There's a lot of disinformation in this thread.
I saw photograph on a billboard the other day which measured about 10m x 3m. How many pixels? Well, if you apply the kind of reasoning that we've seen in this thread (300 pixels per inch) , you come up with a figure of about 4,200 megapixels. Wow! What on earth kind of camera were they using that has 4,200 megapixels?
Obviously they weren't. So obviously you don't need 4,200 megapixels to print an image that big. So obviously the logic is faulty. But where?
The key issue you need to take account of, when you're thinking about a print - and which most people haven't mentioned so far in this thread - is this: how far away from the print do you want to look at it?
People talk about 300 pixels per inch because that's roughly the maximum resolution the human eye can handle. But you need to be really close to the picture to do that. On the
Cambridge In Colour web site, there's a resolution calculator. For somebody with 20/20 vision, it suggests:
- 350 ppi when viewing at 25cm
- 175 ppi when viewing at 50cm
- 87 ppi when viewing at 1m
- 18 ppi when viewing at 5m
(ppi = pixels per inch)
So that billboard can get away with far less than 300 ppi because I wasn't pressing my nose up against it. If I had been, it would have looked horribly pixellated. But if I'd been that close, I wouldn't have been able to see the whole image, so there would have been no point.
It's the same with ordinary sized prints. Research in art galleries has suggested that for most people, the optimal viewing distance for a picture is roughly the same as its diagonal measurement. Closer than that, you can't see the whole picture properly.
So if you put that fact together with the resolution calculator, and crunch the numbers, you come up with this rather surprising result:
For a picture to look sharp (i.e. no pixellation) when viewed from the optimal distance, you never need more than about 6 megapixels.
Of course, that bit I've underlined is crucial. If you want a big print that will look super-sharp even when a camera club judge looks at it closely (and then he's looking at the pixels, not the picture!), more pixels will help.
The biggest print I've ever made had 72 megapixels (it was a panorama) and even on a 40" by 24" print the detail was staggering. But again, that's looking at the details, not the whole picture.
So the the OP: For A2 prints, 16 megapixels is fine - more than enough really - so long as you want to put the picture on the wall and actually look at it. If you want to inspect it with a magnifying glass, well, the more pixels the better!