The Exposure Trio

The Exposure Trio

A Three Part Series

Part One: The Basics


Photographers who understand how ISO, Aperture and Shutter Speed work together can become masters at controlling exposure and take complete creative control of the image making process.

Lets start this short discussion on the exposure trio by defining ISO.

ISO is a measurement specified by the International Organization for Standards for light sensitivity of photographic emulsions, or in the digital camera world, the light sensitivity of the image capture sensor.

In simpler terms, examples of film ISO ratings include 50, 100, 200, 400, 800 . . . and up. This ISO rating, formerly ASA, rates the speed—sensitivity—of the film emulsion. I.e., ISO 200 is twice as sensitive to light (commonly termed, “twice as fast”) as ISO 100. And ISO 400 is twice as fast as ISO 200, and so on.

In a digital camera we can adjust the light sensitivity of the sensor to these same ISO standards. This is a huge benefit over film. I can use ISO 100 outside in bright sunlight, and then walk into a dark auditorium and bump the ISO up to 800 without changing film. All I do is change a setting on my digital camera.

Sounds great, but in practice, there’s a challenge. The higher I push my ISO setting the more digital noise the camera’s sensor creates. Digital noise is similar to film grain—only with colored dots replacing the grain. (Compare Image 1 made at ISO 100, and Image 2 made at ISO 1600.)

Image 1 ISO100 (Low Noise)

Image 2 ISO 1600 (High Noise)

Needless to say, noise is not desirable. The challenge is to use as low an ISO as is possible to create the image I want.

As a rule start with your camera set at it’s lowest ISO setting, in my case ISO 100. Only raise the ISO when your camera tells you it needs more sensitivity. And yes, as we’ll discover later in this series, your camera will tell you when to increase ISO.

So if ISO controls how sensitive your camera is to light, what does APERTURE do?

A film or digital SLR camera lens has an iris style diaphragm inside. This diaphragm opens and closes in steps from largest opening (maximum aperture) to smallest opening (minimum aperture). The size of the aperture opening is measured in f-stops.

Here’s where aperture gets a little confusing. As Images 3 and 4 show, the smaller the f-number, the larger the aperture opening. So a lens aperture of f/2.8 lets in more light than an aperture of f/8. In fact, f/2.8 lets in eight times the volume of light as f/8.

F/4 Aperture

F/16 Aperture

In this table the lens opening—aperture—becomes smaller from left to right. Each preceding f-stop allows twice the light through the lens as the one after it.

F/1.4… f2… F/2.8… f/4… f/5.6… f/8… f/11… f/16… f/22

You also need to realize all lenses are not created equal. When comparing lenses, one with a larger maximum aperture is said to be faster. So a lens with a maximum aperture of f/2.8 is faster than a lens that has a maximum aperture of f/5.6.

Generally, the faster the lens, the more money it costs. Later in this series well talk about when it’s an advantage to own fast glass.

So now we know we can use the ISO setting to adjust our camera’s sensitivity to light, and we can change how big of an opening we allow light to pass through by adjusting the aperture.

All that’s left of our Exposure Trio is SHUTTER SPEED.

Simply put, each time you press your camera’s shutter release you open a curtain (shutter) and allow light to strike the sensor for an exact amount of time. Many of today’s cameras can automatically open this curtain for as brief as 1/8000 of a second to as long as 30-minutes.

Let’s summarize…

ISO is a measure of how sensitive our digital sensor is to light. The goal is to keep the camera set at as low an ISO as possible to limit digital noise.

Aperture refers to an adjustable iris inside the camera lens that controls the size opening we allow light to pass through. This opening is adjusted by changing the aperture size as measured by f-stops.

Shutter speed is how long we expose the camera’s sensor to the light passing through the aperture opening.

I know, I know, for many of us this seems very basic. But for all of us, understanding these three basic adjustments not only gives us the ability to control exposure, but more importantly, the key to actually creating images.

The next two installments will cover the creative use of aperture to control depth of field, and shutter speed to control motion.

Until then,



One Response to “The Exposure Trio”

  1. rommelrutor Says:

    Reblogged this on Words and Images.

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