I’ve talked about 4K before (here, here and here), but I’ve recently done some more 4K jobs that have me thinking again. 4K means different things to different people and in terms of dimensions, there’s the issue of cinema 4K (4096 pixels wide) versus the UltraHD/QuadHD/4K 16:9 (whatever you want to call it) version of 4K (3840 pixels wide). That really doesn’t make a lot of difference, because these are close enough to be the same. There’s so much hype around it, though, that you really have to wonder if it’s “the Emperor’s new clothes”. (Click on any of these images for expanded views.)
First of all, 4K used as a marketing term is not a resolution, it’s a frame dimension. As such, 4K is not four times the resolution of HD. That’s a measurement of area and not resolution. True resolution is usually measured in the vertical direction based on the ability to resolve fine detail (regardless of the number of pixels) and, therefore, 4K is only twice the resolution of HD at best. 4K is also not sharpness, which is a human perception affected by many things, such as lens quality, contrast, motion and grading. It’s worth watching Mark Schubin’s excellent webinar on the topic to get a clearer understanding of this. There’s also a very good discussion among top DoPs here about 4K, lighting, high dynamic range and more.
A lot of arguments have been made that 4K cameras using a color-pattern filter method (Bayer-style), single CMOS sensor don’t even deliver the resolution they claim. The reason is that in many designs 50% of the pixels are green versus 25% each for red and blue. Green is used for luminance, which determines detail, so you do not have a 1:1 pixel relationship between green and the stated frame resolution of the sensor. That’s in part why RED developed 5K and 6K sensors and it’s why Sony uses an 8K sensor (F65) to deliver a 4K image.
The perceived image quality is also not all about total pixels. The pixels of the sensor, called photosites, are the light-receiving elements of the sensor. There’s a loose correlation between pixel size and light sensitivity. For any given sensor of a certain physical dimension, you can design it with a lot of small pixels or with fewer, but larger, pixels. This roughly correlates to a sensor that’s of high resolution, but a smaller dynamic range (many small pixels) or one with lower resolution, but a higher dynamic range (large, but fewer pixels). Although the equation isn’t nearly this simplistic, since a lot of color science and “secret sauce” goes into optimizing a sensor’s design, you can certainly see this play out in the marketing battles between the RED and ARRI camps. In the case of the ALEXA, ARRI adds some on-the-sensor filtering, which results in a softer image that gives it a characteristic filmic quality.
Why do you use 4K?
With 4K there are two possible avenues. The first is to shoot 4K for the purpose of reframing and repositioning within HD and 2K timelines. Reframing isn’t a new production idea. When everyone shot on film, some telecine devices, like the Rank Cintel Mark III, sported zoom boards that permitted an optical blow-up of the 35mm negative. You could zoom in for a close-up in transfer that didn’t cost you resolution. Many videographers shoot 1080 for a 720 finish, as this allows a nice margin for reframing in post. The second is to deliver a final 4K product. Obviously, if your intent is the latter, then you can’t count on the techniques of the former in post.
When you shoot 4K for HD post, then workflow is an issue. Do you shoot everything in 4K or just the items you know you’ll want to deal with? How will this cut with HD and 2K content? That’s where it gets dicey, because some NLEs have good 4K workflows and others don’t. But it’s here that I contend you are getting less than meets the eye, so to speak. I have run into plenty of editors who have dropped a 4K clip into an HD timeline and then blown it up, thinking that they are really cropping into the native 4K frame and maintaining resolution. Depending on the NLE and the settings used, often they are simply blowing up an HD shot. The NLE scaled the 4K to HD first and then expanded the downscaled HD image. It didn’t crop into the actual 4K native resolution. So you have to be careful. And guess what, if the blow up isn’t that extreme, it may not look much different than the crop.
One thing to remember is that a 4K image that is scaled to fit into an HD timeline gains the benefits of oversampling. The result in HD will be very sharp and, in fact, will generally look better perceptually than the exact same image natively shot in an HD size. When you now crop into the native image, you are losing some of that oversampling effect. A 1:1 pixel relationship is the same effective image size as a 200% blow-up. Of course, it’s not the same result. When you compare the oversampled “wide shot” (4K scaled to HD) to the “close-up” (native 4K crop), the close-up will often look softer. You’ll see defects of the image, like chromatic aberration in the lens, missed critical focus and sensor noise. Instead, if you shoot a wide and then an actual close-up, that result will usually look better.
