

[PeterBM]

1) Question for the powershot A80. For example in Tv mode with enabled flash:
- Is the flash power always fixed at max, so the aperture would be determined from focus distance?
- Or is the flash power variable, so how is the aperture determined?

2) Same question for the G3 with inbuilt flash.

3) Same question for the G3 with an external Canaon flash.

[scottbergerphoto]

I own a G2 and 10D, but here's how Canon flash works: Flash pictures are two exposures in one. The first exposure is the exposure of the ambient light and background. This is controlled by the aperture and shutter speed set by you in Tv or Av, or the camera in Auo mode. The second exposure is the exposure of your subject which is controlled by the flash. When you depress the shutter release button all the way down, the flash fires a preflash which is read by the camera meter before the shutter opens. The computer in the camera measures the amount of light reflected off of the subject and tells the flash how long to fire. This is known as ETTL. All Canon digital cameras operate flash in ETTL or manual mode. The resulting image is a combination of the two exposures. In Tv, you set the shutter speed you want up to the max sync speed of the camera and the camera will choose the aperture to suit the ambient light. The flash, using ETTL will control the exposure of the subject in the foreground. In Av you set the aperture and the camera selects the shutter speed based on the ambient light. In manual mode, you set the aperture and shutter speed for the ambient light and the flash takes care of the subject.
For some bizarre reason, the G2 when set to manual exposure, fires the flash only at maximum. That is not the way SLR's or DSLR's work.
The only relation that aperture has to your flash is that the maximum distance the flash can travel is controlled by the aperture(Distance=GN/f stop). As long as you are within the maximum distance for a given f stop, ETTL will properly expose the subject. You can adjust it with FEC.
For a more comprehensive review, read:
http://photonotes.org/articles/eos-flash/
Scott

[PeterBM]

Thank you, Scott. I'll have a look on the site you recommend. Your explanation is clear. This means that in low light, aperture is always at maximum with flash. I am surprised I get so an important depth of field. May be it's normal with f/3.2.at 13.6 mm (equivalent 76mm).
Peter

[scottbergerphoto]

PeterBM wrote:
Thank you, Scott. I'll have a look on the site you recommend. Your explanation is clear. This means that in low light, aperture is always at maximum with flash. I am surprised I get so an important depth of field. May be it's normal with f/3.2.at 13.6 mm (equivalent 76mm).
Peter
Peter,
You are welcome. Just remember that the camera meter is showing you how your settings(aperture, shutter)match up to it's reading of ambient light and not flash.
How did you get from 13.6mm to 76mm? Are you talking about the 1.6 crop factor?
Scott

[PeterBM]

Thank you, Scott. I have read Part I of the pages you recommended. Very interesting, also because I own an EOS 500N. About Digital cameras they never mention Powershot serie A cameras, only Dxx and G serie. Canon do not specify flash operation is E-TTL with in-built flashes in their documents (either A80 or G5). I also had a look on some reviews on line. Only one I read (Imaging Resource) briefly mentions for the G5 it is a TTL operation. Why Canon do not specify it? Is it so obvious? I read in above mentionned site that Nikon uses focal distance for computing the flash power. From where can we know it's E-TTL operation for the Powershot A serie?

About getting 76 mm from 13.6 mmm they are the two numbers in the EXIF data from a photo I considered. I supposed 76 was the equivalent of 13.6 for 35mm films. But when I compute, from my lens 7.8-23.4mm said equivalent to 38-114mm, I found 66 instead of 76 for 13.6. Why??? (by the way, dealing with angles in degrees would so much simpler to feature lenses, they are independant of film sizes and CCD sizes - just a personal remark)

About depth of field, for a given aperture, say f/2.8, is it the same with my A80 at 7.8 mm than with a 35mm camera at 38 mm?

Peter