Sony RX1R II Variable Low-Pass Filter

Away, moiré: Sony's electronically variable low-pass filter tech

by Dave Etchells

We've seen cameras with low-pass filters, ones without them, and recently, the Pentax K-3, that had a mechanically-simulated one. Sony's just taken the whole thing up a notch, though, with technology that lets them vary the strength of the low-pass filter electronically. Also, unlike Ricoh's approach in their Pentax SLRs, Sony's is a true optical low-pass filter (OLPF), so there's no limit on maximum shutter speed.

This is really quite an innovation, as the amount of the effect is continuously variable, and as noted, it avoids any limit on maximum shutter speed.

It turns out that Sony's illustrations in the product presentation and in the demo room afterwards were simplified for the sake of communicating the concept, and so left out some details. I managed to get a more complete story by talking with Mr. Tsutomu Hamaguchi, the General Manager of Sony's DSC (Digital Still Camera) Business, who came from Japan to attend the event. (It speaks volumes that Sony's top manager for the DSC business as well as the company CEO both attended the RX1R II rollout in New York.)

How OLPFs work

To understand what Sony's doing, let's very quickly review how conventional OLPFs work.

The illustration below shows the light path through a conventional OLPF filter stack. The very selective blurring -- the OLPF's job -- is produced by filters made from birefringent materials, which bend or diffract light differently, depending on its polarization. The incoming light is randomly polarized, so is split into two half-images, shifted from each other very slightly in one direction. (Horizontally, in the illustration below).

The so-called "wave plate" is technically a quarter-wave plate, which changes linearly-polarized light back into circularly-polarized light. This means that each of the half-images coming from the first LPF will be split again by the second LPF, this time vertically.

So the net effect will be to create very slightly-offset copies of the image in both horizontal and vertical directions. The resulting blurring of the image removes high spatial frequencies that would otherwise cause moiré patterns when divided up into pixels by the image sensor.

Conventional either/or OLPFs

One more piece of useful background information: Some current cameras are available in versions both with and without OLPFs. (The original Nikon D800 and D800e are good examples.) It turns out that just removing the OLPF stack would result in changes in the focus properties of the camera, so in order to make a non-OLPF version of a camera, manufacturers left the filter stack in place, but just made the second LPF in the stack reverse the effects of the first one. So rather than splitting the horizontally-doubled image vertically, the second LPF element in cameras like the Nikon D800e simply reversed the effects of the first one. The net result is essentially the same as having no LPF present, but all the other optical properties remain the same, so the camera behaves the same optically as one having a normal filter stack.

Switchable low pass filtering X 2

OK, now on to what Sony's doing in the RX1R II...

The illustration above is from one of Sony's presentations in the demo room, following the product announcement. In the "off" case, it shows the light rays being doubled, but then recombined, the same as we just described in the case of the Nikon D800e. In the "on" case, it shows the light being further split, to produce a doubled image, just as in a conventional OLPF.

What wasn't clear from Sony's presentation graphics, but is important to note, is that this on/off behavior is provided for both the first and second LPFs, allowing the LPF to be controlled for both the horizontal and vertical directions.

So in actuality, the three-layer stack of birefringent material/LCD/birefringent material in the illustration above exists for both the horizontal and vertical LPF elements. The net result is a 7-layer stack; one of the variable LPFs shown above for the horizontal direction, a quarter-wave plate, and then another variable LPF sandwich for the vertical direction.

(This raises the interesting possibility that LPF behavior could be controlled separately for horizontal and vertical directions. I'm not sure if being able to split the two orientations would have a lot of real-world application, but it's interesting to contemplate. Want to eliminate moire from venetian blinds, but keep maximum sharpness horizontally? No problem!)

How do they do it? The magic of liquid crystals...

So how do liquid crystals make it possible to vary LPF behavior like this?

It's basically the same idea as the do/undo LPF-cancelling approach that Nikon used in the D800e. The two birefringent halves of each of the RX1R II's LPFs are set so undisturbed light that's split by the first is recombined by the second.

The key is that liquid crystals work by rotating the polarization of light. In an LCD display, there are two polarizers, with the liquid crystals sandwiched between them. The amount of light that's transmitted depends on how much the liquid crystal layer rotates the polarization of the light. If the two polarizers are aligned with each other, when the liquid crystals are off, the light just flows on through. When the liquid crystal layer is turned all the way on, though, the polarization is rotated 90 degrees after exiting the first polarizer, so none of it gets through the second one.

Much the same thing happens here: If nothing is done to the light leaving the first half of an LPF element, it just gets recombined when it passes through the second. If, on the other hand, it's plane of polarization is rotated by the LCD layer, the light won't recombine, and the full low pass filter effect will be felt. If the LCD is only partially activated, it only partially rotates the polarization, so the LPF effect is less.

Low-pass bracketing!

A lot of times, you're not sure whether a subject will moiré or not. That's where one of the RX1R II's cool features comes into play; low-pass bracketing.

It's just what it sounds like. The RX1R II can shoot at 5 fps, and can adjust the amount of OPLF on the fly, so there's a low-pass bracketing mode that snaps three shots, with the LPF on, off, and set to a mid-range level. This strikes me as a great feature(!) I've generally been against the trend towards the complete elimination of LPFs, because once moiré happens in an image, you're pretty well stuck with it, at least not without spending a ridiculous amount of time in Photoshop, and even then, you may not be able to fully vanquish it. But LPF means losing detail and crispness in your image, and that's a tough bargain to accept.

With the RX1R II, you don't have to make that choice, or rather, only have to make the trade-off when you really need to. Just buy a big memory card, enable LPF bracketing, and be happy :-)

Overall, this is yet another innovation from Sony that will make a real, practical difference for real-world photographers. And the RX1R II is just the first implementation of it; I think it's safe to say that we'll see this feature implemented across Sony's mirrorless line going forward. Kudos (again) to Sony for bringing innovation with real impact for photographers!