I’ve explained this so many times over, and every time it’s for the same reason: people dive into technical stuff way too early on and get confused to the point of quitting. It’s hard for somebody who doesn’t do well with electronics to understand sensor sensitivity or how the lens opening affects the depth of field. That is understandable because most of us are artists not professors in physics, after all.
However, in order to master photography you will need to understand the exposure triangle and how each part of it affects your exposure and your image:
As Pye explained in the video, imagine one of the cups being the sensor, the water being the light, the pipe is your aperture, and the faucet is the shutter.
The idea is that you can control the sensitivity (the size of the cup), the aperture (pipe width), and the shutter speed (the time when the faucet is open). Using those three you can affect the exposure any way you want. But before you do that, you need to understand the limitations. This way you will know how to manipulate each variable in order to get the best result.
Sensitivity (marked as ISO on the camera) works by amplifying the light by a factor of two, so if ISO 100 is the baseline, ISO 200 is double that, ISO 400 is twice as bright as ISO 200, and so on. The higher the ISO number, the more detail you lose in noise, but you also need less light to expose properly.
ISO noise comparison
The shutter is basically like a curtain that exposes the sensor to light focused from the lens for a certain amount of time. The faster the shutter closes, the less time the sensor has to gather light. However, faster shutter means frozen action, while slower shutter means exactly the opposite. This is measured in fractions of a second—1/200 for example.
Aperture represents the width of the lens, measured in f stops. Even though it is counter-intuitive, the smaller number means more light. For example, f/2.8 produces twice as much light as f/4. This is due to the fact that it is a fraction of 1. The smaller the aperture number, that more background blur you have. The larger the number, the bigger field of focus is (more area being in focus).
Now let’s go back to the analogy used in the video. If he were to use the ISO 100 cup and open the faucet to maximum, he would be utilizing the lowest aperture number possible of the lens, and that way he would need 6 seconds to fill the cup. When the faucet is closed halfway, the cup needs 12 seconds to fill. However if he were to use the ISO 200 cup, which requires half of the amount of water (light) to fill, with the faucet half way closed it would take 6 seconds to fill the cup.
So if you need a 1 second exposure to expose properly on ISO 100 with f/2.8 in the real world, you will need ISO 200 if you want to stop down the lens to f/4 and retain the same exposure time. However, you’ll lose some detail and you’ll get less background blur.
The idea is to keep the ISO as low as possible (to retain cleaner image) while having fast enough shutter speed to avoid motion blur (except if having motion blur is your intention of course).
What other analogies help you remember or explain the exposure triangle?
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Oh Oh – you made a mistake – two times you said “f/2.8 produces twice as much light as f/5.6″ when you should have said ” f/2.8 produces twice as much light as f/4″.