Why photos come out too dark, and what Auto fix really does

A photo usually comes out too dark because the camera's light meter measured a bright area, such as a window, sky or snow, and set the exposure for that. The main subject ends up underexposed. Detail that is there but dark can often be brought back with Auto fix and the sliders. Pure black cannot, because the camera recorded no usable light there, and brightening also raises the grain that was already in the shadows.

Why the camera gets it wrong

A camera's meter estimates how bright the scene is and sets an exposure that averages it to a middle grey. Light from a window, a white wall or a bright sky pushes that average up, so the camera uses less exposure and the subject in front of the bright area goes dark. This is the classic backlight problem. The meter is doing its job; it cannot tell that the bright area is not the subject.

Dim scenes cause a different problem. A small sensor gathers less light per pixel than a large one. To get a usable exposure, the camera needs a longer shutter time, a higher sensitivity or a wider aperture. When it cannot raise those enough, the result is dark and noisy, and brightening it later brightens the noise too.

Auto fix and the sliders

What Auto fix does

Auto fix works from the brightness of each pixel, measured on a preview copy of at most 1,600 pixels on the longest side. It uses the standard brightness weights for red, green and blue, and it ignores see-through pixels. Then it does two things.

1. It stretches the tones. It finds the darkest and brightest 0.5 per cent of pixels and maps the range between them onto the full scale. The darkest parts become nearly black and the brightest nearly white.
2. It lifts the middle, if the photo still looks dark. When the middle tones are still below middle grey after the stretch, it applies a gamma curve that raises them towards it. A gamma curve lifts the middle of the range and leaves black black and white white. The lift is capped, so a very dark photo comes out clearly brighter without looking washed out, though it can stay a little darker than an average scene.

Auto fix then sets the sliders back to zero, so the sliders work on top of the automatic result. Reset clears it.

The sliders

• Brightness adds or removes up to 128 levels on the 0 to 255 scale, across its minus 100 to plus 100 range.
• Contrast spreads tones away from mid-grey, which sits at 128. At minus 100 the photo becomes flat grey. At plus 100 each tone's distance from mid-grey doubles.
• Warmth moves red up and blue down, or the reverse, by up to 40 levels.
• Saturation mixes each colour with its brightness. At minus 100 the colour is gone.
• Black and white sets all three colour channels to the same brightness.

Steps

1. Drop the photo. The preview appears.
2. Tap Auto fix. Hold Compare to see the original. With a keyboard, focus Compare and hold Space or Enter.
3. Adjust Brightness, Contrast or Warmth if the result needs it. Reset clears everything.
4. Download. The file is saved at the photo's full size, as a JPG, or as a PNG when the photo has transparency.

If the form also sets a size limit, make the file smaller after brightening.

Why shadows cannot be fully recovered

Pure black means the sensor recorded no usable light there, so there is no detail to bring out. A curve can stretch or lift values, but it cannot create detail that was never recorded. Auto fix keeps pure black at black. The Brightness slider can lift it, but then the area becomes a flat grey with no detail, and the noise that was hidden in the dark becomes visible as grain. The more a shadow is lifted, the more its noise shows.

When black and white helps

Dim photos often carry colour casts from indoor lighting, and coloured speckle in the shadows. Coloured speckle is usually more distracting than grey grain. The Black and white option sets every pixel to a single brightness value, which removes coloured speckle and casts. It does not remove brightness grain. Use it when the photo is about light and shape, such as a person beside a window, and colour adds little.

Because Auto fix measures the whole frame, a large bright window can pull its result away from the subject. Cropping to the subject first gives Auto fix a histogram that suits the subject.

Edge cases

• Already bright photos. When the middle tone is already bright, Auto fix only stretches the tones, so the change is small.
• Blown highlights. A bright area that is pure white in the original has no detail to recover, and Auto fix keeps it white.
• Very large photos. The download is made at full size. Only when the device cannot process a photo that large is it scaled down, and the result says so.
• Preview and download. The preview uses a smaller copy, but the full-size download uses the same settings and the same automatic curve.