Exposure: the basics

The act of making an exposure (taking a picture) is collecting light in our cameras. Every time we push shutter of our cameras to take a picture, we’re opening up the “eye” of our cameras and collecting light inside.

a balanced exposure

To make a proper exposure, we have to let just the right amount of light accumulate in our camera. If we open that “eye” too wide or for too long, and let in too much light, our picture will be overexposed (too bright). If open that “eye” too narrow or for not long enough, our picture will be underexposed (too dark).

overexposed

underexposed

a balanced exposure

Our task, as photographers, is control our cameras to let in just the right amount of light, not too much and not too little. We do this by balancing two factors: rate and time. Rate of light refers to how much light we let in to our cameras at a time (how wide we open the eye), and time refers to how long allow light in to our cameras (how long we open the eye).

(rate of light) x (time) = (accumulated light)

At first it can be a bit difficult to wrap one’s mind around the concept of “accumulated light.” We aren’t accustomed to thinking of light in accumulated amounts. To help yourself understand this concept, let’s draw an analogy with something we do think of in amounts: water.

full glass - the right amount of accumulated light

Imagine that you want a full glass of water (our amount of accumulated light). You go up to the tap, and you can fill that glass a couple ways:

  1. Turn on the tap just a trickle, and fill the glass for a minute

    dripping tap

  2. Turn on the tap really strong, and fill the glass for just a moment

    strong tap

In the end, you have a full glass of water.

In the case of taking pictures, the strength of the tap is like how wide we open the “eye” of our cameras, how long we fill the glass is like how long we open that “eye.” You have to balance these too factors or else you’ll overfill the glass (your picture is overexposed – too bright) or you won’t fill it enough (your picture is underexposed – too dark).

So let’s look at how we actually control this on our cameras:

  1. “How wide we open the eye” / “how strong we turn on the tap” is controlled with a setting called Aperture or F-Stop.Aperture has a range of numbers, usually between 1.8 and 32, though these vary depending on the lens you use.A low number (1.8, 2.8, 3.5…) is open WIDE

    strong tap

    wide aperture

  2. A high number (18, 22, 28…) is open NARROW (dripping tap)

    dripping tap

    narrow aperture

  3. “How long we open the eye” / “how long we fill the glass” is controlled with a setting called Shutter Speed.Shutter speed has a range of numbers, usually ranging between 1 and 2000, approximately doubling with each step (1, 2, 4, 8, 15, 30…)Shutter speed numbers refer to fractions of a second so…A low number (8, 16, 30, 60…) refers to a LONGER/SLOWER timeA high number (500, 1000, 2000…) refers to a SHORTER/FASTER time(note: if you’re holding your camera by hand, it’s unwise to use anything slower/longer than 60)

But how do you know what the right amount of accumulated light is (or how big the glass we’re filling is)? The answer to this is with a tool inside your camera called a Light Meter. A light meter will measure the light in a given scene, compare it to your camera’s current aperture and shutter speed and tell you if your picture would be underexposed, overexposed or just right. You can look at your light meter and adjust accordingly. How a light meter looks will vary depending on your camera make and model, but usually they run along the bottom of the viewfinder and involve a plus (+) and minus (-) symbol.

light meter

So let’s try putting this to the test…

Suggested Activity:
(for the following activities, ensure that your ISO is set to 400 or 800, and that you’re shooting outside and in daylight)

  1. find a landscape scene (or at least a scene where you can focus far into the distance)take a picture with the aperture at its lowest
    take another picture with the aperture at its highest
    take another with shutter speed 250
  2. find a close-up scenetake a picture with the aperture at its lowest
    take a picture with the aperture at its highest
    take a picture with shutter speed 250
  3. shoot one each of the following:
  • something delicious with widest aperture
  • something disgusting with widest aperture
  • bird’s eye view at shutter speed 125
  • worm’s eye view with narrowest aperture
  • something fast at shutter speed 1000
  • something fast at shutter speed 60
  • something that makes you jealous with a fast shutter speed
  • something through a hole with a narrow aperture
  • something hidden with a narrow aperture
  • something in a reflection with a slow shutter speed

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