Roxie2401 wrote in post #16066785
This will sound silly, but how do you suggest that I "check that the flash actually was able to provide enough light?"
By looking at the lamp, of course. The flash has one LED to tell you it's ready to fire, and another which turns on for a few seconds after the shot, if the flash power used was sufficient, according to the metering done by the E-TTL system.
If you don't want to take your eye of the viewfinder, then you can use FEL. That will tell you in the viewfinder if flash power will be sufficient, prior to taking the picture. This is possible since FEL (as already stated above) separates flash metering from flash firing for exposure.
When using M mode together with flash, something which is a good idea, by the way, you must dial in an exposure that's more or less underexposure. If you expose the subject correctly already with ambient light, there's no room to add any flash (unless the flash illuminate just some minor part of the subject, usually a part that's in a shadow). If you overexpose the ambient then of course adding flash will just make it worse. That's why the camera normally lowers the shutter speed a bit, when using Av mode together with flash, compared to without flash. To give headroom for adding some flash light.
Thus centering the meter, assuggested by Tommy, will not match ambient and flash. Setting one stop below center does, however, at least on a more "standard" subject, where both ambient and flash will be able to illuminate the part of the subject you see from the camera's point of view.