Modern Sensors Missing half the AA: in the Horizontal or the Vertical direction?

Started Jan 14, 2014 | Discussions thread
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Jack Hogan Veteran Member • Posts: 7,671
Modern Sensors Missing half the AA: in the Horizontal or the Vertical direction?
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An article pointed out by jtra (use google translate) confirmed the unusual Vertical/Horizontal spread in MTF50 spatial resolution measurements that I had noticed a few months ago in recent sensors. I am now wondering whether the latest generation of non-AAless sensors does not simply have weaker Anti Aliasing filters - but whether they have actually done away with 1/2 of them. Specifically in one direction only. What do I mean? Read on.

Here is a graph of MTF50 readings by MTF Mapper off the slanted edges in DPR's new studio scene for the Nikon D610 and Sony A7:

Recent FF Sensors with AA - Why such a large spread in Horizontal and Vertical MTF50 Spatial Resolution readings?

The A7 graph (same vintage sensor by the same manufacturer) looks very similar although curiously the vertical and horizontal lines are switched. On the other hand the AAless Sony Rx1R (similar sensor by the same manufacturer) vertical and horizontal readings overlay each other as expected around the 2500 lw/ph level.

FF Sensor without AA: No material Spread in Horizontal and Vertical MTF50 Spatial Resolution readings as expected

But here's the interesting thing: Allowing a small margin for lens/operator differences the two sensors with an AA filter (D610 and A7) show similar MTF50 readings as their AAless version (RX1R), albeit only in one direction (vertical in one and horizontal in the other). Could it be that in this new generation of sensors the AA was eliminated in one direction only - leaving the other intact?

It would certainly be easy to accomplish this because most modern AAs perform their function by splitting the incoming light four ways: first two-way horizontally followed by two way vertically through a so-called 4-dot beam splitter made of subsequent lithium niobate plates. Here is how Nikon puts it:

It would therefore be easy to remove entirely (or replace with a neutral plate) one of the two Low-Pass filter plates shown transforming the 4-dot beam splitter into just a 2-dot - effectively removing the anti-aliasing action in one direction only. This would explain why the D610 and A7's readings match the RX1R's in one direction only. In this scenario the D610 would have had the horizontal split removed while the A7 the vertical, also explaining the curious switch of the vertical and horizontal lines seen in the two graphs.

Here is another example where this might be happening: the Fujifilm XA1 with AA versus the XM1 without: the higher horizontal readings of the XA1 are again close to the values of the AAless XM1*. Looks like the vertical plate was removed/replaced here.

So it is possible that cameras like the D610, the A7 and the XA-1 come with only half an AA. The absent (or lower) AA strength in one direction apparently results in better spatial resolution in that direction only - but at the usual cost of increased artifacts in that direction, as shown in the article referenced above.

Thoughts on the above and related implications? Should owners of such cameras choose to shoot a scene in horizontal vs vertical orientation according to the direction of the detail contained therein?

Jack

* Excluding operator error both cameras' readings appear to be affected by shutter shock up to and including 1/250s, not an uncommon occurrence in ligher bodies with mechanical shutters.

Fujifilm X-A1 Fujifilm X-M1 Nikon D610 Sony a7 Sony RX1R
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