Does converting image formats, such as from JPG to PNG, have any effect on image quality? I know converting from RAW to JPG has disadvantages, but what about other format conversions?
Going from JPG (which is a lossy format) to any lossless one (like PNG) does not.
Going from any format to a lossy one, yes, including JPG to JPG. It could be too little to notice, and using the same compression ratio loses a lot less on the second saving than on the first one, but yes, it is cumulative.
But beware... Some image formats store more information than others. For example, a CMYK JPG file will be ruined if you save it to an RGB-only file format, like PNG.
If you have something like transparency, you will lose it by saving in a file format that doesn't support transparency.
16-bit images will lose their extended range saving into an 8-bit per channel one, like saving a 16-bit TIFF image as a PNG.
Layers will be lost saving in almost any generic file format.
Effects could be lost saving as an older version of the native file format of some program, like Adobe Photoshop's PSD format.
If you drop the color profile, the image could render very different on some applications.
Some of this changes are not necessarily about "quality", but about the "information" inside the file.
I pretty much agree with Rafael on the following points:
Colorspace conversions will result in some losses (mainly rounding errors).
Decreasing color depth loses color information.
Transparency may be lost.
Layers and effects may be lost.
Metadata may not be preserved. (Even if a program doesn't remove any tags, most cannot help but to add something.)
However, regarding JPEG, I disagree somewhat.
The losses that result from repeatedly saving JPEGs at the same setting is limited. Eventually a steady state is reached where no further losses occur. I avoid saving over original out-of-camera JPEGs, but if it happens accidentally, it's not worth worrying about too much.
Saving from a JPEG to a lossless format does lose information, in particular, the invisible quantization tables used during the compression process. Losing this information prevents the use of future processing improvements, such as by JPEG decoders like knusperli and jpeg2png. If the only format your camera (phone) produces is JPEG, it is prudent to preserve that file as if it were a raw. See Is a JPG guaranteed to produce the same pixels?
There are three main types of image formats:
Best format is RAW image format
RAW is a file format that captures all image data recorded by the sensor when you take a photo. (More detailed image)
When shooting in a format like JPEG image information is compressed and lost. Because no information is compressed with RAW you’re able to produce higher quality images, as well as correct problem images that would be unrecoverable if shot in the JPEG format.
Please see other image format details given below.
Uncompressed (bit-map, a.k.a BMP) - storing the value of each pixel directly in a file. This way the image quality is maximal. - Good
Lossless compression (TIFF, PNG): In an image there are some redundancies that can be exploited so that its representation in some file is smaller. For instance, if we seek constant areas and replace them with a single identifying symbol, then we have compression. This has the exact same quality as uncompressed (1), but the file size will be smaller. The compression here is done similarly to data file compression - we get a smaller file size but we have to get perfect quality (not data loss). Obviously, the file size here will be smaller than uncompressed but not significantly so (for a typical image). :- Loss image data
Lossy compression (JPEG): This type of compression relies on image redundancies and distorts the data in favor of a smaller file size. The algorithms try to make the most visually pleasing way to distort, The file sizes here are significantly smaller than (1) or (2). Depending on the image content, the overall quality will vary from reasonable to almost indistinguishable from (1) or (2).:- Loss image data
SVG format is a special purpose format that is designed for line art. Many SVG images can be zoomed into infinitely and never become blurry. :- Loss some of image data.
5.GIF format are limited to a palette of 255 colors. Complex images such as photos do not compress well under GIF due to the limited amount of colors. :-