RGB to Grayscale without adding noise

Question

You can convert an RGB image into grayscale in Photoshop easily.

Here's a swatch of 16 colours. The values of the colours are important.

However, later versions of Photoshop add noise to the image.

The image now has 41 colours. The noise was NOT in the source image, created by hand.

If we look closer at one of the colour swatches you can see it better: I've increased the contrast - for clarification.

There are many ways to remove colour from an image. As there are ways to remove noise. That's not what I'm asking.

Using Photoshop, is it possible to convert to grayscale without the seemingly additional noise?

Image > Mode > Grayscale > No Noise, thank you.

Answer 1

The image in your post already contains noise/artefacts — maybe it's because it was compressed by GDSE. Here's one of the image channel compressed with Levels:

If your original image doesn't have this noise and contains only pure colors, the issue you have is probably with color profile. Default color setting in Photoshop has a Use Dither conversion option — this results in the noise you see. Also if you're converting from sRGB image make sure you're using Gray Gamma 2.2 profile for correct lightness.

Here I remade your image with pure colors:

Converting to Grayscale with Use Dither set to on, noise is visible after compressing the colors:

After converting to grayscale with Use Dither set to off no noise is present:

Answer 2

The problem is almost certainly caused by JPG artifacts in your original image. I concur with the others who have responded here. I checked and can see them in your coloured image, so they are already there, although not really visible until you apply levels/curves/equalize or try to manipulate the colours.

Don't edit JPGs in Photoshop unless you have no other choice. Instead, create your original image as a PNG or PSD instead. When you use JPGs, the images undergo lossy compression. These compression artifacts are pretty normal for JPG, it's why JPGs can be so small. JPG artifacts can get much worse when you try to change colours/edit stuff. Also every time you save a JPG it adds more compression and more artifacts.

The JPG format should only really be used as a final output format, for example for photographs on web pages, or for print. Because of this problem, editing JPGs is something you should really never do if you are looking for quality results. Don't use them for small web graphics either. Save these in a lossless format instead, such as PNG.

Also note that if you are uploading a PNG image as an icon/avitar or images for social media sites, these are often converted to JPG by the website/app. Social media sites such as Facebook and Instagram also convert PNGs to JPG, and this can cause quality issues.