The 48Mb file size came about when Canon 1Ds MK II entered the market. 11" x17" @300ppi comes up to that file size. That file size also refers to 8bit TIFF, not JPEG. The agency is saying they will accept JPEG so long as the TIFF version is 48Mb. Usually the JPEG will be around 5-6Mb at maximum quality ie. "Quality=12" in PS. Hope this helps.
As Henry stated above the requirements come down to the image being approx 11"x17" at 300ppi. Or if you prefer it in pixel terms approx 5120x3413 pix. The magical reason for this is that 11"x17" = a double page magazine spread. As well if you look into their technical requirements most reputable agencies do in fact state what is required in much greater depth than "file size 48-52 Mb," They really don't want to waste time having to review and reject files that do not meet their technical requirements.
Not to be argumentative, but the 48MB file size has been the requirement for a Double Truck page (2 8.5x11 pages printed side by side on one sheet or across the fold, A3) for a lot longer then the Canon 1DsMkII has been around. This was the specification for Double Truck even ten years ago when I was working from drum scans and CG. I believe that the IDs MkII is the first DSLR to provide that resolution, not to set the standard for the resolution. As stated before, the 48MB to 50MB is based on a RGB TIFF at 300 ppi (default for 150-line screen) for print which is where all these MB resolution standards actually started.
You are exactly right Rick. It is no coincidence that 35mm film scanner technology pretty much topped out at the resolution that will provide this size image.
Sure does take me back though thinking about all the page layouts and the bad scans that had to be retouched all the time:-( This new digital age is truly remarkable in what it allows us to do almost effortlessly now days. The amount of time needed to do a page today versus ten years ago is truly astonishing and I have to say I dont miss the old days:-) It's amazing that you can say the 'old days' in regards to digital isnt it? Seeing as I have C41 and E6 packs in storage still.
