Last night I printed an image I took of the Winged Victory of Samothrace statue at The Louvre in Paris. A nice 13x19" print on my favorite Red River Ultra Pro Satin paper. It came out beautifully. Honestly I was impressed how well it came out considering the size of the file. You see, I took the picture early 2006 with a Canon 20D and a 17-40/4L zoom that I used to own before I got crazy about primes. Nice piece of glass and good camera for it's day, but certainly nothing like the resolving quality of my current 5D Mark III. The 20D is an 8MP cropped sensor camera which spits out a file of 2336x3504 pixels. Approximately the same number of pixels as my current iPhone 5 camera does. And there is a tendency for people to equate the two. Megapixels are megapixels? Right?
Wrong. Megapixels are not megapixels. I once printed out a photograph from my iPhone 4. It was a shot of the inside of the Capitol dome in Washington. The camera had everything going for it. The light was adequate, I held steady. It's as nice of a file as I've seen from a phone. And it printed a nice 8x10" print. Not an AMAZING print, but something you could certainly frame and put on your wall as long as you didn't look at it from a couple inches away. Because at that distance, as it does at 100% on screen, it shows it flaws. It's got that 'digital' look of early digital cameras. And the whole thing has the slightly smeary glaze of overzealous noise reduction, and details aren't really that sharp anyhow. And why would we expect them to be? Physics is physics. A lens and sensor the size of a pencil eraser is not going to resolve as well as one 10 times the size. Megapixels may not be megapixels, but photons ARE photons. Computing power can pick up some of the slack, but at the end of the day, size does matter.
How much it matters is up to you. One could say that the 24x36mm full frame sensor in my 5D is not big enough. That the same amount of pixels in a medium format camera would resolve more. They might be right. In fact that are right, but since medium format cameras tend to use CCD sensors and no AA filters gives it something of an apples to oranges comparison. Huge sensors make beautiful images, but at a monetary and handling cost that I'm not prepared to pay except in specific circumstances.
This is all to say that had I taken the photograph above on my iPhone, there's no way in hell it would look that good as a 13x19" print. 8MP or not.
Oh and one more thing, The 17mm lens I was using with the cropped sensor gives almost exactly the same field-of-view as my trusty 28mm prime does on my full frame body. Apparently I liked that look even back then.