Why do some images look worse after a simple format conversion?

One thing that surprised me when working with image conversions is that changing the format isn't always as simple as changing the file extension.

For example, converting a PNG screenshot to JPG can make the file much smaller, but text and sharp edges can also become noticeably softer.

The reason is that PNG and JPG handle image data very differently.

PNG keeps image details and sharp edges much better, while JPG trades some of that information for a smaller file size.

So when converting an image, I think the better question isn't:

"Which format is better?"

It's:

"What am I going to do with this image afterward?"

If it's a screenshot, graphic, or image with text, preserving sharp details may matter more.

If it's a photograph that needs to be shared or stored, reducing the file size may matter more.

It's a small distinction, but it's easy to overlook when you're just trying to convert an image quickly.

Have you ever converted an image and noticed a bigger quality difference than you expected?