Image brightness contrast analyzer that measures and fixes
Add a photo on the left and the right side immediately shows the preview together with brightness and contrast figures, a luminance histogram and an exposure verdict. The numbers follow the sliders live, and auto levels stretches the tones across the full range. Your photo never leaves the browser.
Add a photo
Drop an image here
JPG · PNG · WEBP · AVIF · BMP · up to 20MB
Adjust
Measured result
- Source brightness
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- Current brightness
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- Source contrast
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- Current contrast
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Verdict
Luminance histogram
Dark pixels sit on the left, bright pixels on the right. A pile against either end means tones are already clipped there.
Showing a sample image. Add your own photo and it takes over right away.
Measuring and adjusting happen inside your browser on a canvas. Brightness is the mean Rec.601 luminance (0.299R + 0.587G + 0.114B) mapped to 0-100%, and contrast is the standard deviation of that same luminance. The verdict uses fixed cut-offs: under 35% underexposed, over 65% overexposed, contrast under 40 low and over 70 high.
ready to use.
- Visible firstKeep the input and result positions clear.
- Results firstPut the main number up front and keep the process secondary.
- Less to askNo sign-up or extra information before using the tool.
How to use the image brightness contrast tool
When a photo feels dark, pushing brightness blindly blows out the highlights, and pushing contrast crushes the shadows. To decide what to touch and by how much, you first need to see the current state as numbers and as a graph.
This tool measures the mean luminance and its standard deviation, reports them as brightness and contrast, and draws a luminance histogram so you can see where the tones actually sit. Adjust from there and the figures follow live; press auto levels and the tones stretch across the full range without clipping. Everything runs inside the browser.
Brightness and contrast are different measurements
Treating them as one knob usually makes the photo worse. Separate what each number describes first.
- Brightness — the mean pixel luminance, how bright the photo is overall
- Contrast — the standard deviation of that luminance, how far apart light and dark sit
- Two photos with the same mean but different deviation look nothing alike
- Many photos that read as dark are actually short on contrast, not brightness
The formulas this tool uses
You can only trust a number if you know how it was produced. These are all published standard formulas.
- Luminance = 0.299 x R + 0.587 x G + 0.114 x B (Rec.601 weights)
- Brightness (%) = mean luminance across all pixels / 255 x 100
- Contrast = standard deviation of that luminance, range 0 to 127.5
- Adjusting multiplies each pixel by the brightness factor, then applies the contrast factor around 128
Follow the screen in order
The left column holds input and adjustment, the right column holds the measurement. One pass from top to bottom is enough.
- Drop a photo on the left panel or press Choose file
- Read the source brightness and contrast figures and the histogram first
- If the verdict says underexposed or low contrast, press auto levels for a baseline
- Fine-tune to taste with the brightness and contrast sliders
- Press save to download a PNG
How to read the histogram
A histogram counts how many pixels sit at each brightness level. Black is on the left of the axis, white on the right.
- A wall against the left edge means the shadows are already crushed to black
- A wall against the right edge means the highlights are already blown to white
- A narrow pile in the middle means low contrast, which reads as hazy
- An even spread to both edges means the tonal range is being used properly
What the verdict thresholds are
The badges on screen use these fixed cut-offs, so the label comes from a computed value rather than a guess.
- Brightness under 35% — underexposed, shadows are at risk of clipping
- Brightness 35% to 65% — well exposed
- Brightness over 65% — overexposed, highlights are at risk
- Contrast under 40 is low, over 70 is high, and in between is fine
What auto levels actually does
Auto levels trims the two tails of the histogram and stretches what is left across the full range. Knowing the steps makes the result predictable.
- It finds the 0.5% and 99.5% points of the cumulative distribution as the effective range
- It computes the contrast factor that maps that range onto 0 to 255
- It decomposes that mapping exactly into this tool's brightness and contrast factors
- Both factors are clamped between 0.5x and 2x like the sliders, and when contrast hits the cap the brightness factor instead centres the mean
Which control fits which problem
The same darkness has different causes, and each cause has a different fix.
- Shot indoors and uniformly dark — raise brightness
- Hazy as if through fog — leave brightness alone and raise contrast
- Backlit with a dark subject — raise brightness a little and lower contrast to recover the shadows
- A scan that came out grey — use auto levels to pull the paper back to white
A worked example
Say an indoor photo measures 28% brightness and 31 contrast.
- The verdict shows underexposed and low contrast together
- Auto levels raises the contrast factor first, spreading the histogram sideways
- It then raises the brightness factor to bring the mean toward the middle
- Anything left is taste, so nudge the brightness slider in 5% steps
Common mistakes
Adjustment leaves losses you cannot undo. These four come up most.
- Raising brightness alone — the highlights stick to white and the detail is gone
- Overdoing contrast — the shadows crush to black and cannot be recovered
- Re-adjusting an already adjusted file — the losses stack, so start from the original
- Judging by screen brightness — devices differ, so trust the figures and the histogram
Check before you save
Walking this list right before saving avoids a second pass.
- Has a wall formed against either end of the histogram?
- Has the verdict moved into the acceptable range?
- For a portrait, has the skin become unnaturally bright?
- Do you still have the original file stored separately?
- Does the saved format open where you intend to use it?
Frequently asked questions
Is my photo uploaded to a server?
No. Every step from picking the file to saving happens inside the browser on your own device, so portraits and unreleased images are safe to work with here.
What does brightness 100% mean?
It is the neutral point of the slider, not a measured value. At 100 the image is untouched; at 120 each pixel luminance is multiplied by 1.2. That is a separate number from the brightness percentage in the measured results.
What contrast number counts as normal?
It is a standard deviation, so the theoretical range is 0 to 127.5. Ordinary photographs land between 40 and 70; below 40 looks hazy and above 70 looks harsh. Logos and documents with strong black and white are naturally higher, and that is fine.
I pressed auto levels and almost nothing changed.
That means the tones already fill the range. Auto levels stretches the span between the 0.5% tails, so if the original already reaches near 0 and 255 there is nothing left to stretch. Only taste adjustments remain.
Does adjusting cost quality?
Yes, because pixel values are recomputed. Pushing already bright areas further sticks them to white and that detail cannot come back. Reach your target in one pass from the original rather than re-adjusting an adjusted file.
My histogram is piled against the right edge.
The highlights are already blown to white. Lowering brightness only darkens everything; the lost information does not return. The only real fix is reducing exposure when shooting.
Does it work on black and white photos?
Yes. Only luminance is computed, so the absence of colour changes nothing. Contrast is arguably easier to judge on monochrome because the number matches the impression directly.
How are transparent PNGs measured?
Transparent pixels still contribute their stored colour values. If a large area is fully transparent the figures can drift from what you see, so flatten the background first when you need an exact measurement.
What format does it save?
PNG. It is lossless, so the adjustment survives exactly and nothing compounds if you pass the file to another tool afterwards. If you need a smaller file, run the saved PNG through the image compressor.
References
Official documentation for luminance calculation and pixel processing.