Bits per pixel: A lot of small frame SLRs (your Canons and Nikons) offer 12 or 14 bits per channel for a combined 36 or 42 bits per pixel. Medium format digital

Bits per pixel: A lot of small frame SLRs (your Canons and Nikons) offer 12 or 14 bits per channel for a combined 36 or 42 bits per pixel. Medium format digital backs now offer 16 bits per channel. Layer size: Layers add more data on top of this: alpha channel (8-16 bits/pixel), clipping mask and maybe half a dozen other things, easily bumping the whole thing from 6 to 9 or 10 bytes per pixel. Layer count: It's fairly common to have multiple layers that are copies of the original photo, with different processing applied and blending them into the final one. I'm very much an amateur and when I'm serious about just making a photo look (not even creating something new) I end up with around 10 layers. Undo step memory: a lot of work in the photo processing workflow is global: color correction, brightness, contrast, layer blending settings and modes, filters (including sharpening or blur) apply to every pixel of a layer. Each confirmed change (by releasing mouse button / lifting stylus / confirming dialog) is likely to have to store an undo step for entire layer. Of course you can just persist some of that to disk, but if a single layer/undo step can be 800MB this will hurt productivity - only very recently we got drives that can write this much fast enough and that's why just a couple years ago, when having enough RAM was not really an option a lot of pro photographers had 10 or 15k RPM HDDs running in RAID0 in their workstations.