The Exposure Triangle: Aperture, ISO, and Shutter Speed Explained

The Exposure Triangle 

Are you one of the many photographers who prefer to shoot in fully automatic mode, but are curious about learning how to manually control your camera’s settings?  If you are, then learning about The Exposure Triangle is an essential piece to the puzzle.

Fully automatic mode is a quick and easy way to capture photographs, but it severely limits the creative possibilities of an image.  If you manually control your settings, you can purposefully under/overexpose parts of a photograph.  You can determine how much of your foreground and/or background is in focus (by controlling depth of field).  You can slow down the exposure to add the element of movement into the image.  And, with some lenses, you can even determine how soft or sharp the image will appear.  Furthermore, manually controlling your settings also permits the use filters on the front of your lens, vastly increasing your opportunities to manipulate the characteristics of a photograph.

I’ll readily admit it:  When I first began learning about photography, I was a bit overwhelmed, even intimidated, by the perceived complexity of manually controlling the exposure of a photograph.

But, my anxiety was quickly put to rest.  Basically, all it boils down to is understanding The Exposure Triangle (see the image above).  Once you learn the relationship between ISO, shutter speed, and aperture, you’ll be off and running with full command over your camera.  After some practice, you’ll know before you even take the shot how the exposure will look, based on the settings you choose.

The Essential Definitions of The Exposure Triangle.

Sensor:  For all intents and purposes, the sensor is the retina of the camera.  The sensor detects and conveys the information that constitutes an image.  In a digital camera, light passes through the lens, into the camera, and strikes the sensor.  The sensor converts these light waves into bursts of electric current which are sent to the camera’s processor.  The processor then does exactly what its name suggests: it processes the electronic information sent from the sensor into a digital image.

The imge sensor in a Nikon D600 and a Nikon D3200. Note the difference in size of each sensor.

Shutter:  There are two main types of shutters:

1) Mechanical shutters are what they sound like…they are sophisticated pieces of equipment designed to open and close with incredible precision.  Similar to the shutters you see in front of a window of a home, the camera’s shutter is responsible for blocking light from reaching the sensor.  While open, light is able to reach the sensor.  When closed, no light can pass through to reach the sensor.

When you press the button to take a picture with a camera that has a mechanical shutter, the little “click-clack” you hear is the shutter opening and closing.

There are different kinds of mechanical shutters, but that’s unimportant for now.  What’s important to understand is that all mechanical shutters, when closed, prevent any light from reaching the sensor.

A mechanical shutter.

2) Electronic shutters, when “open”, allow electricity to flow through the sensor (i.e. the sensor is “turned on”), thereby making the sensor sensitive to the light which passes through the lens.  When electricity stops flowing through the sensor, the shutter is “closed” and the sensor does not record light.

Many cameras employ a hybrid shutter combining the characteristics of both a mechanical and electronic shutter.

If you’re interested in learning about the advantages/disadvantages of a mechanical vs. electric shutter, check out this article.

Aperture:  Essentially, this is the pupil of your lens.  It is an opening, or a hole, where light passes through the lens.  Generally speaking, a wider aperture means more light will pass through the lens, a more narrow aperture means less light will pass through.

The aperture of a lens. Notice how the aperture decreases in size as the f-stop increases.

F-Stop:  The “f-stop” is a numerical value which denotes both the speed of the lens and the relative size of the aperture.  The lower the f-stop value, the wider the aperture and the faster your lens will operate.  The higher the f-stop value, the more narrow the aperture and the slower your lens will operate.

For example, let’s say your lens has a minimum f-stop of 2.8.  When you set the f-stop to 2.8, the aperture of your lens is as wide as it can get, and your lens will operate as fast as possible.  If you raise the f-stop value, you’re literally creating a more narrow aperture in your lens, while simultaneously slowing down the lens.

In case you’re curious, the actual f-stop numerical value is the measurement of the ratio of your lens’ focal length to the diameter of the aperture.  If this sounds confusing, don’t worry about it.  Just remember two important things about an f-stop: 1) The lower the f-stop number, the wider the aperture of your lens will be…and 2) The lower the f-stop number, the faster your lens will be.

