The brightness slider is usually the wrong one
A dark photo, the brightness slider dragged right, and the result is grey and lifeless. This happens to everybody and it is not your judgement, it is the tool.
Brightness is additive. It adds the same amount to every pixel in the picture, so a pixel that was pure black at 0 becomes 40, and 40 is grey. Every shadow in the photograph lifts off the floor at once and the image loses the deep tones that were holding it together.
Exposure multiplies instead. One stop is a doubling of the light, which is what the dial on a camera does. Multiply zero by two and it is still zero, so black stays black while everything with light in it comes up.
That is why the same photo fixed with exposure looks like a better photograph, and fixed with brightness looks like a photograph behind a net curtain.
Done in linear light, which is not the obvious way
There is a subtlety underneath that most web tools skip, and it shows.
The numbers stored in an image file are not proportional to light. They are encoded with a curve, roughly a power of 2.2, because that spreads the available values where the eye can see differences. Mid grey is stored as 128 but represents only about 21% of the light of white.
So doubling the stored number does not double the brightness. A tool that multiplies the stored value goes muddy through the midtones and clips far too early. This one converts to linear light, does the multiply there, and converts back, and that is how one stop on mid grey lands at 176 rather than shooting to white.
Clipping is the thing nobody shows you
There is no value above 255 or below 0. When a pixel gets pushed past either end it stops, and every pixel that reached the end becomes identical to every other one that did.
A sky with cloud detail in it, pushed one stop too far, becomes a flat white shape. Pull the slider back and it stays a flat white shape, because the differences that made it cloud were averaged into a single number and thrown away.
That is why there is a readout under the sliders telling you exactly what percentage has gone. A few percent of blown white is completely normal with a bright sky in frame. Fifteen or twenty percent means the picture has been damaged, and it is better to know before you save than after somebody prints it.
Contrast pivots on the middle
Contrast pushes tones away from mid grey: the dark half goes darker, the light half goes lighter, and mid grey does not move at all.
Which means contrast is a very efficient way to clip a photograph. Every step of contrast moves the extremes closer to the ends of the scale, and a photo that had a little headroom at both ends can lose both after a fairly modest push.
Negative contrast does the opposite and is genuinely useful. A harsh, over-contrasted phone photo of a sunny scene often improves by having contrast taken away, which is not something most people think to try.
Auto levels is a starting point, not an answer
Auto levels looks at the darkest and brightest pixels and stretches them to fill the full range, ignoring a fraction of a percent at each end so one blown reflection does not decide everything.
Its assumption is that your photograph is supposed to contain pure black and pure white. Many are not. Fog, snow, mist, a deliberately soft portrait, anything shot in flat overcast light: all of those are meant to sit in a narrow range and stretching them looks unnatural.
So this one reports what it measured, tells you how much of the range the photo was actually using, and moves the visible sliders rather than applying something invisible you cannot adjust or undo.
Save once
A JPEG is re-encoded every time it is written, and every encode throws away a little more detail. It is not noticeable once. It is noticeable after ten rounds of open, tweak, save.
Make all the adjustments together and export once. Keep the original file, because nothing you do here can be undone from the exported copy.
Common questions
Why does brightening make my photo look washed out?
Because you are using the wrong slider. Brightness adds a fixed amount to every pixel, so pure black becomes grey and the whole picture loses its foundation. Exposure multiplies instead, the way a camera does, so black stays black and the midtones lift. On a dark photo, exposure is almost always the right choice.
What is the difference between exposure and brightness?
Exposure is multiplicative and measured in stops, where one stop is a doubling of light. Brightness is additive and just shifts everything up or down. Multiply zero and you still have zero, which is why exposure cannot lift a pure black pixel and brightness can. That sounds like a limitation and it is the reason exposure keeps a photo looking like a photograph.
What does clipping mean?
A pixel has gone past pure white or pure black and stopped. There is nothing above 255 or below 0, so any difference between those pixels is flattened into one value and the detail is gone. Pull the slider back and the flat area stays flat. The readout under the sliders tells you what percentage you have lost, which almost no free tool bothers to show.
Can I recover a blown-out sky?
Not from a JPEG, no. If the sky is pure white in the file, there is no information there to recover and darkening it produces grey, not cloud. Shooting RAW matters here: a RAW file often holds a stop or two beyond what the JPEG kept. From a JPEG, the honest answer is that it is gone.
Why did auto-correct make it worse?
Auto levels stretches the darkest and lightest pixels out to the full range, which assumes your photo is supposed to contain both pure black and pure white. Plenty do not. Fog, snow, a low-contrast portrait and anything shot in flat light are all meant to be soft, and stretching them looks wrong. It is a starting point, not an answer, which is why this one shows you what it measured and moves the sliders rather than doing it invisibly.
Does editing a JPEG lose quality?
Every save does, because a JPEG is re-encoded each time and each encode discards a little more. One edit and one save is fine. Opening, adjusting, saving, reopening and saving again ten times is visible. Make all your changes in one sitting, and keep the original.