When saving for web, you have to save 2x the resolution to view it correctly on a rMBP. Say, you want to have a 100px width for an image, you'll have to save it as 200px and use css to make it preview correctly at 100px (best for retina scale). If you save the image for web at 100px, it's 2x smaller when opening afterwards in preview on a retina screen. The photoshop canvas shows you retina quality image, but when exporting you have to make sure you have twice the pixel count, otherwise your actual image wil be smaller on your rmbp. Now, if you just "save as" then the image is as big as it was on the canvas on the rMBP screen. So using that wil make the images twice the size on regular screens.
does this make any sense?
I hardly ever use save for web.
As a test i created 1024x768/72 canvas, this was quite small... when i save this (save as) on retina it will come out as 512x384 ? and on non-retina it will display normal. So to accomodate for retina i need to create my documents at 2048x1536px/72 .. when i save this on retina it will give me 1024x768px, but on non-retina it will be twice the size (2048x1536px) .. to over come this i just need to set the size of the image on the web so that it displays correctly?
Do we keep the resolution at 72?
What about background images, we can only use background-size in IE9+ so thats not a viable solution.
I try to keep image use low and use CSS where possible so i'm hoping it's not going to be a major issue.
Finally, what about print? If i create an A4 poster at 210x297mm/300, is this exactly what i'm getting?