The a9 has no AA filter!

Started Mar 31, 2018 | Discussions thread
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JimKasson Forum Pro • Posts: 37,064
The a9 has no AA filter!

At first shipment, there were questions about whether or not the a9 had an anti-aliasing (AA) filter -- aka Optical Low Pass Filter (OLPF). The consensus seemed to be that, with only 24 MP the camera needed one. I remember people saying that it did have one. When I tested the camera, I had bought into the conventional wisdom and didn't test to see if such a filter was in place.

Fast forward to a couple of weeks ago, when the brouhaha about the a7III PDAF pixel artifacts started. People thought that it might have something to do with the a7III AA filter. People wondered about its characteristics. I said that I thought it was probably a one-direction filter like all the other a7x cameras that had AA filters, and pointed to a test that I'd done with the a7 to show the effect of such a filter.

Then I thought to do some more extensive testing. I'd sold my a7, a7S, and a7II cameras and the only camera that I had that I thought had an AA filter was the a9, so I tested it. I started with a simple visual test using a Siemens Star and the Sony 35 mm 1/1.4 FE lens. I set the lens to f/4.

Overall setup

Enlarged star

Well, gee! I see monochromatic aliasing, and it's the same horizontally and vertically.

I set up another target, this time a slanted edge, I chose a sharper lens, the CV 65mm f/2 Apo Lanthar set to f/4, and I let Imatest do undemosaiced development with dcraw.

Here is what I got for a vertical edge. Imatest calls this a horizontal edge profile.

My that's a sharp lens! If there were an AA filter in this direction, there would be a zero between 0.6 and 0.7 cycles/pixel, and there is not. Don't worry about some of the Imatest warnings above; that is usually the case with you don't demosaic the files.

For a horizontal edge:

Very similar.

I now know that something that I had reflexively accepted for a year just isn't true.


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Nikon Z7 Fujifilm GFX 100 Sony a7R IV Sony a9 II +1 more
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