ChobialWhat exposure is: light, shutter speed and ISO
Every photograph is a record of light. Exposure is how much of it the camera collects, and it decides whether a photo comes out too dark, too bright or just right. Everything else in these lessons builds on it.
The idea
A camera's sensor is a grid of millions of tiny light buckets. While the shutter is open, light falls into them. More light means fuller buckets and a brighter photo. Too little and the photo is dark and murky. Too much and the buckets overflow: the bright parts turn to flat white with no detail left.
A camera has three ways to control how bright the photo comes out:
- Aperture: how wide the opening in the lens is. On a big camera you can change it. On an iPhone each lens has a fixed aperture, so you can't.
- Shutter speed: how long the sensor collects light, from a tiny fraction of a second to about a second.
- ISO: how strongly the sensor's signal is amplified afterwards. Higher ISO makes a brighter photo from the same light, at the cost of noise.
So on an iPhone, exposure is a balance between two things you can set: shutter speed and ISO.
Stops: the unit of light
Photographers measure light in stops. One stop is a doubling or halving. Going from 1/250 s to 1/125 s keeps the shutter open twice as long: one stop more light. Going from ISO 100 to ISO 200 doubles the amplification: one stop brighter.
Because both are counted in stops, you can trade one for the other. Make the shutter one stop faster and the ISO one stop higher, and the photo comes out just as bright. What changes is how it looks: less motion blur, a little more noise. That trade is the heart of exposure.
The meter
How does a camera know how much light it needs? It has a meter. The meter measures the scene and picks settings that turn an average scene into a mid-tone, neither dark nor bright. That works most of the time. It's fooled by scenes that aren't average: a snowfield comes out grey because the meter darkens it, and a person in front of a bright window comes out dark.
That's why you'll often want to tell the camera "a bit brighter" or "a bit darker". That's exposure compensation.
In Chobial
Chobial starts in P (Program): the camera picks shutter and ISO for you. You can still steer it:
- Tap the viewfinder where the important part of the picture is. An orange frame shows where the camera focused and metered.
- The EXP dial comes up on the right. Drag up to brighten, down to darken. Each step is a third of a stop, up to three stops either way.
- Tap RESET on the dial to go back to zero.
When you want to set things yourself, open the controls and tap MODE:
| Mode | You set | Chobial sets | Use it when |
|---|---|---|---|
| P | Exposure compensation | Shutter and ISO | Most of the time |
| S | Shutter speed | ISO | Motion matters: freeze it or blur it |
| ISO | ISO | Shutter speed | You want the cleanest file |
| M | Both | Nothing | The light is steady and you want full control |
The readout at the top shows what the camera is using right now: ISO, shutter speed and the lens's aperture. Values you set yourself are shown in orange.
Balance an exposure by hand
- Point the camera at something still, in steady light. In P, note the ISO and shutter speed in the readout.
- Tap MODE and choose M. Set SHUTTER and ISO to the same numbers. In M, a meter appears under the readout and should sit near the middle.
- Move the shutter one step faster. The meter moves one stop darker, and so does the viewfinder.
- Now move ISO one step higher. The meter comes back to the middle: same brightness, different settings.
- Take both photos and compare them in the Library. Tap ⓘ on each to see the settings.
Next, the first of the two controls: shutter speed.
Questions
Why doesn't Chobial let me change the aperture?
Every iPhone lens has a fixed aperture, so there's nothing to change. Chobial shows it in the readout (for example ƒ1.8) so you know what it is, but exposure on an iPhone comes down to shutter speed and ISO.
What is a stop?
A doubling or halving of light. One stop brighter means twice the light; one stop darker means half. Shutter speeds and ISO values are laid out in stops so that one step on one can be traded for one step on the other.
Is there a correct exposure?
There's a sensible starting point, which is what the meter aims for: an average scene coming out as a mid-tone. The right exposure is the one that makes the photo you want. A dark, moody street at night should look dark.