The Basics of Exposure

Lesson #1 in Photography for Ignorant Geniuses

Exposure is the one of the elemental theories of photography. Even though it is quite a complex subject, it is all perfectly logical. By the end of this first Photography for Ignorant Geniuses lesson you should be able to grasp the theory behind it.

Let’s start by looking at the basics of how a photo is taken. The principle of a camera has remained the same for centuries – we have a lens, a shutter and a sensor (or film, the concept remains the same).

The sensor records the light that lands on it through the lens, while the shutter keeps the light out. So when we press the shutter release to take a photo, it opens the shutter at a set diameter for a set period of time to allow the sensor to capture the image.

The basic concept of a photo

Exposure is the quantity of light we need to arrive at the photo we set out to take. An over-exposed photo has too much light and will contain areas which are washed out (white). An under-exposed photo is the exact opposite – too dark, usually with areas that are completely black.

Sometimes you might want to under-expose or (rarely) over-expose a photo for effect though. You wouldn’t do it with snap-shots of your children playing in the park, but a night shot can still look good if under-exposed in areas (as long as the subject of your photo is well-exposed).

An under-exposed vs a well-exposed photo

Cameras do help you out with achieving the right exposure, and every new generation of cameras brings about improvements to the technology used to select the best settings possible for the situation. Nevertheless, knowing what goes on behind the scenes will help you greatly for two main reasons:

  1. The ideal exposure for a camera’s “brain” is usually an evenly well-lit photo, but you might be after something which doesn’t fit the parameters the camera deems right – a silhouette is a classic example.
  2. You need more control over a camera’s settings in order to take good photos in every situation.
This photo is terribly under-exposed for a camera. It would never have taken it on an automatic setting, but it is exactly the effect I was after.


To understand exposure better, imagine that you have a glass you need to fill from a faucet. You must fill the glass up right to the brim, but ideally not splash any water on the surrounding areas. The size of the glass might vary – just like the correct exposure of the photo you need will change from situation to situation – but your goal is always the same: filling the glass to the brim without splashing.

The three factors (settings) that will determine a photo’s exposure are similar to the three variables you have in your average water system:

  • Aperture – This is the equivalent of how wide you open the faucet.
  • Shutter speed – Yes, you guessed it – this is the time you leave the faucet running.
  • ISO – Slightly more complicated to explain, but I like to compare this to adding a pressure pump to your water system. It will allow you to open the faucet for a shorter span, but there is a caveat – the more pressure you add to the water the more you risk splashing all over the place.

To get to the right exposure you can usually use a combination of values, the trick is finding the right one for the setting you are after.

Going back to the glass analogy can help us understand how these three work together:
I need to fill a beaker with water. Let us imagine that if I open the faucet completely it will take 5 seconds to fill. That means that if I opened it half way I would need to leave it open for 10 seconds. The result is the same, the glass is now full. If I turn the pressure pump on at 200% and leave the faucet half open, however, I can still bring my time back to 5 seconds – this is the equivalent of going from ISO 100 to 200.


Out of all the three settings, ISO seems to be the one that confuses people most. I personally  think it is definitely the most simple to understand. I will go into the nuances of aperture and shutter speed at a later stage, but to understand ISO I will just give one thing away. If you use a shutter speed which is too slow you risk ruining a shot because of camera shake.

ISO simply increases the sensor’s sensitivity artificially to help you take photos with less light. That is the only difference it makes on a photo, and there is only one caveat – the higher you pump up the ISO, the “grainier” your photos get. This happens because the camera pushes itself electronically to improve light, sacrificing quality in the process.

A crop of two photos, one taken at ISO 100 (top) and another at ISO 1600.
A crop of two photos, one taken at ISO 100 (top) and another at ISO 1600.

So the only lesson you need to know about ISO is that you should keep it as low as possible when you have enough light, and push it up as needed when you are in darker situations. Not all cameras perform the same under higher ISOs, in general the more you pay the higher you can afford to push it.

Remember: It is always better to have a noisy photo than not to have one at all, so don’t ignore a good opportunity just because you might compromise some of the quality.



It will take me a few more lessons to go into how aperture and shutter speed affect the aesthetics of your photos, but until next week try to set some time to browse your photos.

If you use a program like Picasa (which can be downloaded for free here) you can see a box with the basic settings (including the aperture, shutter speed and ISO) at which the photo was shot.

Look out for different exposures in photos you took – are some of your photos under- or over-exposed? Was your camera pushing ISO up too much, or not enough?

You can also take it a step further and browse photos on Most of them give the settings they were shot at if you click on “Actions -> EXIF info”.


If you have some spare time, look for the ISO feature on your camera. Shoot some photos around dusk on a setting that allows you to override the ISO setting. Start at the lowest ISO and shoot the same scene over and over again, going up a notch at a time up till the very top of the range available.

Once done, sit at a computer, view the pictures at full magnification (100%) and get a feeling of how bad “noise” in the photos gets as you increase ISO. Take note of the point at which photos are no longer acceptable and keep it in mind for the future.


Photography for Ignorant Geniuses is a set of tutorials I give out on a weekly basis about the science behind photography. For more information (and to subscribe) click here.

(#21 of 366 X 2012 project)

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