Comparing two images sounds simple until the difference is small: a compressed copy that might look worse, a design with one changed detail, two screenshots taken a week apart. Looking back and forth between two windows misses a lot. Here’s what works.
Three ways to compare
| Method | Best for | How it works |
|---|---|---|
| Side by side | Overall look, colour, composition | Both images next to each other |
| Swipe | Small changes in a known area: sharpness, compression, retouching | One image over the other, with a divider you drag across |
| Difference map | Finding every change, however small | Each pixel that differs is highlighted |
Compare Images does all three, in your browser: drop in both images and switch between the views.
How to tell how different they are
Looking only tells you so much, so the tool also measures:
- Different pixels: the share of pixels that changed by more than the sensitivity you set. 0% means none did.
- Largest difference: the biggest change in any single pixel, out of 255.
- Similarity (PSNR): a standard score for how close two images are, in decibels. As a rough guide:
| PSNR | What it usually means |
|---|---|
| Identical (∞) | Every pixel is exactly the same |
| 45 dB or more | Visually identical |
| 38–45 dB | Very close; differences are hard to see |
| 30–38 dB | Differences are visible up close |
| Under 30 dB | Clearly different |
PSNR measures pixel differences, not how the eye judges them, so treat it as a guide alongside your own look, not the final word.
Common uses
- Checking compression. Compare the original with the compressed copy from the Image Compressor or Compress to File Size. Above about 40 dB, few people will notice.
- Spotting changes between versions of a design, a document scan or a product photo. The difference map shows exactly where something moved or changed.
- Testing screenshots of a website or app before and after a change.
- Checking for duplicates. Two files that show the same picture can still differ in size, format or metadata; the comparison tells you whether the pixels match.
Why tiny differences show up everywhere
- Re-saved JPGs and WebPs. Saving again changes pixels slightly across the whole image, even when it looks the same. Raise the Sensitivity to ignore these and see only real changes.
- Different sizes. If the images aren’t the same size, the second is scaled to match the first, and scaling alone changes many pixels. Compare images of the same size when you can; Image Info shows each image’s size.
- A one-pixel shift. If the same picture is moved by even one pixel, every edge shows as different. Check that both images are cropped the same way.
- Metadata doesn’t count. The comparison looks only at pixels, so camera details, dates and colour profile tags don’t affect the result.