When I use
convert image.png image.jpg to convert a PNG to a JPEG, does
convert perform any lossy compression? How much?
You can define the amount of JPEG compression with
convert image.png -quality 100 image.jpg
100 is no compression. The default used by
92, which is a very common value in digital cameras.
There are several arguments on what amount of JPEG compression is a good amount and I am not planning to start another one. I'll just point out that people most commonly use values between
75. The amount of filesize reduction by the amount of compression depends on the content of the file, therefore there is no magic rule.