Exposure refers to the amount of light allowed into a camera shot. Vision is based on our ability to perceive light. When the eye looks at an object, we’re seeing the light reflected from it and that reflection is interpreted by the brain. That’s why it’s difficult to see in the dark - not enough light. Cameras, whether digital and film, still and motion, are based on the same principal: the lens is the eye, and the film or digital storage is the brain.
Exposure is a bit more complicated than that, though. When a camera operator thinks of exposure, they’re thinking of the amount of light let in through the lens based on the aperture setting used. The aperture is a part of a camera that functions more or less like your irises: the wider it is, the more light is let in. This also relates to the ‘depth of field,’ the area around the focal point that remains sharp.
When legendary director Stanley Kubrick was filming his adaptation of the 18th century novel 'Barry Lyndon' he, as lens expert Ed DiGuillo who worked on the extraordinary camera used on the film said, “wanted to preserve the natural patina and feeling of these old castles at night as they actually were”. Typically, a candle in shot may motivate the lighting, but in fact the illumination actually comes from an orange-gelled lamp just out of frame. Kubrick, however, wanted all the light in these scenes to come only from the candles themselves.
This required an innovative solution. Without going into the really technical aspects, the most sensitive film available in the mid 1970s had a stock exposure index of 100. This would require an aperture of 0.7. Remarkably Kubrick discovered that NASA had commissioned Carl Zeiss to build ten Planar 50mm f/0.7 stills lenses in the sixties, which were used to take photos of the dark side of the moon. Kubrick bought three of them.
The lens and BNC camera were then heavily adapted by the aforementioned Ed DiGuilio who had to adapt the mount and hack pieces from the camera to bring the lens close enough to the film plane to register the light! It was then a case of recalibrating the focus scale of the lens. The tiny depth of field that this resulted in massive challenges in keeping focus but the resulting softness of the low light gives the scenes a Vermeer like quality.
ISO and Depth of Field
If you adjust the sensitivity of the camera, called the ISO, you can use a smaller aperture to bring more of the image into focus. In this still from 'Moonlight' (2016), Mahershala Ali is in sharp focus while both the scenery behind him and the character at frame right are blurred. This is called ‘shallow’ depth of field because the zone in focus is very small, and it’s achieved by widening the aperture, allowing for a higher exposure.
As with all things photographic, balance is key. Too much light, and you risk overexposure, just as too little might leave your image underexposed. Overexposure renders highlights as disproportionately bright, while underexposure will result in muddy, blurry patches. Neither of these is necessarily a bad thing, however - depending on what the intention behind their use is. For example, cinematographer Vilmos Zsigmond overexposed shots of the aliens in 'Close Encounters of the Third Kind' (1977) to deliberately cast them as mysterious and otherworldly…
Dinner by Candlelight in 'Barry Lyndon' (1975)
The heavily customised BNC camera and NASA lens used on 'Barry Lyndon' (1975).
The Carl Zeiss Planar 50mm f/0.7. NASA used the 50mm to photograph the dark side of the moon in 1966.
Over Exposure in the Sci-Fi Classic 'Close Encounters of The Third Kind' (1977)