Imatest interpreting

The various specs are so confusing. Imatest which and uses and dxomark which dxo and dpreview now uses are different metrics for the same thing. How good is a particular body and lense combination. Truth is that a great body (NEX-7) can turn in a low resolution camera with a mediocre lenses (it’s kit lense) produces an effective 9-10 megapixels whereas a very good lense (Olympus prime) and body (MD5) can produce a 12 megapixel image.
What is more complicated is that resolution changes dramatically from the center (where it it shapest) to the edges (worse) and the corners (even worse). And the focal length on a zoom and the aperture have a dramatic effect (on a Sony RX-100, F/5.6 is 30% sharper than wide open at F/1.8).
The most confusing part is that the measures are different so Imatest likes to talk about lp/ih that is line pairs per image height and DXOmark likes to talk about effective pixels (without defining what that means), although the idea of having a single figure of merit is sure useful when you realize that resolution varies substantially across lenses, apertures and focal lengths!
So how is anyone supposed to think about this? Well, perhaps the first thing is to get some sort of simplification across Imatest and DXOMark particularly in those cases where the Imatest thing has me starting at acutance charts looking at resolution maps across an image. Ugh.
First question is how to think about Imatest in terms of effective resolution. So what is the actual line height for a given sensor. You can look at this as a pretty easy rough math problem (because hunting for the exact resolution vs image height spec is actually pretty hard, but here are the rough numbers)
image height = sqrt( megapixels / (aspect ratio))
In most modern cameras, they still use the 35mm film aspect ration (24x36mm or 4:3 aspect)

8 sqrt(8*3/4)=sqrt(6) 2,449 lines
12 sqrt(12*3/4)=sqrt(9) 3,000 lines
16 sqrt(16*3/4)=sqrt(12)=2*sqrt(3) 3,464 l/ih
20 sqrt(20*3/4)=sqrt(15) 3,872 l/ih
24 sqrt(24*3/4)=sqrt(18)=3*sqrt(2) 4,242 l/ih
36 sqrt(36*3/4)=sqrt(27) 5,196 l/ih

What does this all mean, well if you look at Imatest results from PC Magazine, the resolutions

I’m Rich & Co.

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