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Exposure Explained

This post is part 1 of a 4 part series:

  1. Exposure Explained
  2. Considering the Aperture Setting
  3. Selecting the Shutter Speed (coming soon)
  4. ISO (coming soon)

Exposure Explained

There are three variables you can manipulate in a camera when making a photograph--aperture, shutter speed and ISO speed. Striking equilibrium between the three is what gets you the ideal exposure. I refer to it as an equilibrium because you can raise one and lower another and still maintain the same exposure. Changing each of those variables however affects the image in differing ways.

First, let me try to explain a way to think about exposure, then I'lltouchon the most obvious ways that changing each of the three variables affect your captured image. I'm going to use the analogy of filling a container with water from a faucet. Think of the water in the container as light in your photograph. Too much water is overexposure and a half-full container is underexposure. So let's take a look at the three variables:

Aperture:The aperture is the diameter of the opening of your lens. (For the geek types reading this, it is expressed as a ratio of the focal length of the lensto thediameter of the opening.) Think of the aperture as the faucet in my analogy. If you open it only a tiny bit, the water trickles in, but if you open the faucet full blast, the water gushes out.

Shutter Speed:The shutter speed is a measure of time (most often expressed in fractions of a second). It is the length of time that the shutter remains open exposing your film or digital sensor to light. In our analogy, shutter speed equals the amount of time you leave the faucet running.

ISO Speed:ISO speed, is the measure of the film or sensor's sensitivity to light. So for our analogy, think of ISO speed as the size of the container. Higher ISO speeds require less light (water) so think shot glass. A lower ISO speed requires more light (water), think swimming pool. ISO stands for International Organization for Standardization.

So, if I have a bucket that I want to fill and I put it under a faucet set to a trickle, it will take a relatively long time to fill. If I twist that faucet wide open, the bucket will fill in a fraction of the time. Now, if I open the lens on my camera all the way (ƒ/1.4), it will expose my film in much less time than if I let in a trickle of light (ƒ/22). Make sense?

In upcoming posts, I will explain how changing each of these variables affects your images. This will help you determine which variables you'll want to change in which direction to achieve the optimum exposure and the desired visual effect.

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Reader Comments (2)

This will help me out, thinking like your analogy. However, a picture would be worth a thousand words, as you said you can get pretty much the same result with different mix of shutter; aperture; and ISO speeds; but they are not exact the same and can change the picture results. Can you find a venue that or website that has pictures of what the differences look like?

I am a complete newbie -- using a camera a long time with the AUTO function on, and experimenting with those other functions -- so what would be really useful is a list of different locations, venues, etc.. and what the different setting should be if you want to mimic or produce better results than the AUTO mode on my point and shoot camera (Kodak Z1015 IS).

June 3, 2009 | Unregistered CommenterGreg Jewett

Hi, I was looking for your writeup and explanation of shutter speed and ISO which were listed as coming soon. I couldn't find them. Have you posted it at all? Thanks. Your explanation of exposure and aperture helped me quite a bit in understanding as I'm a beginner and can't grasp the concept of exposure.

October 8, 2010 | Unregistered CommenterRachel

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