A phone screen is a poor judge of exposure. It's bright, it changes with the room you're in, and it can make a ruined sky look fine. The histogram shows the photo's brightness as numbers, so the room and the screen don't change it.
What a histogram shows
A histogram sorts every pixel by brightness. The left edge is pure black and the right edge is pure white. Everything else is in between. The taller the graph at any point, the more of the photo is at that brightness.
Three common shapes:
- Too dark. The graph is piled against the left edge, and the shadows are crushed to black.
- Well exposed. Nothing is piled at either edge.
- Clipped highlights. The graph is piled against the right edge, and the sky and lights have gone to white.
There's no single right shape. A photo of a black cat on a dark sofa should lean left. A beach at noon should lean right. A snowy field should sit to the right and a night street to the left. A hill in the middle is right only when the scene is mostly midtones.
How to read clipping
Look at the edges first. Clipping is a pile-up hard against either edge. Everything in that pile is the same pure black or pure white, with no detail left in it.
Many histograms also show the red, green and blue channels. They differ because the scene has colour. A red flower can clip in the red channel while the overall brightness looks fine. Then the petals lose their detail. If one colour hits the right edge, bring the exposure down a little.
Why highlights matter more than shadows
The two edges aren't equally bad. A RAW file holds a lot in the shadows. What looks black on the screen can often be lifted with Exposure or Shadows in the editor. Highlights are different. Once the sensor is full, it's full. A clipped sky is white in the RAW too, and no editor can bring its clouds back.
That leads to the most useful rule for shooting RAW. Expose so the important highlights are just safe, and lift the shadows later. The photo will look too dark on the screen, but it keeps the detail you need. If you're deciding between RAW and HEIF, iPhone RAW vs HEIF for a film look has the trade-offs and the file sizes.
Zebras show where the clipping is
A histogram tells you that something clipped. Zebra stripes show you where. They draw diagonal stripes over any part of the viewfinder that's brighter than a set level. If the stripes are on a white shirt or the sky, that detail is at risk. If they're only on a lamp bulb or the sun, that's fine, because those should be white.
How to use a histogram while you shoot
To see one while you shoot, you need a camera app that shows it. The steps below are the same in any of them.
- Frame the shot and look at the right edge of the histogram first.
- If something important is piled against it, lower the exposure until the pile moves away from the edge.
- Check the left edge. A pile there is often fine on RAW, because you can lift the shadows later.
- If the app has zebras, check where the stripes are before you take the photo.
Practice: expose for the sky
- Find a scene with bright sky and something darker in front, like a tree, a building or a person in shade.
- Turn on the histogram and zebras. Tap the dark subject. The camera brightens for it, and the sky probably fills with stripes.
- Lower the exposure until the stripes on the sky disappear, or cover only the sun. Take the photo. It'll look too dark, and that's expected.
- Open the photo in an editor. Raise Shadows and Exposure until the subject looks right. The sky keeps its clouds.
In Chobial
- Tap Controls and then Guides. A tile comes up for each guide. Tap a tile to turn its guide on or off, and a guide that's on is orange.
- Turn on Histogram. It appears in the corner of the picture, with the overall brightness and the red, green and blue channels.
- Turn on Zebras. Set how bright a highlight has to be before the stripes appear in Settings › Zebras.
For the practice above, drag down on the picture to lower the exposure. After the shot, tap Edit and use the Light tool to raise Shadows and Exposure.
The viewfinder's histogram measures what the sensor records, before the film is applied. That's what ends up in your RAW. So the histogram doesn't change when you switch films. After the shot, open a photo and tap ••• › Show Histogram in the editor. That histogram measures the developed photo, with the film and your edits. It shows what will clip in the file you export.
To edit the sky and the subject on their own, use masks. A sky mask changes only the sky, and a people mask can brighten only a face. For the film side of the picture, see how to make iPhone photos look like film.
