Shedding Light on the Exposure Triangle

I had someone ask me a question about exposure the other day, and it became clear that they did not understand how aperture, shutter speed, and ISO all worked together to create (or not) a good exposure. Photographers refer to these three variables as “the exposure triangle.” Here’s how they work.

First you have to understand that a good exposure is made simply by allowing the right amount of light to reach the sensor (or the film plane, if, like me, you hearken back to primordial times). The key phrase in that last sentence was “the right amount of light.” Too much light, and your image is overexposed. Too little, and it is underexposed. Simple enough, right?

So, how do we control the amount of light that reaches our sensor? Through the combination of aperture, shutter speed, and ISO. Here’s a quick breakdown of each:

Aperture: Aperture is simply the size of the hole through which light passes to reach the sensor. Obviously, a larger hole lets in more light, a smaller hole lets in less light. Aperture is measured in f-stops, where the larger the number (for instance, f/22), the smaller the hole, and the smaller the number (for instance, f/2.8), the larger the hole.

Shutter Speed: Shutter speed is simply the length of time the shutter stays open to allow light to pass. Think of your shutter as the curtain on your window. Open it and more light washes into your room, close it and less light gets in. In your camera, the shutter opens when you press the shutter release. The longer the shutter stays open, the more light reaches the sensor, and the less time the shutter stays open, the less light reaches the sensor. Shutter speeds range from very long (for instance, 30 seconds) to extremely short (1/8000 of a second).

Now, before we get to ISO, let’s play with these two variables a bit. You’ll often hear photographers talk about “stops” of light, which is a measurable amount of light. As it pertains to aperture, the f/stops you use to control the size of the hole through which light passes on its way to the camera’s sensor allow measured amounts of light. The difference between f/2.8 and f/4 is exactly one stop of light, and the difference between f/4 and f/5.6 is exactly one stop of light.

Here are your basic f/stop settings, each one representing, by exactly one stop of light, an increase (if you move toward the left) or decrease (if you move toward the right) in the amount of light reaching the sensor.

f/1    f/1.4    f/2    f/2.8    f/4    f/5.6    f/8    f/11    f/16   f/22    f/32    f/45

As we said, the amount of light hitting the sensor at f/4 is one stop less than the amount of light hitting the sensor at f/2.8. It’s important to note that these stops represent a doubling (if you’re moving toward the left) or a halving (if you’re moving toward the right) of the amount of light reaching the sensor. Thus, the amount of light reaching the sensor at f/8 is exactly half the amount of light reaching the sensor at f/5.6, and the amount of light reaching the sensor at f/4 is exactly double the amount of light reaching the sensor at f/5.6.

Be aware that most digital cameras have the ability to adjust the aperture of a lens in increments of 1/3 or 1/2 stop as well. The 1/3-stop increments between each of these numbers are as follows:

f/2.8    f/3.2    f/3.5    f/4    f/4.5    f/5.0    f/5.6    f/6.3    f/7.1    f/8    f/9    f/10   f/11    f/13    f/14    f/16    f/18    f/20    f/22

Now, your shutter speed also measures light in stops. Here are the standard shutter speeds, in seconds, each one representing, by exactly one stop of light, an increase (if you move toward the left) or decrease (if you move toward the right) in the amount of light reaching the sensor.

1    1/2    1/4    1/8    1/15    1/30    1/60    1/125    1/250    1/500    1/1000

And, just like with aperture, each stop doubles the amount of the light reaching the sensor if you move toward the left, or halves the amount of light reaching the sensor if you move toward the right.

Okay, now let’s put this into practice. Say you’re shooting your son’s football game on a Saturday afternoon. For the sake of illustration, let’s say you’re shooting with your camera on manual mode. You take a shot of your son running the football toward the end zone, check the LCD on the back of your camera and notice that, while the photo seems to be exposed correctly, your son is blurry. You check the shutter speed and aperture and notice that you’re shooting at f/11 and 1/125 of a second. Clearly you need a faster shutter speed. So you increase your shutter speed to 1/500 of a second. You’ve now decreased the amount of light hitting the sensor by two full stops. If you leave your aperture at f/11, your next image will be underexposed by two full stops. The photo will look bad and your spouse will be mad at you for botching little Johnny’s football photos. So you decide to balance the two-stop loss you’ve created when you increased the the shutter speed by opening up the aperture by two full stops. So you switch the aperture from f/11 to f/5.6, which gives you a nice, properly exposed photo once again.

Johnny looks much better, but he’s still a bit blurrier than you’d like, so you think about dropping the shutter to 1/1000 of a second, losing another stop of light. Quickly, because you’re getting the hang of this now, you attempt to open the aperture another stop, to f/4, which should make everything wonderful.

But, the kit lens you’re using with your camera doesn’t open up any further than f/5.6. And you really need a faster shutter speed to freeze Johnny’s motion as he breaks for the winning touchdown. What on earth do you do?

This is where ISO comes into play. Remember, ISO is the third corner of the exposure triangle. Back in the days of yore, film had a speed rating, measured as ASA, which stands for American Standards Association. ASA basically told you how sensitive the film was to light. ASA 800 speed film was “faster” than ASA 400 speed film, meaning that you could use it in lower light (because it was more sensitive to light) and thus use a “faster” shutter speed. When we changed to digital, the ASA system was superseded internationally by the International Standards Organization (ISO). With digital cameras, you can basically adjust your camera sensor’s sensitivity to light. As you might guess, ISO settings are also measured in stops of light. Your basic ISO settings are as follows:

100    200    400    800    1600    3200    6400

And, just like aperture and shutter speed, most digital cameras today allow you to adjust ISO in 1/2 or 1/3 stops as well. So, a setting of ISO 800 gives you one stop more light than a setting of ISO 400, because the sensor is now twice as sensitive as it was at ISO 400. Note though that unlike aperture and shutter speed, more light doesn’t reach the sensor at ISO 800 than it does at ISO 400. The same amount of light is reaching the sensor. But the sensor at ISO 800 is more sensitive to the light that does reach it, so it will expose the image at a higher relative light value. So an image at f/4 and 1/125 at ISO 800 will be one stop brighter in appearance than the same image at f/4 and 1/125 at ISO 400.

So, back to the football game. You’re at f/5.6 and 1/500. Let’s say your ISO is set at 200. You want to move the shutter speed to 1/1000, which gives you one stop less light. You’d open up the aperture to f/4, but the lens you’re using doesn’t open up any wider than f/5.6. So you need one stop of additional light to let you go to 1/1000. Quickly (because Johnny’s only 20 yards from the end zone and running quickly) you dial up your ISO to 400, which effectively gives you one stop more light, move your shutter speed to 1/1000 and fire off a quick burst of photos as your manchild crosses the goal line for the winning touchdown. Your spouse is thrilled with the photos, your son has a new manly man photo for his Facebook page, and you’ve become the best photographer in the world. Marital and family bliss has been cemented, and all is right with the world.

All because you understand the power of the exposure triangle.

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