Shutter Speed:  Basically, this is the measure in time of how fast the shutter in your camera will stay open.  For example, if your camera’s shutter speed is set to 1/30, when you press the button to take a photo, the shutter in your camera will open for exactly one thirtieth of a second.  A faster shutter speed means less light will reach the sensor, a slower shutter speed means more light will reach the sensor.

ISO:  The numerical measurement of the sensitivity of your camera’s sensor to light.  The higher the ISO value, the more sensitive to light your camera’s sensor will be.  In other words, if you lower the ISO, less light will reach the sensor.  If you raise the ISO, more light will reach the sensor.  

Think of it this way: Let’s hypothetically say your shutter speed is 1/30, f-stop is 5.6, and your ISO is 200.  You take a photo with these settings, but the photo is too dark.  You want to keep the shutter speed at 1/30 and f-stop at 5.6, but want a brighter photograph.  One way you can accomplish this is by raising ISO.  The sensor then becomes more sensitive, and will collect more light while the shutter is open for that same 1/30 of a second.

Unfortunately, raising ISO comes at a price.  When the ISO goes up, and the sensor becomes more sensitive to light, it is sensitive to all kinds of light…both good and bad.  In other words, at a high ISO, the sensor will pick up more unwanted “noise” (noise in a photograph looks similar to the static you see on an analog TV when it gets poor reception) in the image than it would at a lower ISO.  For this reason, photographers usually prefer to work at the lowest possible ISO setting they can get away with.  In a brightly lit scene (i.e. shooting outside in the middle of the day), you can usually work at or near the lowest ISO setting your camera allows.  But in a darker scene (i.e. inside your home without a flash), you may need to bump up the ISO.

I like to use the following metaphor to describe the relationship between aperture, shutter speed, and ISO:  

Imagine holding a water hose…and imagine that the amount of water coming out of the hose is the same thing as the amount of light reaching the camera’s sensor.  

The size of the hose opening is the aperture, the amount of time you open the water valve is the shutter speed, and the pressure of the water passing through the hose is the ISO.

If you widen the diameter of the hose opening (i.e. widen aperture), more water will pass through the hose.  If you leave the water valve open for a longer period of time (i.e. slow down shutter speed), more water will pass through.  If you make the pressure of the water higher (i.e. raise ISO), more water will pass through.

Now, try this exercise with your camera:

To start, put your camera in Aperture Priority Mode.  This mode gives you manual control over the f-stop, while the camera automatically chooses shutter speed.  Then set your camera to allow manual control over ISO.

While looking at your camera’s shutter speed, raise your f-stop (and keep ISO constant). As you raise the f-stop, you will see the camera slow down the shutter speed to compensate.  This demonstrates the relationship between aperture and shutter speed.  As f-stop goes up, shutter speed must slow down to maintain proper exposure.

This makes sense if you think about it:  Raising f-stop narrows the aperture (slows down the lens), which causes less light to reach the camera’s sensor.  If less light reaches the sensor, the shutter needs to stay open longer.  So, to compensate, the camera will automatically slow shutter speed.

Now, while still looking at your camera’s shutter speed, raise ISO (and keep aperture constant).  When you bump up ISO, you will see your camera’s shutter speed get faster.  Think about it:  Raising ISO means the sensor is more sensitive to light (it will pick up more light).  Since the sensor needs less time now to collect light, the camera quickens the shutter speed to maintain proper exposure.

I know this is quite a bit of information to absorb if you haven’t read about these topics before.  I still remember the very first night I began learning about photography.  I sat down in front of my computer and googled “Learning About Photography”.  The first article that came up discussed the relationships between aperture, ISO, and shutter speed.

(The first article I read was on Ken Rockwell’s website.  Ken’s online personality is intentionally abrasive and pretentious, and his website looks like it was designed in 1986 on an Atari 5200, but I have to hand it to him: He does a great job explaining the fundamentals of photography)

I discovered that the quickest and most efficient way to memorize the relationship between aperture, ISO and shutter speed is to go out and practice taking shots in Aperture Priority Mode.  Between shots, adjust the f-stop and/or ISO to see how the camera compensates by changing shutter speed.

Once you get The Exposure Triangle down cold, you essentially have complete control over your camera!

Happy Shooting!






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