Megapixels vs Compression Artefacts

Question

Assume everything is equal such as lens quality, sensor etc, except the megapixels. Does cameras with more megapixels produce pictures with less compression artefacts than cameras with less megapixels?

Answer 1

More and Less.

More artifacts because there's more pixels to have compression artifacts. If 1% of an image is artifacts, 1% of a bigger image is more than 1% of a smaller image in terms of number of pixels of image noise.

Less artifacts because each 8x8 or 16x16 block where artifacts live represents a smaller proportion of the image the larger the image is.

In short, once printed into, say an 11x17, the higher MP image will (likely) have more artifacts over-all, but they will be less visible to the eye when looking at the print as each artifact chunk is smaller relative to the scene.

TLDR: there is an effect, but it's a trade-off and therefore mostly a wash in terms of image quality, so it's fine to ignore.

Answer 2

If you are speaking about lossy image output it totally depends on image settings. All consumer cameras can produce images with no visible artefacts at highest quality level.

If you asked specifically about JPEG it can be produced either by a standard encoder or by some replacement codec (for example, Mozilla has an optimized JPEG encoder), and it's totally transparent to end user which one is used in camera as it's usually undocumented and unimportant.

In general, cameras can have different artifacts which are not direcly linked to compression losses. For example: 1 2.

Also, hypothetically, if the image is compressed with same quality settings, whatever artefacts it have they will be less perceptible if the resolution is higher. On the other hand if you try to fit two photos with different resolutions into same file size you will often find out the bigger image has more artefacts. However, that second case does not apply to cameras. Cameras do not have any specific setting to fit images into specific file size.

Answer 3

TL;DR: No, megapixels and compression artifacts are unrelated.

Compression atrifacts come from the lossy compression algorithm that reduces the original RGB image to a smaller (e.g. JPEG) format. And all these compression algorithms have tunable parameters how aggressively they do the compression.

The more aggressively the algorithm is set up, the smaller the file size gets, and the more compression artifacts you'll see, so it's a tradeoff.

• Using the same compression settings, in pixel-peeking view you'll get the same amount of artifacts regardless of the megapixels.

• If you want to compress to a given file size, a higher-resolution image is worse, as it forces you to use a much more aggressive compression, producing more prominent artifacts.

• Given the same compression settings, an image with higher resolution can be better, as it allows you to scale it down to the lower resolution after compression, and thus level out some of the compression artifacts - but scaling down an image produces its own artifacts, so it might not really be an improvement.

If you are concerned about compression artifacts, use the least aggressive compression that the camera provides, or shoot in raw (uncompressed) mode.