How many megapixels do you need? Well, for most people, as I said in my camera buying guide, the answer is 6. But what if you’re an artist, and you’re making big prints and trying to approximate the effect of using a medium or large format camera? How many megapixels of digital resolution would you need? This has always been a moot point, because the answer was “much more then any digital camera has.” But with the introduction of the Canon 5d Mark II and the Nikon D3X (21 and 24 megapixels, respectively), it deserves to be revisited.

Online photographer takes an analytical approach, attempting to determine what, for an 8 × 10 inch print, constitutes the highest theoretically possible resolution. The answer: 100 megapixels. Maybe. (It might also be 400.) Ken Rockwell compares the D3x to 35mm film and determines them to be very close in resolution. But we’re not interested in an 8 × 10 inch print, and we’re not interested in matching 35mm film. We’re interested in matching big prints made from medium and large format cameras.

I’ve spent a lot of time looking at these big prints (recommend visiting the Margulies for a near-lethal dose). It seems to me that in the best of the 30 × 40 inch prints, the resolution on an per-square-inch basis nears that seen magazines images. Since magazines typically consider 300 dpi to be the minimum resolution for print, it’s easy to calculate 30 × 40 × 3002 = 108,000,000 — 108 megapixels.

But looking at real-world results belies the mathematical precision that is suggested by talk of dots per inch, etc., and the fact is that the actual resolution of those large format print-based prints also varies greatly. This has everything to do with the subject matter, and in what an artist considers acceptable. A 6 megapixel image printed 16 × 24 inches is 120 dpi, yet looks surprisingly good. (A print from the D3x at the same resolution would be 34 × 50 inches.) It’s imperative for each person interested in this to familiarize themselves with what 100 dpi, 150 dpi, etc. looks like for their subject matter, and with today’s technology it’s trivially easy to do this.

Just for fun, check out the picture above. It’s a tiny crop from a picture off my Canon pocket camera, scaled up by 150%. Not fantastic, but at this resolution (assuming a 100 dpi monitor), a print of the whole picture would be 50 inches across.