I hope your flash shooting is going well. If you have questions be sure and let us know.
I noticed that a few of you are having a problem that I have had in the past, and it is regarding accurate exposures. There are so many variables with flash that can affect your exposure, and these are usually the result of all the automatic features. For example, a highly reflective surface could have an altered reading, as well as the camera metering missing as well.
I have used the same test for years no matter whether it was flash or studio strobes: I create a small still life setup, and I try to use a neutral background. In my studio I have grey seamless background paper. If you want to do this test, a grey poster board from the dept. store will work. On this background, I place something black and something white, and I also have 18% grey cards (from camera store) and I shoot a ETTL exposure in P mode.
I will shoot the test, download this to the computer, and make a careful evaluation. Is the white really slightly grey? Is the black not black, rather dark grey? The idea is to see how accurate the auto metering is. Some pros actually claim that Canon 550EX is 1/3 stop darker, so they use +1/3 for everything. Take your test if you wish, and if it looks a bit dark, adjust your exposure +1/3 and see what you get.
I noticed years ago that my Canon cameras, in Av mode, will adjust the shutter speed when I turn on the flash. For example, if my meter reading for ambient light was 1/60th @ f/8 and then I turn on the ETTL flash, the shutter speed automatically changes to 1/100th second. I have spent some time researching this off and on and never have found a definitive answer. My guess is that the camera is programmed to do this so that the flashed subject stands out from the background more. Check your camera, if this interests you, by placing the camera on a tripod, pointed at a neutral subject, like the side of the house or something with not a lot of highlights or deep shadows. Meter as you normally would and note the s.s. and aperture. Turn on the flash, and see what the new s.s. is. If it changed, then you would need to use Exposure Compensation to do a +2/3 to get the s.s. back to your normally metered settings.
Guide Numbers? Keep in mind that these are the industries evaluation of the power capabilities of a flash unit. Beyond that, they really don’t matter. So, if this is confusing, skip it.
Remember that shutter speed controls the available light or what some call the background exposure. Aperture controls the flash exposure. However, when in ETTL, the flash will adjust to any changes you make in the aperture. If you move the fstop from f8 to f11, the flash will automatically increase flash output to compensate for your changes. Same going the other direction; if you change fstop from f8 to f5.6 it will reduce flash output automatically.
Shooting modes; P mode is for quick shooting and only uses shutter speeds from 1/60th to 1/250th (depending on your camera). So, if you are in a dark room, chances are that the backgrounds will go dark. In AP mode, the camera uses a shutter speed for a proper background exposure. In SP mode the camera selects the proper aperture for correct background exposure. Manual mode is when you want to control both shutter speed and fstop.
Another thought: Flash with ETTL is the best that it has ever been in my 30 years of shooting. Very accurate-most of the time! ETTL is convenient when you are shooting fast and have no time to think about anything, but getting your shots. However, when I have time to think about it, and time to setup my shots using flash, I always choose to use the flash manually, and test my exposures even more accurately. You will see many example of this in the upcoming lessons.
Have fun! Charlie