Fundamentals of exposure
Sometimes, a friend asks me how to take some shot or why a photo looks awful even if the scene was superb. I have to confess that, often, I know too well the reason of the disaster and the source of embarrassment. The fact is, simply speaking, a poor understanding of exposure.
The whole idea of a photo is to capture a quantity of light with a light-sensible medium. Today, this is the sensor of our camera.
The right quantity of light is given by the light meter, which is integrated in our camers. Well, this is not exactly true, but let’s assume we know the right quantity of light we need – maybe, I will write more about how to use the light meter another time…
- ISO speed: it measures how much light the sensor captures per time unit. Traditionally it gets measured in STOPs: …, 100, 200, 400, 800, …
- shutter speed: it measures how much time the shutter is left open, allowing light to reach and impress the sensor. It is measured in fractions of seconds, and traditionally ordered in STOPS: …, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, …
- diaphragm aperture: it measures the diameter of the hole left open to allow light to pass through the lens. It is measured by a strange ratio, indicated by f/4, where the number indicates the value. Traditionally (surprise!), the measured in STOPS: …, f/11, f/8, f/5.6, f/4, …
A simple trick to remember the values of the various STOPS is to remember just one base number, e.g., 100 for ISO, 1/125 for shutter speed, f/5.6 for aperture. Then, as most cameras allow to increment/decrement STOPs by one-third, simply starting from the base number, we count three ticks of the camera to get to the next stop. Depending on the direction, we may go up or down, i.e., to greater or lower values.
Fixed a triple of these parameters, we have fixed the quantity of light that will impress the image on the sensor. But more than one triple will lead the same quantity of light!
In fact, STOPs have exactly this purpose: to help us to discover all the possible triples which allows the same quantity of light to be collected by the sensor. The idea is simple, yet powerful: whenever I step down one parameter by n STOPs and I step up another parameter by n STOPs (the same n), the quantity of light collected by the sensor will be the same.
OK, keep calm! First, what does it mean to step up or down? Step up by one STOP means to change the value of a parameter to next one in the range of STOPs, stepping down means to change the value to the previous one in the range of STOPs.
So, if I have the camera set to ISO=200, shutter speed=1/125, aperture = f/8, this is equivalent to ISO=200, shutter speed=1/250, aperture = f/5.6 (stepped down shutter speed and stepped up aperture by 1 STOP), which, in turn, is equivalent to ISO=100, shutter speed=1/250, aperture = f/4.
Notice that these exposures are equivalent only in the sense that they convey the same amount of light to the sensor. But they do their job in very different ways, leading to very different images!
Next time, I will try to explain a bit better what is the difference between two equivalent exposures and how to control slightly better the final result of a photo, before pressing the shutting button.
In any case, it is very instructive to fix a scene, put the camera on some support so that its view doesn’t change in time, and try to shot with all the possible equivalent exposures the camera allows to: the resulting images will give a very direct experience of what is the ‘quantity of light’ and how equivalent exposures are similar and different at the same time.