Getting Correct Exposures in Photography

A correctly exposed photo can be defined as one that is neither too bright nor too dark to see the subject(s) of the photo clearly. A photo where the subject is too bright is said to be ‘overexposed’, while one where the subject is too dark is said to be ‘underexposed’. Exactly what sort of exposure you get depends on the camera’s settings for ISO, aperture, and shutter speed. This is because these three things determine respectively how sensitive the sensor is to light, how much light hits the sensor, and for how long.

If the scene that is photographed has a wide range of tones from light to dark (think of a person at the snow wearing a dark jacket, standing with their back to the sun on a sunny day), then it’s quite possible that some of the elements in the scene will be either overexposed on underexposed, because the camera is physically incapable of capturing the full range of tones. That might not matter so long as exposure is ok for the subject. It might be a problem if the desire was to take a photo where all the details were visible, but this might be impossible. The user in this case has to make a choice of what to expose correctly.

In digital photography, the most important rule to observe when thinking about exposure is generally not to allow the highlights (i.e. the brightest points) in a scene to become overexposed to the point where they are what is called ‘blown out’. Highlights are blown out when they are so overexposed that no detail is recorded in them at all – they are completely white. While underexposure can often be rescued in image adjustment software such as Photoshop, blown highlights are impossible to recover. To guard against blown highlights, some cameras have a feature where they flash on and off when a photo that has them is displayed on the rear LCD screen.

Of course, exposure is also an aesthetic choice. Sometimes making something over- or under-exposed is part of the art of photography. For example, in ‘high key’ portrait photography, the background behind the subject is generally so overexposed that it is rendered completely white, but in this context the overall result is effective as it makes the subject stand out.