Does interpolation, the upsizing of digital images, work, is it useful or should it be avoided at all costs?

Anyone who has been involved with submitting images to picture libraries over the years will probably have done quite a lot of it. Certainly in the early days of digital photography, many libraries asked photographers to upsize so as to be on a level playing playing field with film, i.e. to be able to supply the same sizes for print publication. I certainly in the past have upsized many images quite substantially.

One library required 48MB files as a minimum. Using 4MP (11MB) and 6MP (17MB) cameras required a fair amount of upsizing and the images certainly looked somewhat soft and noisy in their blown up state. However I have sold quite a lot of these, including some used at their enlarged size. I've even seen some of them in print, and am always quite surprised at how good they look.

Since I'm currently using a couple of 12MP cameras I was interested to see just what happens when you upsize the files.

Here's a picture I took on my Olympus E-P2 (12MP) Panasonic GX1 (16MP) and Sony NEX-7 (24MP)

Here's the Panasonic GX1 version compared with an upsized version from the E-P2.

Here's the Sony NEX-7 version compared with the E-P2 picture upsized to the same file size as the Sony.

Finally here's the Panasonic GX1 version upsized to the same size as the NEX-7 file.

So the original is "better", but by how much? When the files are reproduced just how much difference will there be? From my experience not that much.

For information, I used an Olympus 45mm f/1.8 on the m4/3 cameras and a 55-210mm zoom on the Sony. Tripod mounted at f/11 and ISO 200. In all examples the upsized file was slightly sharpened as interpolating does slightly soften the image.