Return of Interesting article on DxO about 5D III and D800...

Started Apr 10, 2013 | Discussions thread
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Erik Magnuson
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In reply to John Sheehy, Apr 10, 2013

John Sheehy wrote:

For a regular array of photosites, however, in columns and rows, a single metric, either in the form of DxOMark's "Perceptual Megapixels", or a resolution at a single contrast level, tell a very fractional story, and can often imply quality when, in fact, the result is actually aliasing.

It should be obvious that any single figure of merit can tell only a fractional story.  The more important question is does a story involving "Perceptual Megapixels"  correlate well with the perception of detail in real world usage?  In this case, aliasing or not is mostly irrelevant because it contributes to the "perception" of detail.   Certainly when comparing lenses with the same sensor, aliasing is important as it can only occur when the lens images enough detail to alias.

The problem is that we have inherited methods of measuring resolution from film,

Lens resolution has been measured without using film.

So, a monolithic measurement leaves the rest of the contrast vs frequency curve undefined.  The real curve is actually not a single curve, as it should split at the frequency where aliasing starts to take effect, with a higher trend for maximum contrast due to ideal pixel-to-subject alignment, and the lower trend for unlucky alignment (a transient right in the middle of a pixel).

Other than for bragging rights, how much does this matter?  In the real world there is also copy variation, field curvature, and even rounding error (P-Mpix is only reported to at most two digits: is 16 vs. 17 really 16.4 vs. 16.5?)

Of your points, the fact that P-Mpix is computed over the entire frame while most subjects are not is probably the most perception relevant.

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