One of the main elements in producing a film of high-quality is exposure. It is therefore important to understand what good exposure is, how to avoid burned images, overexposure and underexposure. Contrary to popular belief, not everything can be fixed in post-production! As a definition:
Exposure is the amount of light reaching the sensor for a certain amount of time.
Hence, the art of getting good exposure is to control that amount of light. Not enough light can lead to a noisy, flat and grainy image whilst too much light can lead to both a faded and aggressive image. Either way, under or over-exposed images means that you will lose a lot of colour information from the image.
Four tools to control exposure: aperture, shutter speed, neutral filter (ND) and gain (ISO)
1. The most important is the opening of the optic (or”iris“). It is a circle in the optics, of variable size, through which light passes. The opening size is known as”f“. Paradoxically, the lower this f value is, the bigger the hole is (as in f/2.0 for example), letting a lot of light through. Conversely, the smaller the hole, the higher the f-value (e.g. f/11), leaving little light through. It’s what we call the diaphragm in photography. The opening also affects the depth of field. The larger the aperture, the shallower the depth of field, which means the area where the subject is in focus is restricted. Optics also do not have the same performance at all openings: they can lose sharpness, produce chromatic aberrations and so on. As a rule of thumb, the best performance is generally from f/2 to f/5.6.
2. The shutter speed controls how long light is allowed to enter the optics. The slower and shutter, the more light “stays”, although the blurrier a moving subject will be.
From a moving image perspective, we will play very little on this parameter because it affects the rendering of an image – however, it is far more crucial for stills photography for example. If for example you wanted to have a lot of light yet keep the lens wide open to have an artistic blur behind the subject, you may be tempted to increase the shutter speed (1/200th for example) to avoid burning your images – however, you risk obtaining an image that glitters and a very clinical render with all images movements being ultra sharp. The general rule to apply therefore is that of 180 degrees, and to use a shutter setting at twice your recording rate. For example, if you are shooting at 4K/25p your shutter should be at 1/50 (2×25). So what do you have to do to get a well-exposed image with a large aperture and without touching the shutter?
3. This is where the neutral density filter comes in. It’s a bit like a pair of sunglasses but without side effects (colour change, polarization etc.). They are used in very bright situations such as on a sunny day. This filter (which is fixed by step or variable), will thus allow to compensate the overexposure without touching either the opening, nor the shutter. An EVA1 has 3 integrated ND filters for example.
4. Gain or ISO – If you are in the opposite situation i.e. when you’re fully open, without an ND filter and the image is underexposed, the only way is to use an electrical amplification of the signal behind the sensor. By “boosting” the signal, we can compensate for the lack of light. But it has a price: the addition of digital noise to the image. That’s why Panasonic invented the dual-native ISO technology to restrict this noise to the image.
It is by playing on these 4 parameters (or rather 3 if we do not touch the shutter) in order to obtain a good exposure.