It’s a question one can find all over the Internet, and one that I get asked all the time. My favorite answer is generic: The better the image quality, the more leeway you have when it comes to printing – regardless of the number of pixels. It’s my way of trying to get people to be less fixated on the gear they’re carrying and more attentive to what they’re photographing.
Okay, so image quality isn’t the only thing – there’s also the issue of viewing distance.
I know from the example above that images from my 12-megapixel Canon EOS 5D can hold their own on a billboard – and the shot used for that billboard wasn’t even a full crop. But I also know that no one but the sign maker and the sign hanger are ever going to get close enough, long enough, to notice much of anything but the message.
The good thing about high-hanging signs is that they’re harder to “pixel peep”, but the same question applies to photographs that you want to enlarge for display in you home or office: From what distance are they meant to be viewed? That – and the image itself – will determine how large you can go.