The title of this post is probably deceiving (a camera needs enough light to produce an image correctly exposed), but it give us a reason to learn more about exposure, aperture, sensitivity and noise.

Despite the nature of the photo sensitive surface that captures the projected image in the camera – film, plate or digital sensor – we will agree to name it photo-sensor or simply sensor. And, for the moment, we will ignore any technical details related to the sensor itself other than its role in capturing the image when a certain amount of light enters into the camera.

Here “amount of light” refers not to the intensity (i.e. brightness) of the image (as some may wrongly believe) but to the luminous exposure (directly related to luminous energy) that is the product between the illuminance (a measure of the intensity of the incident light) and exposure time (measured in seconds):

H_v = E_v × t

where

• H_v is the luminous exposure (usually in lux seconds);
• E_v is the image-plane illuminance (usually in lux);
• t is the exposure time (in seconds).

I know, it sounds a bit too academic but, believe me, this is the essence of the photo exposure process. In plain English, to obtain “a certain amount of light” (H_v), we either need a brighter image or the image has to stay longer on the photo sensor in order to produce the same result. As consequence, if more light is needed to produce the right image, we have essentially two options:

• enlarge the hole or;
• keep the hole open a longer time.

For example, to achieve the same result you can either use a hole that provides an image two times brighter or use an exposure interval two times longer (the reciprocity principle that works really well under normal conditions).

If not enough light reaches the sensor we will get a darker image or an underexposed image. In such cases, the details in the shadows are lost (they all look black). If too much light is present we will obtain a lighter image or an overexposed image where all the details in bright light are burnt out (they all look white). Please remember that when we refer at light in photography we think at the received luminous energy that takes into consideration both the intensity and the exposure interval.

Normal exposure

Overexposure

Underexposure