The one thing correction cannot undo

Almost everything a dull photograph is missing is still in the file, waiting to be stretched back out. There is exactly one exception, and it is worth understanding before you expect any tool to fix it.

What the right-hand wall means

A histogram counts pixels by brightness: how many are nearly black, how many are mid-grey, how many are nearly white. A well-exposed photograph produces a shape that tapers off before it reaches either end. A clipped one produces a shape that stops abruptly at the right, often with a spike sitting on the final column — and that spike is not a lot of very bright pixels. It is every pixel that was brighter than the sensor could record, all of them written down as the same single value.

The distinction that matters is between compressed and absent. A dark photograph has compressed shadows: two hundred different tones squeezed into forty levels, hard to see but recoverable, which is what a shadow lift does. A clipped highlight is not compressed. The sky, the window frame, the reflection on the glass and the lamp behind it all arrived at the same number, and no operation performed on that number can tell you which of them a particular pixel used to be.

Two failures that look similar and are not
Symptom	In the file	Recoverable
Dark, muddy shadows	few levels, plus noise	mostly
Flat, washed-out midtones	narrow but complete	almost entirely
A grey cast over everything	complete, wrong gains	entirely
A white sky with no edge	one value, repeated	no

Why the pass holds back when it sees one

Clipping changes what the correction should do, not just what it can achieve. The obvious way to brighten a dark picture is a linear gain, which multiplies every value — including the ones already sitting at the ceiling. Since they cannot go any higher, what actually happens is that the values just below them arrive at the ceiling too, and the flat white area grows. The photograph gets brighter and the blown region gets larger, which is a poor trade.

So the pass checks first. When more than about one and a half percent of the frame is already sitting on pure white, the linear gain is capped well below what the median alone would ask for, and the remaining work is handed to a midtone curve instead. A curve lifts the middle of the range and leaves the top exactly where it is, so the room gets brighter and the window does not spread. Wherever that cap was applied, the panel prints the proportion of the frame that was already clipped when it arrived.

The white point is read at the 99.5th percentile

For the same reason, the level the stretch treats as white is not the brightest pixel in the picture. One specular glint on a car bonnet would otherwise define the entire top of the range and the stretch would do nothing at all. Reading the level that only half a percent of the frame exceeds ignores the glint and finds where the picture really ends — at the cost of pushing that half percent to pure white, which is a clip the tool is choosing deliberately rather than inheriting.

What can still be done

We can't rebuild a blown highlight. Where the sensor clipped to pure white there's no detail left in the file, and no amount of correction invents it — what we can do is stop the rest of the photo from suffering for it.

In practice that is more useful than it sounds. A backlit photograph taken against a window is usually two problems stacked: a blown window, and a subject three stops too dark because the metering was protecting a window that blew anyway. The first is permanent. The second is entirely fixable, and fixing it is what makes the picture worth keeping — nobody looks at a portrait to examine the view outside.

If the highlight matters, the fix belongs at capture rather than here: expose for the bright part and lift the shadows afterwards, or take two frames at different exposures. Both are decisions made before the shutter, which is the honest answer and the reason this page tells you the number instead of quietly working around it.

Take it back to the picture

Corrected on your own machine — we never get a copy.