Image Quality After Converting: A Repeatable Way to Judge It

What a conversion keeps and what it can lose

Think of a conversion as decode, then re-encode. The picture is first decoded to plain pixels, then written in the new format. If the new format is lossless (PNG, BMP, the PNG-compressed icon), the pixels you wrote are exactly the pixels you decoded, so nothing new is lost. If it is lossy (JPEG, WebP), the encoder rounds fine detail and you get a smaller file with small differences.

The converter's quality slider spans 10 to 100, opening at 90, and only JPEG and WebP outputs respond to it; PNG, BMP and ICO ignore it. Whatever you pick, the source's earlier damage comes along: the encoder cannot see what an earlier save removed.

A five-step test you can repeat on any picture

1. Choose a hard file. Use a picture with the things that break first: a smooth sky or wall, some text or thin lines, and strong red or saturated edges. If it survives, easier pictures will too.
2. Make three versions. Convert at 90, 70 and 50, and rename the outputs so you can tell them apart. Note the size of each.
3. Compare at 100 percent zoom. Open two versions in separate windows or tabs and flip between them on the same spot. Fit-to-screen views hide almost everything.
4. Look in three places. Flat color areas (blocky banding), text and edges (speckle around letters), and saturated color borders (muddy fringes).
5. Pick the lowest setting where you cannot tell the difference at the size people will see. Then go one step up for margin, because a different screen or a second recompression can reveal what yours did not.

Also check the source before blaming the converter. If blocky squares or fuzzy halos are already visible in the original at 100 percent zoom, they came from an earlier save and every format will reproduce them. If the colors look slightly different afterwards, that is a color profile matter rather than compression, covered in color profiles and sRGB. The size people will see matters more than anything else. A picture shown 800 pixels wide on a web page hides flaws that are obvious when the same file is shown full screen.

What the slider does to size and measured fidelity

To give you a feel for the curve, I encoded two synthetic 1920 x 1080 pictures with Pillow at six JPEG settings. Picture A has flat color blocks and text. Picture B has smooth gradients and blurred shapes with fine noise. PSNR is a rough score in decibels where higher means numerically closer to the original. Browsers use their own encoders, so the same slider number gives different bytes there; look at the shape, not the exact figures.

Quality	A bytes	A PSNR	B bytes	B PSNR
95	276,041	44.0	606,672	36.3
90	219,783	42.4	330,904	36.1
80	173,308	39.3	172,596	35.8
60	134,939	35.6	80,860	35.4
40	114,437	33.8	57,068	35.0
20	88,523	31.0	45,516	33.7

Two things stand out. Size falls fastest at the top: from 95 to 80, Picture B loses 72 percent of its bytes while its score moves half a decibel. And the score barely moves for Picture B at all, since the noise I added (which JPEG largely discards at any setting) dominates the difference, whereas the text-and-blocks picture degrades steadily. Different content, different curve, which is why the five-step test uses your file.

Starting points by purpose

• Default 90: a good first try for anything; hard to fault on photos.
• 75 to 85: pictures for web pages, email and messaging where size matters and viewing is at moderate size.
• Below about 50: expect visible blocking and ringing on text and gradients; acceptable only for small thumbnails or previews.
• 95 to 100: rarely worth it. In my test, going from 90 to 95 added 83 percent to Picture B's size for a 0.2 dB gain. If you need exactness, a lossless format is the right tool, not the top of a lossy slider.

These are starting points to test, not guarantees. If a file is too large, reduce its pixel dimensions before you lower the quality: halving both sides of Picture B (960 x 540, a quarter of the pixels) at quality 90 gave 64,294 bytes against 330,904, an 81 percent reduction, paid for in resolution instead of compression artifacts.

Generation loss: what repeated saving really does

Re-saving a JPEG is often said to destroy it quickly. My measurements say the truth is more nuanced. I re-saved the flat picture 20 times in a row at quality 90 and tracked PSNR against the original: 42.35 after one save, 41.36 after two, 39.8 after five, 38.62 after ten and 37.92 after twenty. File size stayed almost constant (219,783 down to 219,175 bytes). The noisy picture hardly moved, from 36.12 to 36.06, because its first save had already removed what the later saves could.

So identical saves at the same setting wear a picture slowly and the wear flattens out. Trouble starts when something changes between saves. Alternating quality 90 and 70 dropped the flat picture to 36.79 after just two saves, as if a single low setting had been applied. Passing through different formats compounds in the same way: JPEG at 85, then WebP at 80, then JPEG at 85 again ended at 38.28 for the flat picture, worse than a single JPEG save at quality 80 (39.3). Cropping or resizing between saves moves the 8 x 8 blocks, so each round rounds fresh detail.

Practical consequence: pass a picture through lossy formats as few times as possible, and make the last conversion the only one. Sending a screenshot of a screenshot is the same problem in disguise. For what happens to size, see why a converted image can get bigger; for the rounding step behind all this, lossy vs lossless.

Why matching a number across programs fails

Quality belongs to the encoder, not to the picture, so a JPEG saved at 90 by one program is not the same as 90 from another, and re-converting to match a number is pointless. Compare results by looking, and write down the file size next to the setting that produced it so that a later batch can reuse the choice. When the same picture is going to several destinations, test the strictest one first: the form with the small size limit, or the screen that will show the picture largest. If transparent areas turn solid when you convert to JPEG, that is not a quality problem but a background fill, explained in transparent backgrounds when converting PNG to JPG.