On Sharpness, ISO and Shutter Speed

Started Nov 29, 2013 | Discussions thread
OP Jack Hogan Veteran Member • Posts: 6,461
Difference Between Horizontal and Vertical Readings

Bernard Delley wrote:

In this thread we have the riddle of 20% difference between vertical and horizontal resolutions occurring one way with A7 and the other way around with D610. Obvious possible causes include vertically running shutter shake, decentered lens, misaligned Bayer filter etc.

The lens used with the D610 appears to be the same as that used with the D4, which resulted in a 'perfect' graph, so I would exclude lens defects like astigmatism/distortion. I would also tend to exclude shutter shock because the difference seems to be constant throughout the measured shutter speed range.

This leaves out-of-focus effects and sensor related differences (microlenses, filters, fill factor).

Assuming the same 85mm:1.8G was used, I believe the Df's DPR studio scene captures are slightly out of focus:

D4 average MTF50 reading ISO 50-12800: 1834 lw/ph

DF average MTF50 reading from ISO 50-12800: 1701 lw/ph - Why the difference?

Such different readings using the same lens+sensor would suggest that the Df was not focused properly, unless there is something else at play? And the spread between the vertical and horizontal readings in the Df is minimal, therefore reducing out-of-focus' chances as a primary suspect.

If one excludes all of the above the likely culprit starts to look like the sensor, even because the Df and D4 apparently use the same one made by Nikon as opposed to a Sony in the D610 and A7.

Some AA filters are known to produce a directional response (oval shaped for instance). Whether this is desired or the unwanted result of misalignment of the beam-splitting plates is anyone's guess. The vertical/horizontal switch observed between the A7 and the D610 could simply be due to the AA plates being 'misaligned' 90 degrees off in one vs the other.

It would be harder to explain the A7/D610 V/H switch as a result of the shape of the 'fill' factor, since both sensors were designed by Sony and it is suspected that they come off the same fab line.  And I am not sure how the CFA could cause such an effect, especially given how the MTF50 measurements are made off the white balanced raw data only.


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