Calculating Stops

Understanding stops (and how to calculate them) is a very important skill that is becoming all but lost in the age of built in exposure meters. Not that there’s anything wrong with letting the camera do the mundane work, but what happens when you’re going for a shot that’s outside the ability of the camera to calculate? For instance, with night photography or when using a neutral density filter to get that nice cotton candy water effect your camera may not have enough light to figure out what’s best and suddenly the weight is on your shoulders. The good news is that it’s pretty simple if you know how to multiply and divide by 2.

A Brief Aside on Correct Exposure
Correct exposure is subjective, but when speaking in normal terms, a well exposed image has the shadows just above black and the highlights just short of being blown out. (This post is about figuring out equivalent exposures so I’m going to assume you already know how to identify a good exposure.) When taking a picture we have three things we can adjust: the amount of light we let onto the sensor (Aperture), the length of time that light is allowed in (Shutter Speed), and how sensitive our sensor is to the light hitting it (ISO.) When you combine those three things you get the Exposure.

The Stop
When talking to other photographers you’ll hear things like “expose it a stop higher” or “when shooting HDR, shoot a normal exposure and shots a few stops higher and lower.” What does that mean? It means make the exposure lighter or darker, but beyond that, it means by a specific amount. A stop is a relative measurement of half or double an amount of light and it’s useful for quantifying how much of an adjustment we’re talking about. (BTW, you’ll sometimes see stops referred to as an Exposure Value or EV.)

The Equation
Here’s what you need to know. Doubling any one of the three exposure controls increases the exposure by one stop while halving them decreases the exposure by one stop.

• Shutter speed is easy. If you’re shooting 1/100, then 1/200 is a stop darker and 1/50 is a stop lighter.
• ISO is easy too. If you’re shooting at ISO400 you can stop down to ISO200 for a darker image or up to ISO800 for a brighter image.
• That leaves us with aperture.  Aperture is referring to the size of hole in the lens that the light passes through. If you double the size of that hole, you let in double the light. Halve the size and you halve the light. Easy! Aperture is a simple concept, but can be confusing because of how it’s measured. Let’s stick with simplicity for the purpose of this post and not dive into why those numbers are measured the way they are (I’ll cover that in-depth elsewhere, but the short version is that it’s a fraction.) Just know that the bigger number means a smaller opening and a smaller number means a bigger opening (so f/2.8 lets in a lot more light than f/11.) Some of the common numbers you’ll see are: f/2.8, f/4, f/5.6, f/8, f/11, f/16. (Each jump in that list represents a stop, so f/2.8 to f/4 is one stop. f/5.6 to f/11 is 2 stops. And if you have trouble with memorizing that list, just remember 2 and 2.8 and you can figure out the rest from there by doubling – 2, 4, 8, 16 and 2.8, 5.6, 11)

Scenarios
So, you’re shooting at night and you take a shot at 30 seconds, F/11, and 200ISO. Your image looks okay, but your friend shooting nearby says it’d look great if it were 2 stops brighter. So what can you do?

– Change the shutterspeed by 2 stops. Doubling it once gets you 1 minute. Again gets you 2 minutes.

– You could increase the ISO by 2 stops. From 200 to 400 to 800.

– You could open your aperture by 2 stops. From f/11 to f/8 to f/5.6.

All of those examples will work to brighten the exposure, but you could also make smaller adjustments to multiple settings to get two stops. For example, increase the shutter speed to 1 minute and increase the ISO to 400.

Go Practice!
The math is easy, it just takes a little bit of practice and you’ll be calculating stops in your head in no time. One of the best resources I’ve found for online practice is the DSL Simulator at CameraSim.com. Give it a try!