Method to Madness: The Z7's Insane Sharpness.

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Jack Hogan Veteran Member • Posts: 7,277
Method to Madness: The Z7's Insane Sharpness.
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Like many around here I have been literally blown away by the level of sharpness the Z7 is producing. I thought that my surprise might be the result of moving up from 24 to 45.7MP, or the excellent pin-point focusing mode, or the lack of an AA. Well, it turns out that there is probably more at work than that.

This weekend I pulled out the largest cutter blade I could find and set it up rough and tumble* to take a peak at what the MTF curves that produce such sharp results might look like.

I was again blown away, clocking at over 100 lp/mm the 50mm/1.8's green raw channel MTF50 at f/4 in the center (that's over 0.45 cycles per pixel, 2500 lp/ph, 5000 lw/ph).

Green raw channel MTF curve of Z7 + 50mm/1.8S @f/4, vertical slanted edge, courtesy of open source MTF Mapper.

When I first saw these results I thought I did something wrong because, you see, these results are not just insane, they are at first glance theoretically impossible. This is what theory says is the best a perfect lens and perfect square pixel can be expected to do:

Solid curves: MTF of otherwise perfect circular aperture lens @ f/4 in the center combined with perfect square pixel of 100% fill factor with increasing perfectly focused 3rd order spherical aberrations; Dots: measured MTF curve of Z7 with 50mm/1.8S @ f/4 in the center of the FOV. Units are lp/ph.

The solid lines represent the combined MTF of a perfect pixel and a perfectly focused lens with increasing spherical aberration. The dots is one of my actually measured MTF curves at f/4 in the center of the Z7 as described.

You'll note that where the Z7+50mm MTF dots meet the 0.5 level (MTF50) the curve is actually above the nearest solid line, which represents a pixel seeing pure diffraction. And that's physically impossible, because diffraction puts an upper limit on the actual performance of practical lenses.

Well, it's impossible if the combined effect of sensor photodiode, microlenses and filters is a perfectly even square filling the entire area of a pixel. Such a perfect pixel acts as a low-pass filter, somewhat smearing the image. But if the effective fill factor is reduced, so will be the smearing. For instance, this is the same perfect lens setup as above, with a 75% fill factor:

Same as above but with a 75% effective fill factor. Units are lp/ph.

Aha, now my z7+50mm/1.8 @f/4 in the center are back in the realm of (still to me mind bending) reality. We've never had it this good with Nikon.

Nikon is not the first to have thought of this trick, actually having taken a page from some Medium Format backs and Fuji (GFX-50). If your audience is mainly landscapers, who capture mainly natural scenes where aliasing and moiré are hard to spot - and you are trying to reduce sensitivity to achieve those sub 100 ISOs - it makes sense to reduce the effective area of your pixels. This also in part may explain why the Z7's effective QE seems to be about 20% lower than the Z6's.

Conclusions:

  1. The effective fill factor of the Z7's pixels is apparently less than 100% and most likely somewhere between 65-80% **
  2. This results in insane 'sharpness', so go easy with capture sharpening
  3. The downside is greater potential aliasing and moirè

So it seems to me more and more that the Z7 was designed with landscapes and Medium Format in mind: lower ISO and better 'sharpness', aliasing and QE be darned.

Jack

PS. There is a companion article to this post.

* So caveat emptor
** Better controlled testing will confirm the actual effective Fill Factor figure

Nikon Z7
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