Comments from Chuck Westafll of Canon posted by Scott Blankenship in the* newsgroups

The EOS Elan II is consistent with other EOS cameras in the variety of exposure
modes that can be used for flash photography depending on the desired results.
The following table shows how aperture and shutter speeds are set according to
the camera's shooting mode when used with most EOS dedicated Speedlites or the
camera's built-in flash: 

(Program AE): X-sync shutter speed automatically set to 1/60~1/125 sec. based
on A-TTL or TTL program. Aperture value automatically set according to A-TTL or
TTL program.

Tv (Shutter-priority AE): X-sync shutter speed manually set to any shutter
speed of 1/125 sec. or slower.* Aperture value automatically set according to
ambient light level and shutter speed.

Av (Aperture-priority AE): X-sync shutter speed automatically set between 30
sec. and 1/125 sec. according to ambient light level and set aperture value.
Aperture value manually set to desired aperture.

M (Manual): X-sync shutter speed manually set by main input dial to any shutter
speed of 1/125 sec. or slower.* Aperture value manually set by quick control
dial to desired aperture.

*The camera automatically resets the shutter speed to 1/125 sec. if a faster
speed is set.

Now that you know how the camera behaves according to the selected camera
exposure mode during TTL and A-TTL flash photography, here's some additional
background as to which exposure mode is best according to the situation at

P (Program AE): With the camera set for fully automatic operation, the EOS and
Speedlite work together while you concentrate on picture-taking. In daylight or
brightly lit indoor situations, the background will always be exposed correctly
and the camera will control the fill-flash ratio for optimum results. Indoors
or at night, the Speedlite becomes the main source of illumination and the
shutter speed will automatically be kept high enough to permit hand-held

Av (Aperture-priority AE): Selecting Aperture-priority AE mode with flash gives
you maximum control over depth of field when it is a concern. The camera will
automatically set a shutter speed to provide adequate background exposure, day
or night. Outdoors, or in relatively bright indoor lighting, if the selected
aperture is unusable, the shutter speed indicators in the camera's viewfinder
and external LCD panel will blink. Just select a smaller aperture, and shoot.
Indoors or at night, slow shutter speeds are likely, so it's best to use a
tripod or pick a different camera exposure mode. 

Tv (Shutter-priority AE): Shooting in Shutter-priority AE mode with flash lets
you select the shutter speed while the camera selects the aperture to give
correct background exposure. High shutter speeds up to 1/125th of a second can
be used in bright light, while slower speeds down to 30 seconds are more
appropriate in dark conditions or for special effects. In low light situations,
the maximum aperture value of the lens may blink in the viewfinder and external
display if the selected shutter speed is too fast to produce a good exposure of
the background. Just set a slower shutter speed and shoot, or switch to manual
exposure mode on the camera.

M (Manual): Manual exposure mode lets you control both the shutter speed and
aperture. This option is important in low light situations when you want to
combine small apertures with high shutter speeds. Keep in mind that manual
exposure mode on the camera can be combined with fully automatic flash
exposure, since the EOS Elan II's metering systems for flash and existing light
are independently controlled.

Here's another related file:

>>when used in aperture priority mode for fill, the camera/flash automatically
goes into slow sync anytime you want to shoot at less than wide open. So with
the 28-105 I use as my standard lens, the camera defaults to f/4 and 1/60; if I
set a smaller f stop, the camera immediately lowers the shooting speed by the
reciprocal -- no matter what I do! That may be fine for tripod mounted
landscapes, but it won't do for hand held or for fidgety live subjects.
What's the deal?<<

It's crucial to understand the concept of fill-in flash, which is the
combination of flash illumination with a proper exposure of the available light
in the scene. If we can agree on that, then it becomes a lot easier to see how
aperture-priority AE (Av mode) and shutter-priority AE (Tv mode) work with
flash. They are intended to be used for fill-in flash at all times, a task for
which they are particularly well-suited because they allow the camera's
metering system to provide correct exposure of the available light
automatically, at the aperture (in Av mode) or shutter speed (in Tv mode) of
your choice. 

Fill-in flash is often undesirable in low-light situations, for reasons you've
discovered: it involves the use of slow shutter speeds and/or wide apertures,
which may be inappropriate when working with live subjects and/or a handheld
camera. However, there are several situations in which it can produce
spectacular results. Consider taking a night shot of a person standing in front
of a beautifully lit building in the distant background; if the flash is set to
expose the person properly, it's far too weak to expose the building. But if
the shutter speed is long enough, the available light will expose the building

When you're shooting flash in low light and decide that you'd rather use a
faster shutter speed than Av or Tv provides, then you're using the flash as a
main light rather than a fill, and the available light will be underexposed.
There's nothing wrong with that; it's just another way of making the picture,
and is obviously preferable when you want to avoid low-shutter-speed blurs
caused by camera or subject movement. At this point, the Elan II gives you 3

1. Set the shutter speed and aperture settings yourself (manual mode on the
camera, E-TTL with the 380EX or TTL with the 540EZ.) 

2. Have the camera make both settings for you automatically (program mode on
the camera, E-TTL with the 380EX or TTL with the 540EZ). 

3. Use Aperture Priority combined with Custom Function 9. During available
light shooting, the camera acts normally. But when using the camera's built-in
flash or an EOS dedicated Speedlite, the shutter speed will automatically be
set to 1/125 as soon as the flash's ready light is on.