In this movie we'll review in more detail how the exposure variables, ISO, shutter speed, and aperture work when using flash. Then in the next movie we'll introduce two new variables, the power of the flash, and the flash to subject distance, which when considered in combination with one another determine brightness or luminance. To review, let's start with the variables you should already be familiar with. First off, ISO. Let's see the visual effect of changing your ISO by looking at a series of images that I have shot in a controlled environment.
I began at ISO 100, and I bracketed the ISO in third stop increments all the way to 1600 without adjusting any other exposure setting. As expected, the mannequin in front is properly exposed in the first frame, while the mannequin in back is underexposed. As I increase the ISO the exposure got brighter until at ISO 400, the mannequin in front is two stops overexposed and the dummy in back is properly exposed. As the ISO continue to increase both manikins became increasingly overexposed.
The practical consequence of all of this is that you can effectively increase the power of your flash simply by increasing the ISO. Now for variable number two, shutter speed. The first thing to note about shooting with strobes is that the shutter speed does not affect the flash exposure at all. Not one little bit. Notice I said flash exposure. If you're shooting in an environment with ambient light, you have to be aware that you can accumulate light from both the ambient source and the flash pop changing both the exposure and the aesthetics.
But now let's review a series of images that I took to illustrate the point that shutter speed does not affect flash exposure. These images were taken using shutter speeds of 2 seconds, 1/20 of a second, 1/200 of a second, and finally 1/500 of a second. Because we conducted our test in the dark, we can also conclude that the exposures were made by the light from the flash itself and nothing else. We see that the first three exposures are identical proving that the shutter speed does not affect flash exposure.
I've included the last exposure because it's above the sync speed on my camera. It's important to see and understand what happens when the shutter speed is faster than the sync speed. See how the image is divided? We see our subject as normally exposed in the top of the frame but nothing is registered in the bottom. That occurred because the shutter curtain wasn't completely open when the flash fired preventing the flash pop from reaching that part of the chip. Remember, the sync speed is the fastest shutter speed that you can use while using your flash.
For most cameras with focal-plane shutters, normal sync speed ranges between 1/200 to 1/500 of a second. Now on to the third variable aperture. Let's review some test images to see how it affects flash exposures. In this series I set my flash to subject distance to provide a normal exposure at f/8, but I began shooting at f/2.8 and bracketed it in third stop increments until I reached f/22. As expected, the series proves that unlike shutter speed, aperture clearly affects flash exposure.
But furthermore, the results are predictable once you learn a few basics. So we've seen that while our shutter speed does not affect flash exposure, both our ISO and aperture settings do. This means that we'll have to coordinate our aperture with the ISO and the illumination level to achieve proper flash exposure. In the next movie we'll see that controlling the illumination level is a function of the flash power and the flash to subject distance. That's a big part of the magic of flash photography.
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