Sony F5 / F55 Optical Low-Pass Filters

Last Friday saw the release of Version 2.0 firmware for the Sony PMW-F5 and F55 Large Single Sensor cine cameras. The new firmware brings many enhancements such as higher frame rates, ‘scopes in the EVF, and 2K XAVC (disclosure: Art Adams and I have been playing with beta versions for a while; Art’s just shot two projects exploiting the new XAVC and raw high-frame-rate capabilities, which I expect he’ll write up in the next couple of weeks. In the meantime, here are some production stills from the music video and  spec spot shoots). I want to focus on hardware: these cameras offer interchangeable optical low-pass filters (OLPFs).

OLPFs suppress high frequency detail that can cause aliasing and moiré, and most cameras have an OLPF carefully chosen to match the characteristics of their sensor’s native resolution (Nikon has a good writeup on OLPFs here). But when a sensor is scanned out in a non-native way, or at a different resolution, the OLPF’s characteristics may no longer be suitable: if it’s too strong, you’ll get an overly soft image; if it’s too weak, you’ll get excessive detail, aliasing, and moiré. For example, the Canon 5D Mk II’s OLPF is optimized for 21 Megapixel stills capture, but it’s woefully inadequate when that camera is used for 2 Megapixel HD capture. Folks like Mosaic Engineering make add-on OLPFs for the 5D Mk II and other cameras to better filter the image for HD capture, but these add-on filters add bulk and change the optical path length of the light behind the lens, so focusing marks shift.

The F5 and F55 cameras presently offer three different shooting modes:

• 4K native capture, with pixel-for-pixel 4K recording options.
• 4K native capture, with HD or 2K recording downsampled from the 4K image
• 2K native capture in high-frame-rate mode, using the full-frame sensor but with a less elegant hardware readout – binning or pixel-skipping or the like – similar to the 240fps mode on the FS700.

The camera ships with an OLPF optimized for 4K capture and 4K recording (4096 x 2160). Downsampled HD (1920 x 1080) and 2K (2048 x 1080) look very good, too, but are very slightly prone to high-frequency-detail aliasing (though in truth it’s no worse than most HD cameras, which use weak OLPFs to maximize the perception of sharpness in their images). In all these cases the F5 and F55 make very crisp images; some might even feel they are too crisp.

In high-frame-rate mode, aliasing is more pronounced: yes, there’s plenty of detail, but there’s also a greater risk of nasty aliases on fine patterns and sharp edges.

What if you could swap out the camera’s OLPF as needed for different situations? What, indeed…

Behind the F55’s FZ interchangeable mount, its Super35mm sensor sits behind an ND filter wheel and the OLPF.

Unlike most cameras, the F55 (and the F5) have removable OLPFs: simply loosen the black screw at the upper right, slide the filter to the right, and lift it off its pins.

Sony sent us a prototype HD/HFR OLPF in a custom case: hand-cut antistatic foam glued to a piece of cardboard, with a bit of clear plastic taped over the top. I think it’s safe to assume that production filters will come in somewhat more formal packages.

Just looking at the OLPF – both the 2K and the original 4K versions – there’s nothing special about them: they look like clear glass filters. Disappointingly, there’s no obvious blurring or birefringence or other whizzy optical effect visible. Darn it.

Installing the new OLPF takes perhaps a minute, start to finish: remove the old one, dust off the new one, slide it into place, and gently tighten the screw.

Might one worry about dust and dirt getting into the optics? I certainly did. But the OLPF stands off a fair distance from the sensor itself; far enough to allow for the ND filter wheel to rotate between ’em, so any dust spots on the OLPF itself should be well out of focus even with long lenses and small apertures.

For what it’s worth, after swapping the OLPF a couple of times in less-than-cleanroom conditions, I put the Sony 135mm PL-mount prime on, stopped it down to T22, and still couldn’t see any dust spots. Your mileage may vary.

Fine, so it slots in easily. Does it make any difference at all?

First, let’s look at 4K native capture with 4K XAVC recording. In all these shots, the F55 recorded XAVC, with default settings for sharpness, and I used no post-processing other than equalization of brightness:

When shooting 4K, the difference is extreme: the crispy-sharp edges of the 4K are rounded off, and the residual Bayer-pattern moiré is entirely suppressed. Note, also, the strong null at 1080 TVl/ph; why, it’s almost as if this filter were designed for 2K / HD imaging!

If you’re not a fan of digital’s hard-edged crispness, you might use the HD/HFR filter – yes, in 4K recording – to capture a softer, more film-like image, yet preserve the fineness of 4K’s sampling grid.

Now, 2K recording from a 4K native capture:

HD or 2K images shot with the 4K OLPF show plenty of detail past 1080 TVl/ph, which gives ’em that digital crispness but also risks plenty of aliasing. With the 2K filter the image is far cleaner. Which is preferable, as with the 4K images above, depends on what you’re looking for.

Bear in mind, also, that test charts are the nastiest thing to shoot. A real-world image shows a more typical comparison:

Even so, if you’re looking at the 2K OLPF pix and thinking, “geez, that’s soft“, remember also that you’re looking at a side-by-side (well, top-by-bottom) comparison. If I were to screen the 2K OLPF footage by itself, you probably wouldn’t see the image as soft at all; if anything, you might marvel at how natural it seemed compared to most sizzly-edged HD origination.

Now, high-frame-rate capture, using a binned or pixel-skipped sensor (Sony hasn’t told me exactly how the HFR capture works). These images are full-frame 2K native:

It’s pretty clear: if you plan to shoot slo-mo subjects with fine patterns on the F5 or F55, the HD/HFR OLPF is a Good Thing To Have.

The HD/HFR OLPFs should be available as I write this. It’s not necessary to have the firmware upgrade to use the filter.

The mere fact that the OLPFs are interchangeable, and that Sony has now made this public, means that the camera isn’t limited to Sony’s own 4K and 2K OLPFs. What if third parties got involved? The sharpness-at-any-cost crowd could commission a clear filter, essentially eliminating the OLPF altogether (a clear glass filter is required to maintain the optical path length and prevent the focus from shifting). Others could fine-tune frequency response to a greater or lesser degree as they saw fit.

And the beauty of the F5 and F55 is that the filter is so easily swapped: you can change the look of the camera with a minute’s work.

About the Author

Adam Wilt is a software developer, engineering consultant, and freelance film & video tech. He’s had small jobs on big productions (PA, “Star Trek: The Motion Picture”, Dir. Robert Wise), big jobs on small productions (DP, “Maelstrom”, Dir. Rob Nilsson), and has worked camera, sound, vfx, and editing gigs on shorts, PSAs, docs, music vids, and indie features. He started his website on the DV format, www.adamwilt.com/DV.html, about the same time Chris Hurd created the XL1 Watchdog, and participated in DVInfo.net’s 2006 “Texas Shootout”. He has written for DV Magazine and ProVideoCoalition.com, taught courses at DV Expo, and given presentations at NAB, IBC, and Cine Gear Expo.  When he’s not doing contract engineering or working on apps like Cine Meter, he’s probably exploring new cameras, just because cameras are fun.