I have seen two metrics used to measure the sharpness of a camera:
- Perceptual megapixels, used on Dxomark.
- Number of lines the camera can resolve in a resolution chart.
But when we compare the number of lines the camera can resolve on a resolution chart, a completely different picture emerges: The Lumix does 22, while the RX1 does only 26. This is only an 18% difference, which is worlds away from the 6x difference we calculated earlier.
I can understand a slight difference between the two measurements, but not 18% vs 6x. Where does this difference come from? Horizontal vs vertical resolution? Sharpness at center vs at edges?
Which number is more useful as an objective indicator of sharpness? If I take a photo using both cameras and view them on a 4K monitor, after downscaling both photos to the monitor's resolution, will I see a huge difference? Going by the perceptual megapixel number, since the Lumix manages only 3MP, while the 4K monitor is 8-9MP, and the Sony does 18MP, yes, I should see a world of difference. But if I go by the resolution chart, the conclusion is that there will only be a small difference.
I'm comparing both cameras at their base ISO, and in either RAW or JPEG -- whichever is sharper.
I know that sharpness varies between the center and the edges, but let's ignore this detail for now. If a camera has a certain perceptual megapixel count, I don't care how those are spread across the sensor. I just care about the total number of perceptual megapixels.
I'm comparing fixed-lens cameras in this example to avoid confusion about which lens is being used.
I'm also ignoring MTF, since it seems like the perceptual megapixel number is just a summary of it.