Equivalent Exposures
An equivalent exposure is creating an exposure that allows an EQUAL amount of light. In order to make an equivalent exposure you need to adjust both the shutter and aperture in the same amount of increments and in opposite directions to get the same amount of light. As you can see from studying the chart below, all of these are equivalent exposures to one another, as they are allowing the same overall exposure/amount of light coming into the camera.
Equivalent Exposures vs. Bracketing Exposures
An equivalent exposure is not the same and should not be confused with bracketing exposures. When you bracket your exposure you only adjust one element of the exposure triangle and you are photographing the same scene, but allowing different amounts of light to come in, thereby, taking 3 different exposures. Usually 1 normal, 1 overexposure and 1 underexposure. With an equivalent, you must adjust at least two elements of the exposure triangle while photographing the same scene, but this time you are allowing the same amount of light to come in, so you are taking the same exposure. So that may lead you to ask... "Why would I need to know how to make an equivalent exposure"? If you haven't already figured it out, take a look at the diagram below for a hint. All the exposures in the diagram below are EQUAL/equivalent exposures to each other.
Why would I need to produce an equivalent exposure?
Creating equivalent exposures gives you more freedom to use the creative benefits of shutters, apertures and ISOs. For example, using the above diagram, let's say that my camera gives me a normal exposure of f 2.8 at 1/60 of a second. And let's say that is the correct exposure for the scene I am shooting but my problem is that I want to shoot at a much slower shutter speed so that the water in my scene will be more blurred. If I simply adjust my shutter speed to 1/4 of a second, I will be severely overexposing my scene by 4 stops. In order to compensate for the 4 stops of extra light that my shutter is now letting in, I need to close down by 4 stops to f 11. f11 at 1/4 is an equivalent exposure to f 2.8 at 1/60 so therefore I will get the same amount of light, but will now be able to also get the motion blur effect I am also looking to create.
Equivalents are useful when you not only need to change the amount of motion or freezing you want to produce with the shutter speed, but it can also be used to adjust the amount depth-of-field via the apertures or adjust the amount of sharpness or grain via the ISO setting. All you need to remember is once you get a normal exposure that is appropriate for the scene, then you need to adjust 2 of the Exposure Triangle elements in EQUAL increments. Make sure you practice making equivalent exposures. Create a shutter speed and aperture diagram to assist you. Use your camera to practice making equivalent exposures and practice calculating them on paper. You will likely see these show up on a quiz! :) Hint! Hint!
Equivalents are useful when you not only need to change the amount of motion or freezing you want to produce with the shutter speed, but it can also be used to adjust the amount depth-of-field via the apertures or adjust the amount of sharpness or grain via the ISO setting. All you need to remember is once you get a normal exposure that is appropriate for the scene, then you need to adjust 2 of the Exposure Triangle elements in EQUAL increments. Make sure you practice making equivalent exposures. Create a shutter speed and aperture diagram to assist you. Use your camera to practice making equivalent exposures and practice calculating them on paper. You will likely see these show up on a quiz! :) Hint! Hint!