It looks like the new test tested 7MP, 10MP, and 16.7MP crops. That would be 135 PPI, 161 PPI, and 209 PPI. Assuming no other weaknesses in the imaging chain, that is, a high quality lens was used, proper postprocessing, and a sufficiently detailed subject (a landscape might have been a better choice than a portrait), etc., it would be a bit surprising if there were no identifieable difference in the 135 PPI image.

But he doesn't say that. He says only that only 3 people correctly identified all three. I'd really like to know the percentage of those that attempted an identification who correctly identified the 7MP image.

With regard to the larger MP images, I'm really not that surprised. 161 PPI is getting pretty close to the point at which it's supposed to be unlikely that most people would notice a difference.

Remember that you need to just about double the MP for the resolution difference to be really noticable. The difference between 10MP and 16.7 MP really isn't that great. But the difference between the 16.7 MP image (209 PPI) and the 7 MP image (135 PPI) really should be expected to be noticeable.

That hasn’t been my experience at lower Mp. There is a “noticeable” difference between a 13 X 19 print from a 5Mp camera compared to one from a 7Mp camera. That works out to a 135 PPI print compared to a 162 PPI print.

Your post did bring the methodology to mind. His original tests cropped a 5Mp image from a 13Mp DSLR shot. If the pixels in the resulting 5:4 print at 16 X 20 resulted in 5Mp he was simulating the results from a 6Mp camera and not from a 5Mp camera.

It seems from the wording that the photographer who extracted the 7Mp image probably started with a 7Mp 3:2 image, resulting in a 5:4 image considerably less than 7Mp. By my calculations he would have ended up with a 5.8 Mp image and a PPI around 135 as you calculated. He might have chosen 7Mp because that would take him closer to the Mp used in the original test.

But if he cropped so that the resulting 16 X 20 used 7Mp he would be producing a 148 PPI print. That represents a 20% increase in pixel density over 135 PPI and makes a difference at those low resolutions.