Why Doesn't Quality Come Back When You Switch From JPG to PNG?

Two different compression philosophies

The most practical way to understand digital image files is to distinguish between the two fundamental approaches to compression: lossy and lossless. That distinction underpins both the difference between JPG and PNG and the answer to the question "does quality improve when I convert JPG to PNG?"

Lossless compression repacks data so it takes up less room, but it discards no information. Think of it like ZIPping a text file: when you unpack it, every character is exactly where it belongs. PNG works on this logic. Lossy compression strikes a different bargain: it deliberately throws away information the human eye struggles to notice, producing far smaller files. JPG took that route, and it is extremely effective on photographs.

How exactly does JPG discard information?

JPG compression works in several stages, and each stage sacrifices a little information.

First the image is separated into brightness and colour components. Because the human eye is far more sensitive to changes in brightness than to changes in colour, colour information is typically stored at half resolution. This step is called chroma subsampling, and right from the start part of the colour detail is gone.

Next the image is divided into 8x8 pixel blocks and each block is decomposed into frequency components. Slow colour transitions inside a block are represented as low frequencies, sharp edges as high frequencies. During compression the high-frequency components are aggressively rounded off or zeroed out entirely — because the eye notices them least.

This is where JPG's characteristic flaws come from: the faint shadowing around sharp edges (ringing), the square patterns that appear in areas of flat colour (blocking) and the dirt around the edges of fine text. The lower you set the quality, the harsher that rounding gets and the more visible the flaws become.

Loss across generations: silent degradation

The most insidious issue with JPG is generation loss. When you open a JPG, make a small edit and save it again as a JPG, the file is compressed from scratch. That second compression takes as its input not the original image but the output of the first compression. In other words, an already degraded image gets degraded again.

In a single pass the difference is usually invisible. But if you edit and save an image five or ten times — or if an image circulates from hand to hand and gets re-saved at every stage — the blocking and colour shift become steadily more obvious. This is exactly the mechanism behind those images that circulate on social media and progressively "melt."

So what does converting to PNG do?

When you convert a JPG to PNG, the pixel values as they stand at that moment are taken into a lossless container. From then on you can save and reopen the file as many times as you like; the pixel values will not change. The generation loss chain has been stopped.

But the damage the chain accumulated up to that point stays in the file. PNG does not erase blocking, does not clean up ringing, does not put back discarded colour detail. It only guarantees that "everything from here on will be preserved." That is why the right moment for the conversion is at the start of the editing process — not the end.

A practical rule: if you are going to work on an image across more than one session, switch to a lossless format in the first step, run all the intermediate stages there, and convert only the final publish-ready version to a lossy format. This workflow keeps quality constant throughout editing while keeping the final output at a reasonable size. If you want to prepare your file this way, you can use the JPG to PNG converter.

Why is the size difference so big?

Lossless compression saves space only by shortening repeating patterns. A screenshot has large white areas, single-colour borders and repeating structures — PNG packs those very efficiently. In a photograph, on the other hand, nearly every pixel differs from its neighbour; even the sky contains hundreds of different shades of blue. With no repetition to find, the compression gain drops.

The result is that for photographic content, the PNG file is several times bigger than the JPG of the same image. For graphic and text content the situation can flip; PNG both looks cleaner and is sometimes smaller. This is the most concrete demonstration of which format was designed for which kind of content.

The right mental model

Think of JPG as the summary of a conversation: quick to read, it conveys the main idea, but some details have been left out. PNG is the full transcript — nothing has been dropped, and in exchange it is much longer. Taking a summary and storing it in full-transcript format does not turn it into a full transcript; it just stores the summary completely.

Keep this model in mind and format decisions become very simple: use a lossless format where you need to preserve information, a lossy one where you need to distribute it. And the direction of conversion always makes sense from lossless to lossy; the reverse is done only to stop the chain.