Opacity Precision
While easy to understand, using integer percentages for opacity does not map well to what typically happens at a technical level — opacity values in 8bit per channel images have a range of 0 to 255. A design tool that uses a 0 to 100 range for opacity can not access 154 of the actual possible values. The percentages just get rounded to the nearest real value.
[…]
Does it matter? Quite often, shadows are incredibly sensitive to opacity changes, and many shadows use values from around 5% to 20%. That means there’s only 15 or so steps in the usable range, and single step jumps can be quite noticeable. This is not the most pressing issue in the design tools we use, but it is a real problem.
This does bother me a lot. And not only in opacity, but in many other places where sliders use percentages.
When dealing with gradients even having 256 steps is not enough, that is why 8bit monitors are not sufficient for dealing with high quality images.