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Comment by dividedbyzero | Hacker News Reader

How would I do this on iOS, if I had just my phone and no CaptureOne on a laptop or the like? Are there apps that let me use a gray card to set the white balance of raw photos?

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When you click Edit on a photo, you will have bunch of edit options. Warmth and Tint sliders represent white balance. You can increase the warmth until it matches what your eye perceives.

Essentially you are telling your camera post capture what the temperature of the source is.

In the iOS camera you can point at something white, like white paper lit with white light, and long-press to get an auto focus and auto exposure lock. Then you can take a picture of the orange sky / landscape and it will use the locked white balance setting.

A white countertop lit with fluorescent light would work.

You can also use a camera app with more settings like ProCam which lets you set the white balance manually.

Or, as briefly mentioned in the article, use Halide, which outputs RAW files, which you can then edit in your app of choice. (Nice thing about iOS: it outputs Adobe DNG files, which is a standardized format, and broadly readable by a bunch of applications.)

Hold a small piece of white paper up in the corner of the frame. Or get a corner of street sign, etc. (may need to make sure it's illuminated though.)

Just set the image's white balance to daylight (3500K). The colors that you see will not be corrected. You can tweak from there.

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