There are 3 factors that control the amount of light that enters your camera: Aperture, Shutter Speed and ISO. Those factors are called the exposure triangle.
My favorite metaphor for the exposure triangle is that taking a picture is like filling a bucket with water, to get the correct exposure you need to get the right amount of light into the camera (or water into the bucket), to control the amount of water you can keep the water flowing for shorter or longer amount of time (shutter speed), use a ticker or thinner water pipe (aperture) or replace the bucket with a smaller or larger bucket (ISO speed).
It’s worth noting that the exposure triangle has nothing to do with the triangle shape we all know and love, but “exposure triangle” is a catchy name (better than the more geometrily correct exposure cuboid).
Changing the amount of light with any one of the 3 factors will give the same brightness as changing the same amount with any other factor – but not the same image because aperture also affects depth of field (among other things), shutter speed controls how you capture motion and ISO changed the amount of digital noise in the image.
To demonstrate this I’ve set the camera on a tripod in front of a toy microphone, set it to manual exposure and manual focus, and I’ve also set the white balance (because auto white balance can change white balance between pictures) – and took 13 photos , 1 correctly exposed according to the camera light meter and then for each of the 3 factors I took 4 pictures at +2, +1, –1 and –2 stops from the correct exposure.
I took the JPEG images right out of camera and combined them into the table below to show that the brightness really does change by the same amount.
The darker band on the right side of the pictures is actually the corner of the room.
You can’t see the changes in depth of field or noise in those small images but that’s ok because you also can’t see them in the full size, the maximum aperture is small enough to get the entire microphone in focus and the background is a solid color – so no visible change in depth of field, also, there is plenty of light and my camera has good high-ISO performance so there’s no visible noise in the high ISO images.
Also, the “correct exposure” picture is too dark and the over-exposed pictures are better – this is because the light background fooled the camera, exactly why this happened and how to deal with this will be the topic of a future post.