Why can exposure be adjusted in a RAW file after the photo is taken?
Asked 8/18/2016
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I’m trying to understand how RAW exposure correction works in software like RawTherapee. If a photo appears overexposed, how can the software darken it after capture? For example, if part of a cloud is clipped to white, how could RAW processing restore detail if those pixels are already white? Is exposure stored as metadata, or is there actually extra image data in the RAW file that allows this? Why is exposure set in-camera at all if it can be changed later when processing RAW?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Very good question :0)
When you expose your image you have more detail than you can normally see on a normal monitor. This is more a limitation of the display technology than the raw sensor.
This technology is limited to the 256 levels of gray for each channel, but your sensor captures more levels on each channel, this means you have more color diferences on the dark areas and bright areas, making it possible to "reinterpret" your image as is it was exposed brighter or darker when you see it again as 256 level image.
I think a key word here is reinterpret. Remember that a raw file is considered an "undeveloped" image.
Remember the old days when you used negatives. You could expose more or less your paper to achive a lighter or darker version of your negative. When you make adjustments you are reinterpreting the raw data, to give it a "flavour". Exposition at some extent, is just one of this flavours.
But if the exposure is really clipped, you can not recover detail because beyond that limit you do not have information at all.
Take a look at this post: What's the point of capturing 14 bit images and editing on 8 bit monitors?
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
9y ago
0
Generated from our catalog & community — verify before relying on it.
RAW exposure adjustment works because a RAW file usually contains more tonal information than a JPEG or a monitor can display. Camera sensors often record 12–14 bits per channel, while a typical JPEG/output image uses only 8 bits per channel. RAW processing maps that larger range into the smaller display/output range, so software can reinterpret the data as brighter or darker during conversion.
But this only works if detail still exists in the RAW data. If highlights are truly clipped at the sensor, there is no real detail to recover—software can only reduce brightness, stretch nearby tones, or sometimes guess missing information, especially if only one color channel is clipped. That’s why highlight recovery has limits and can sometimes look unnatural.
So exposure is not just an arbitrary post-process value. The in-camera exposure determines how much light the sensor actually captured. A good exposure matters because once shadows are too dark or highlights are fully clipped, that information may be lost. RAW simply gives you more flexibility to remap the captured data afterward than JPEG does.
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