EV 1 – What is EV?

When I bought my camera the first thing I did was read the manual–I had to have something to do while the battery was charging. As I read, I kept coming across the designation EV. It was used in sentences like, “Flash compensation can be set to values between -3 EV (darker) and +1 EV (lighter) in increments of 1/3 or ½ EV.”  Of course nowhere in the manual did it tell me what EV was. Having been an electronics nut as a kid, the thought that crossed my mind was, “ElectraVolts”, which made no sense at all.
My photographer friend, Peter Lopez, cleared it up for me. EV stands for Exposure Value. It is a measure of the amount of light that will be used when the sensor of your camera is exposed. Knowing this was not quite enough, but it gave me a starting point for further learning.
By the way; not long after hearing the term EV, you’ll be “well exposed”, to the term “dynamic range”. While it’s beyond the scope of this post, dynamic range in a nutshell is the number of exposure values your camera is capable of distinguishing and registering on the sensor.

Setting Exposure Value
There are three camera adjustments that control the exposure value to be used for a given shot. Each of these adjustments can be used to control the amount of light used in the exposure.

ISO

ISO changes the sensitivity of the sensor. High ISO values like 800, 1600, and sometimes 3200 or more, cause the sensor to be extremely sensitive to light. Low values like 50, 100, or 200, make the sensor less sensitive. ISO 400 is a middle-of-the-road setting, and being the conservative guy I am, its the one I use most of the time.
Low ISO settings are better when you need outstanding detail. They work best when there’s lots of light. High settings are great in low light situations or when you need a shutter speed boost to freeze a fast-moving subject. More on this later.

Shutter Speed

Shutter speed is a measure of how long the shutter will remain open and allow light to fall on the image sensor. My camera provides shutter speeds from 30 seconds to one 4000th of a second. A 30 second exposure will let you begin to see the movement of the stars. Another setting called “bulb” can be used for really long exposures. Bulb simply keeps the shutter open until you tell it to close–or until your battery runs out! A six-to-eight-hour bulb exposure is what they use to take those nighttime pictures where the stars draw concentric (around the North Star) circles in the sky. A 4000th exposure will stop anything moving that I know about.
Obviously a slower shutter speed allows more light into the sensor than a faster speed.

Lens Aperture

Lens aperture is a measurement (in f-stops) of the size of the opening in the lens through which light can travel. This can be a little tricky, so be advised. A large aperture (a big opening) is indicated by a small f-stop like 2.8 or 4. A small aperture on the other hand, is given a large f-stop number like 16, 22, or 32. When your hot-shot photographer friend talks about “stopping down the lens”, he is saying than he has set the aperture at a large-numbered f-stop so that very little light can come through.
Aperture is an extremely important setting for another reason as well. Lens aperture is what controls the camera’s depth of field. Depth of field is a way of discussing how much of an image is in focus from front to back. We’ll get back to this.
Each of the three settings above, ISO, shutter speed, and lens aperture, has one effect in common. Each of them effectively controls the amount of light used in an exposure. What’s more, they work together so that adjusting one up by one EV and another down by one EV will result in exactly the same amount of light being used. Stopping down the lens by one f-stop, from f/8 to f/11 say, will reduce the amount of light in the exposure by the same amount as reducing ISO from 800 to 400, or reducing the shutter speed from 1/250th to 1/125th of a second. Tthese adjustments each reduce the light by one stop, or one EV. Parenthetically, you can use either term to express this (one stop, or one EV), but most people use the term “stop”. If you did all three of these things you would reduce the light used in the exposure by a whopping three exposure values (stops)!
On film cameras (older ones at least) this is a little more straightforward because in general, they can be adjusted only in one-stop intervals. These cameras are where the concept of one stop or one EV came from. Digital cameras can usually be adjusted in intervals of one-third of a stop.
OK, you’ve been effectively introduced to the concept of exposure value. I’ll add to this discussion in future posts on each of the three primary adjustment mechanisms.