What a busy, crazy month its been since I wrote about wanting to post photography fundamentals here on Travelomo.com. Sorry if I’ve kept you waiting, but here we go. I’d like to start out this series with a topic that befuddles many beginners: exposure.

Details in both shadow and highlight make this a properly exposed photo

Definition of Exposure

So, what is exposure? The definition of exposure, according to Wikipedia, is “the total amount of light allowed to fall on the photographic medium  during the process of taking a photograph.” In simpler words, it’s the amount of light that hits your film to produce an image.

Three variables determine a photograph’s exposure. The first is aperture (which is just another word for “opening”). The second is shutter speed (the amount of time your camera lets in light). The third is ISO (the sensitivity of your film to light). The first two variables regulate the amount of light that enters a camera. The third determines how much light is needed to burn an image onto the recording medium (the higher the ISO, the less light needed for a shot). Understanding the relationship among these three variables is fundamental to our understanding of what exposure is and how to control it.

The Drinking Glass Analogy

The Drinking Glass Analogy

There are several analogies used to illustrate this relationship, but my favorite is the one that uses a drinking glass, a faucet and some water.  You can actually go and do this in real life, just to drive the point in.

The drinking glass represents a blank frame of film, while the water represents light. In order to create a photo, you have to fill that glass up by opening the tap and letting the water flow. Now, you can open the tap just a little bit to let the water drip out slowly. Or, you can open it wide to fill that glass up fast. The tap represents your first control, aperture. The second control is shutter speed, which is represented by the time you keep the faucet running. The third variable, ISO, is represented by the size of your glass. If ISO 50 is a huge Slurpee cup, ISO 800 is a shot glass.

In general, a properly-exposed photograph adequately shows detail in both the shadows and highlights of a photo. This means your skies aren’t a blank expanse of white nor your shadows an impenetrable field of black. Going back to the analogy, proper exposure means your glass is full. Pour water to fill up just half the glass and you’ve got a photo that is underexposed. Keep the tap running until the water flows over the brim and your photo is overexposed.

That’s where the relationship of aperture, shutter speed and ISO comes into play. Your job as the photographer is to regulate the amount of light that enters your camera and make sure you don’t put in too much or too little. At this point it isn’t too important to know how long you should keep the tap open or how wide you should open it. All that matters is that you fill up that glass.

Here are some samples of “good” and “bad” exposure from my files.

Good: Average exposure thanks to the Fuji Silvi P&S

Good: Point & Shoots typically expose for even results.

Good: Nice even colors under diffused sun

Underexposed: Carpark details lost in shadow


Underexposed: Neither subject nor background are adequately lit

Underexposed: Totally lost the subject's features

Overexposed: Manual shooting means mistakes will be made sometimes

Overexposed: I underestimated the morning sun and got burned

Overexposed: I meant for this to be slightly underexposed but made an error in aperture selection

Hope these samples help. If you’ve got any of your own, feel free to post them in the comments so we can talk about them.

To recap:

Drinking Glass = Frame of Film

Water = Light

Faucet = Aperture

Time Faucet is Open = Shutter Speed

Size of Drinking Glass = ISO

The wider your aperture, the less time (faster shutter speed) you’ll need to properly expose your film.

The slower your shutter speed (shutter is open longer), the narrower your aperture needs to be to properly expose your film.

The lower your ISO, the more light is needed to properly expose your film.

Now the big question is: How much water does it take to fill up that glass?

We’ll save that for next time. Class dismissed.