Most of the shooting modes on an EOS camera can be used with fill-in flash. Here's a guide to which you should use, and why.
• Fully Automatic (Scene Intelligent Auto):
As mentioned, Full Auto mode activates the built-in flash when the camera decides that flash is needed. It does this in low light, as well as backlit conditions. This is fine if you are in a hurry and need a quick and easy way to shoot with fill-in flash, but it offers no creative control.
• Program mode (P):
This is a good mode for simple fill-in flash. As in Full Auto mode, the camera sets the shutter speed and aperture automatically, but leaves you to choose whether or not to use the flash. If you raise the built-in flash unit, it will fire. Otherwise it won't. Or you can attach a Speedlite and switch it on.
In both Full Auto and Program (P) modes, the shutter speed with flash is limited to between 1/60 second and the camera's flash synchronisation speed (between 1/90 and 1/250 second, depending on the camera). This is a fail-safe mechanism built into newer cameras to avoid images being ruined by the effects of camera shake at slower speeds. Most of the time you will see the lower speed being set. The higher speeds come into use only in really bright daylight. However, 1/60 second is a perfectly good speed for fill-in flash.
• Shutter Priority mode (Tv):
In Shutter Priority mode (Tv), you select the shutter speed and the camera automatically sets the aperture for correct daylight exposure. The camera won't let you select a shutter speed higher than the camera's flash synchronisation speed – if you try, it will set it to the synchronisation speed.
• Aperture Priority mode (Av):
This is a good shooting mode for creative fill-in flash. With Aperture Priority (Av), you select the aperture and leave the camera to set the shutter speed. On the EOS R
and all EOS cameras released since, the same fail-safe mechanism applies in Av mode as explained above in Full Auto and Program modes, limiting the shutter speed. Older cameras can go down to 30 seconds. Aperture is one of the main determinants of depth-of-field (how much is sharp in front of the point of focus and behind it).
However, you don't have total control over the aperture. If you set a very wide aperture on a bright day, the camera might not be able to select a shutter speed fast enough for correct daylight exposure. The result will be an overexposed image. You can avoid this by checking the shutter speed value displayed in the viewfinder before shooting. If the value is flashing, it means you are out of range and should set a smaller aperture.
If you set a very small aperture, you might find that the camera sets a slow shutter speed, especially if the sun is not shining. You should get correct exposure, but the slow speed may mean you need to use the camera on a tripod to avoid the effects of camera shake.
• Manual mode (M):
This allows you to set both the shutter speed and aperture independently of the camera while retaining full autoflash exposure. This is very useful for some flash techniques − but not fill-in flash. Most of the shutter speed/aperture combinations will not balance the flash with daylight. Use Aperture Priority (Av) mode instead.