Mind you, with Best mode and HiDPI resolution, your display is still displaying its native resolution (say UHD, ~4K, 3840x2160 pixels), but the OS X sort of upscales all applications 2x, so they display at seemingly 1920x1080 pixels (but of course much sharper than a native 1920px res, because all the icons, text and graphics are prepared at 3840px, with Retina in mind). Retina aware apps (PM, Photoshop, Photos, ...) may display photos at 1:1 pixels, which uses the true Retina resolution (e.g. 3840px), so a pixel in the photo still equals pixel on the screen. Of course these pixels are really tiny, almost invisible to eye, so in Retina mode a photo that is smaller in size than the screen resolution (say a 6 megapixel photo or a RAW file with not fullsize embedded preview like certain older cameras did) will actually seem to zoom out in Retina mode and 1:1 zoom, since the photo size in pixels is actually smaller than the screen size in pixels.