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Bryan Peterson has written a book entitled Understanding Exposure. I highly recommend you read it if you want to venture off of your digital camera’s Auto mode and start experimenting with its manual settings.
In Understanding Exposure, Bryan illustrates the three main elements that need to be considered when setting your exposure. He calls them the “exposure triangle.”
Each of the three aspects of the triangle relates to light and how it enters and interacts with your camera.
So if you’re ready to become an expert in exposure…
The exposure triangle has three corners:
It is at the intersection of these three elements that an image’s exposure is determined.
Now, exposure refers to the overall brightness of an image.
So depending on your camera settings, you might end up with an exposure like this, which is too bright:
Or an exposure like this, which is too dark:
Or an exposure like this, which is just right:
Here’s the most important thing to remember:
A change in one of the elements will impact the others.
This means you can never really isolate just one part of the exposure triangle. You need to always have each corner of the exposure triangle in the back of your mind.
Many people describe the relationship between ISO, aperture, and shutter speed using different, easy-to-follow metaphors. And in the next section, I’ll share with you three of those metaphors.
A quick word of warning first, though:
Like most metaphors, these are far from perfect and are just for illustrative purposes. So learn from them, but make sure you don’t take them too seriously.
Imagine your camera is like a window with shutters that open and close.
The aperture is the size of the window. If the window is bigger, then more light gets through and the room is brighter.
Shutter speed is the amount of time that the shutters of the window are open. The longer you leave the shutters open, the more light that comes in.
Now imagine you’re inside the room and are wearing sunglasses (hopefully this isn’t too much of a stretch!). The sunglasses desensitize your eyes to the light that comes in, and this represents a low ISO.
There are a number of ways to increase the amount of apparent light in the room. You could increase the time that the shutters are open (i.e., decrease the shutter speed), you could increase the size of the window (i.e., increase aperture), or you could take off your sunglasses (i.e., increase the ISO).
It’s not a perfect illustration, but you get the idea.
Another way to think about exposure in photography is to think about taking a photo as getting a suntan.
Now, a suntan is something I always wanted when I was growing up. But unfortunately, because I was very fair-skinned, it was something that I never really achieved. All I did was get burned when I went out into the sun. In a sense, skin sensitivity is like an ISO rating, because some people are more sensitive to the sun than others.
Shutter speed, in this metaphor, is the length of time you spend outside. The longer you stay in the sun, the higher your chances of getting a tan (of course, spending too long in the sun can mean being overexposed!).
Aperture is like sunscreen that you apply to your skin. Sunscreen blocks the sun at different rates, depending on its strength.
Apply a high-strength sunscreen, and you decrease the amount of sunlight that gets through. As a result, even a person with highly sensitive skin can spend more time in the sun. (In photography terms: Decrease the aperture, and you can slow down the shutter speed and/or increase the ISO).
A third metaphor I’ve heard used is the garden hose.
Here, the circumference of the hose nozzle is the aperture, the time that the hose is left on is the shutter speed, and the pressure of the water is ISO.
If you increase the circumference of the nozzle, increase the length of time the hose is left on, and increase the water pressure, then your garden is going to get really wet (i.e., it’ll get overexposed). But if you decrease the circumference of the nozzle, shorten the time the hose is left on, or decrease the water pressure, your garden will stay relatively dry (i.e., it’ll get underexposed).
The key is to find a nice balance of nozzle size, length of time, and water pressure; that way, you can have a perfectly-watered (i.e., well-exposed!) garden.
As I’ve said, none of the metaphors are perfect. But they all illustrate the interconnectedness of shutter speed, aperture, and ISO on your digital camera.
Mastering the art of exposure is something that takes a lot of practice. In many ways, it’s a juggling act, and even the most experienced photographers experiment and tweak their settings as they go.
Keep in mind that changing each element doesn’t just impact the exposure of the image. Each exposure element influences other aspects of your photo, as well.
The great thing about digital cameras is that they’re ideal for learning about exposure. You can take as many shots as you like at no cost. Plus, digital cameras generally have semi-automatic modes like Aperture Priority and Shutter Priority, which allow you to make decisions about one or two elements of the exposure triangle while the camera handles the rest.
A lot more can be said about each of the three elements in the exposure triangle. So check out these articles, which cover each point of the triangle in greater depth: