Today’s Question: I’m not clear on the reference to the term “Exposure Value”. What does it mean and how do I use it?
Tim’s Quick Answer: The term “Exposure Value” (often abbreviated as “Ev”) is a number representing the overall exposure settings based on specific lighting conditions. You can think of the Exposure Value as representing how much light is being reflected from a scene, and therefore what type of exposure settings can be used to produce a good photographic exposure.
More Detail: For a given scene you can produce the same good overall exposure with a variety of different camera settings. For example, let’s assume you achieved a good exposure with a lens aperture of f/16, a shutter speed of 1/125th of a second, and an ISO setting of 100. You could achieve the exact same overall exposure by opening up the lens aperture to f/11 and changing the shutter speed to 1/250th of a second.
In the above example, both exposures represent an Exposure Value of 15. That also happens to correspond to the “Sunny 16” rule of exposure, which serves as a good rule of thumb for exposure on a sunny day. With “Sunny 16” you use an aperture of f/16 and then use a shutter speed with a reciprocal that matches the ISO setting, such as 1/125th of a second at ISO 100.
When you have known good exposure settings, you can of course adjust the various settings to achieve the same exposure with a different effect, such as to match the exposure but get a long exposure blur effect. The Exposure Value is helpful at the very beginning of that process, when you need to determine good exposure settings to start with.
One of the easiest ways to study Exposure Values is with a table. I find the PhotoPills to be especially helpful in this regard, because it includes both an Exposure Values table as well as an Exposure tool for calculating exposure settings based on existing settings from a test exposure.
You can learn to master the many features of the PhotoPills app with my comprehensive course on the subject, which you can find here: