Brightness is set by exactly three things: how wide the hole is (aperture), how long it stays open (shutter speed), and how much the camera amplifies what came through (ISO). Any camera can get the brightness right. Your job is to decide which of the three pays for it, because each one charges a different price.
aperture → depth of field shutter → motion ISO → noise
The bucket in the rain
Think of the sensor as a bucket and light as rain. To catch the right amount of water you can widen the bucket’s mouth, leave it out longer, or measure the little you caught and multiply it up — accepting that the measurement gets rougher the more you multiply. That is aperture, shutter speed and ISO, and there is nothing else.
It is called a triangle because all three are measured in the same unit: the stop. One stop is double or half the light. f/5.6 to f/4 is one stop more. 1/125 s to 1/60 s is one stop more. ISO 400 to ISO 800 is one stop more. Because they share a unit they trade exactly: open one stop, shorten the shutter one stop, and the brightness is unchanged.
The full-stop ladders, worth memorising
| Step | Aperture | Shutter | ISO |
|---|---|---|---|
| Brightest | f/2.8 (not on this lens) | 1/1000 s | 100 |
| one stop darker | f/4 | 1/500 s | 200 |
| two | f/5.6 | 1/250 s | 400 |
| three | f/8 | 1/125 s | 800 |
| four | f/11 | 1/60 s | 1600 |
| five | f/16 | 1/30 s | 3200 |
| six | f/22 | 1/15 s | 6400 |
What this means on the X-M5 specifically
- The mode dial decides who controls what:
Ayou set aperture,Syou set shutter,Myou set both,Pthe camera sets both. - There is no aperture ring on XC lenses. Aperture is set with a command dial on the body.
- ISO lives at SHOOTING SETTING > ISO, but reach it through the Q menu or a function button instead.
- The screen previews the exposure you are about to take. What you see is roughly what you get — a real advantage over the film era.
One leg of your triangle is short. The kit lens gives f/3.5 at its widest and only f/5.6 zoomed in, so there is not much aperture to spend, and the body has no in-body stabilisation to let you hold the shutter open. In dim light you will lean on shutter speed and ISO. Knowing that now saves a lot of blurry indoor photographs later.
Recommended starting point
A mode with Auto ISO. You make the one creative choice that matters most, and the camera does the arithmetic. Move to M later, when you want it to stop helpfully changing exposure between frames.
Assignment: three ways to the same brightness
Indoors near a window, put a small object on a table with something visually busy about two metres behind it. Mode M, 23 mm.
- ISO 400, f/5.6. Adjust the shutter until the preview looks right. Write the speed down. Shoot.
- Open to f/4. The preview brightens; shorten the shutter until it matches shot 1. Shoot.
- Back to f/5.6, raise ISO to 1600 (two stops), shorten the shutter by two stops. Shoot.
All three are the same brightness. At 100 percent on a screen, shot 2 has a softer background and shot 3 has speckle in the shadows. Same exposure, three different photographs. That difference is the entire point.
Common mistakes
- Chasing “correct exposure” as the goal. Decide the side effect you want first.
- Treating ISO as free. Spend it last — but do spend it rather than come home with blur.
- Assuming f/3.5 is always available. Zoom to 45 mm and you are at f/5.6, having silently lost about 1.3 stops.