Exposure is the amount of light a negative from a roll of film, or a sensor from a digital camera captures when a photo is taken.

If you take a photo that is underexposed, it means that the photo is too dark (compared to what you saw with your own eyes). When a photo is underexposed there is normally not enough detail in the dark areas. A black pair of trousers in an underexposed shot may appear completely black, with no details of the creases or texture. If you shoot Raw images as oppose to JPG you can bring back some of the detail by lightening the shot in Raw processing software, such as Adobe Lightroom 5, but you will introduce some digital noise into the shot by doing so.

An over exposed shot is where the white or light areas in the shot are ‘blow-out’. This means that the lighter areas may have lost detail.
Some photographers believe that it is better to slightly underexpose a shot rather than over expose it. I think this entirely depends on the type of shot you are taking. Slightly underexposing a wedding dress to make sure you have all the details may be a good idea, but underexposing a pair of black trousers in a fashion shoot may render your shots useless. It’s best to get the right exposure in camera in the first place if you can.

Three things can control exposure - your aperture, shutter speed and ISO.

Which one you choose, depends on what you are photographing. If you are a sports photographer that relies on fast shutter speeds, then you are more likely to use your aperture to affect the exposure than the shutter speed. On the other hand if you are a portrait photographer, you may choose your ISO or Shutter speed instead, so that you can still choose the aperture you want for that shot.

There are a few ways to check the exposure is right. I normally use the histogram, which is a graphical representation of the light, mid tone and dark areas in your image. Not all cameras will have the option of turning on the histogram, but if yours does, set it to the black and white (not RGB) histogram.

The left side of the histogram represents the black area and the right the white. If you find it hard to remember just think of black and white film. If you wrote down “black and white film” the black is on the left and the white on the right.

If you have ‘blown-out’ the whites in your shot the data in the histogram will bunch up toward the right. Of course if you were to take a shot of a white piece of material, then you may have a histogram that looks like that too. What you are trying to avoid, is the data in the histogram scaling the right side wall of the histogram, as that represents blown-out highlights. If the data for the white area stays just short of the right side you should be fine. Normally you want the data to be evenly spaced over the entire histogram, but if you have a dominance of light or dark it will not be evenly spaced, so bear that in mind.

The best way to practice understanding the histogram is to take some shots of grey, white and black materials (filling the frame). Look at the histogram after each shot and analyse it. Under and over expose some shots so that you see how the histogram shows the changes.

There are times when you will need to under or over expose shots. A classic example is a shot of white snow. A camera likes to meter off something that is mid grey to get the correct exposure, so if you meter off white snow you will get grey snow. The way to correct this, is either to over expose the shot till you get white snow, meter off something that is mid-grey or use a grey card if you have one (you often get a grey side on the collapsible white balance cards you can buy). Another option is to meter off you're the palm of your hand if you have white skin as this will be closer to mid grey than the snow. When you do get white snow in your shot remember to check the histogram to double check that it is not over exposed and you have all the details.

If you are taking a shot of very dark things, then your camera will tend to over expose them to get the mid-grey that it thinks you are metering from. In this case you will need to under expose to preserve the rich dark colours, or meter off something that is mid-grey.

Not all shots should be as you see them. Occasionally you may want to under expose intentionally, to create a more dramatic result. Think of those great big dark storm clouds you often see in black and white landscape shots. Often these are underexposed to exaggerate the light and shadow and create a more dramatic result.

One thing that you should never rely on are the JPG previews that the camera gives shows you on the screen after you have taken your photo. Even if you are shooting RAW you will still only see JPG previews. These are unreliable and do not give you a good indication of a well-exposed shot.

Trust your histogram and you will soon find that you are getting better shots!