Back to Basics: Exposure

Exposure is one of those fundamental concepts of photography that many people struggle with. Get it right and you have a beautifully lit image. Get it wrong and you end up with a ruined shot because it is either too bright, pure white in extreme cases or too dark, maybe even black. We’ve all had them.

The big question is what causes under or over exposure and how can it be avoided?

A Bucket of Light

One of the best analogies for exposure that I’ve heard was to consider the camera’s sensor or film as a bucket that you need to fill up. When the bucket is full to the brim with water so that not a single drop more can be added without causing it to spill you have the perfect exposure.

If the bucket overflows, the image is over exposed.

If the bucket isn’t full, the image is under exposed.

The trick is to get it just right. The complication is that the tap we’re using to fill our bucket is either on or off. There is no slowing down the flow of water as we near the top of the bucket. The tap is either on or off.

This is exactly how light behaves when we take a photo. It either falls onto the sensor or film or it doesn’t. What the camera does is to determine how long to leave the tap open so the bucket fills to the brim with the correct amount of light to give us the perfect picture. Not a drop more, not a drop less.

Making a Correct Exposure

When you fill a bucket of water you need to keep you eye on the rising water level. If you don’t the water will spill when it overflows.

This is exactly what the camera does, albeit much, much faster. When you press the button to take a picture the camera uses all of it’s clever electronics and software to measure how much light there is and work out how long to leave the tap on for. It knows how big the bucket is, it measures the flow of water and then calculates how long to leave the tap on for. Or, in photographic terms, the camera knows how sensitive the sensor is to light, it has metered the scene to ascertain how much light there is so it can calculate how long to leave the shutter open for to give a perfect exposure.

Simple. Well, not quite. All cameras, no matter how modern or expensive cannot deal with every type of scene. They can and do get it wrong.

Conclusion

This analogy is a big simplification of what really happens but at this stage it is the concept that is important. Once you understand the fundamental concepts of exposure it is then easy to correct for when the camera gets things wrong. And they do. Frequently.

Can you correct for it? Yes. But that discussion will be the subject of another post.