This one fact - that the shutter speed you select and shoot at doesn't affect that portion of your image illuminated by your flash - seems to be the hardest for students in my classes to swallow, that is, believe! Obviously, the shutter speed you use does affect the overall outcome of the shot. But, a very slow or very fast shutter speed will not change how much light falls on your intended subject. Period.
OK, I get it. I understand why folks find this hard to believe. I didn't believe it at first either. After you've become so used to working your aperture and shutter speed, in conjunction with ISO adjustments, to get a pleasing shot, it doesn't now seem to make sense - all of a sudden - to hear you can forget about the shutter speed. No sense at all! But, see, it really does make sense when you consider how fast a flash fires and how fast a shutter can open and close. As Syl Arena states in his latest book, "Lighting for Digital Photography" (Peachpit Press, 2013, p. 176), "For most flashes, the longest flash duration is 1/800" second. So, it does not matter if your shutter is 1/60", 1/125", or 1/250" - the burst of flash will fit into any of those gaps." (Remember: When the lower number in a fraction is larger, it represents a smaller part of 1, of the whole! In other words, in the case of the shutter, it is a smaller slice of time. See the copy of Arena's illustration above. The yellow bars represent how long the flash lasts compared to the two horizontal lines representing 1/60 and 1/250 respectively!)) What this means is that the light present in the scene produced by the flash is there for an incredibly shorter time than your shutter is open due to shutter speed settings. On the other hand, the ambient light in the scene, that is, the constant light (e.g., sunlight, table lamps, overhead fluorescent lights, etc.) stays on and will continue to hit your camera's sensor as long as the shutter is open.
And, likewise, how large or small an aperture you use will influence how much the ambient affects your image. However - and this is the very important part to remember - the flash will always overpower the ambient, i.e., available light, when it comes to what the aperture "sees!"
So, repeat after me: Shutter speed controls ambient and aperture controls flash! Again, and again until it comes out of your mouth automatically! (Check point #14 on this page: http://pixsylated.com/blog/15-insights-new-speedliter/)
As much as I think chanting this sentence to yourself will help it sink in, I do know that seeing is believing. And practice helps us internalize behaviors better than anything. So, let's practice. Simply attach your external flash to your camera's hot shoe and switch it on to your flash's TTL mode. Turn on your camera and put it in Manual Mode (M). Don't worry if this is a mode you've never tried before. Just follow the instructions that follow. The idea in this exercise is to take a shot of exactly the same scene - a person if you have a patient friend or family member or, perhaps, a vase of flowers or some other still life object - so you may want to use a tripod. If you don't, try to frame each shot exactly the same. Set your ISO to 200, make sure your Flash Exposure Compensation is set to 0 (Check it on your flash AND on your camera!), and set your aperture to f/5.6. Let's start with a shutter speed of 1/15, hard to hold steady by hand, maybe, but we're not doing this to make a portrait for the ages. Take a shot. Check your LCD. Now speed up the shutter by one stop (Do you remember your full stops for shutter speed?). That would be 1/30, right? OK, now repeat this through the shutter speed range up to and including your camera's sync speed - e.g., 1/60, 1/125, 1/200 or 1/250, depending on sync speed.
All done? Now, review your shots. In each one of your images, the subject, splashed by the light from the flash, should look almost or exactly the same. Does he/she/it? What do you notice about the background as your shutter speed increased, that is, as the time your shutter was opened got shorter? Did the different shutter speeds seem to have any effect on the subject? Leave a comment and tell us what your experience was or write me with your comment.
In addition to teaching scheduled photography classes throughout the year, I am also available to do one-on-one tutoring or small group lessons designed to meet YOUR needs and what you want to learn in the area of photography, using flashes, or the use of Apple products and software. Give yourself the gift of learning: http://www.infotor.com/photoclasses! Check out my new special tutoring bundle: http://www.infotor.com/photoclasses/tutoringbundle.php!