On the other hand, if you blow up the 4K-to-HD or a native HD shot, you’ll typically see a result that looks pretty good. That’s because there’s often a lot more information there than monitors or the eye can detect. In my experience, you can commonly get away with a blow-up in the range of 120% of the original image size and in some cases, as much as 150%.
To scale or not to scale
Let me point out that I’m not saying a native 4K shot doesn’t look good. It does, but often the associated workflow hassles aren’t worth it. For example, let’s take a typical 1080p 50” Panasonic plasma that’s often used as a client monitor in edit suites. You or your client may be sitting 7 to 10 feet away from it, which is closer than most people sit in a living room with that size of a screen. If I show a client the native image (4K at 1:1 in an HD timeline) compared with an separate HD image at the same framing, it’s unlikely that they’ll see a difference. Another test is to take two exact images – one native HD and the other 4K. Scale up the HD and crop down the 4K to match. In theory, the 4K should look better and sharper. In fact, sitting back on the client sofa, most won’t see a difference. It’s only when they step to about 5 feet in front of the monitor that a difference is obvious and then only when looking at fine detail within the shot.
Not all scaling is equal. I’ve talked a lot about the comparison of HD scaling, but that really depends on the scaling that you use. For a quick shot, sure, use what your NLE has built in. For more critical operations, then you might want to scale images separately. DaVinci Resolve has excellent built-in scaling and lets you pick from smooth, sharp and bilinear algorithms. If you want a plug-in, then the best I’ve found is the new Red Giant Instant 4K filter. It’s a variation of their Instant HD plug-in and works in After Effects and Premiere Pro. There are a lot of quality tweaks and naturally, the better it does, the longer the render will be. Nevertheless, it offers outstanding results and in one test that I ran, it actually provided a better look within portions of the image than the native 4K shot.
In that case, it was a C500 shot of a woman on a park bench with a name badge. I had three identical versions of the shot (not counting the raw files) – the converted 4K ProRes4444 file, a converted 1080 ProRes4444 “proxy” file for editing and the in-camera 1080 Canon XF file. I blew up the two 1080 shots using Instant 4K and cropped the 4K shot so all were of equal framing. When I compared the native 4K shot to the expanded 1080 ProRes4444 shot, the woman’s hair was sharper in the 1080 blow-up, but the letters on the name badge were better on the original. The 1080 Canon XF blow-up was softer in both areas. I think this shows that some of the controls in the plug-in may give you superior results to the original (crisper hair); but, a blow-up suffers when you are using a worse codec, like Canon’s XF (50 Mbps 4:2:2). It’s fine for native HD, but the ProRes4444 codec has twice the chroma resolution and less compression, which makes a difference when scaling an image larger. Remember all of this pertains to viewing the image in HD.
So what about working in native 4K for a 4K deliverable? That certainly has validity for high-resolution projects (films, concerts, large corporate presentations), but I’m less of a believer for television and web viewing. I’d rather have “better” pixels and not simply “more” pixels. Most of the content you watch at theaters using digital projection is 2K playback. Sometimes the master for that DCP was HD, 2K or 4K. If you are in a Sony 4K projector-equipped theater, most of the time, it’s simply the projector upscaling the content to 4K as part of the projection. Even though you may see a Sony 4K logo at the head of the trailers, you aren’t watching 4K content – definitely not, if it’s a stereo3D film. Yet, much of this looks pretty good, doesn’t it?
Everything I talked about, regarding blowing up HD by up to 120% or more, still applies to 4K. Need to blow up a shot a bit in a 4K timeline? Go ahead, it will look fine. I think ARRI has proven this as well, taking films shot with the ALEXA all the way up to Imax. In fact, ARRI just announced that the AMIRA will get in-camera, on-the-fly upscaling of its image with the ability to record 4K (3840 x 2160 at up to 60fps) on the CFast 2.0 cards. They can do this, because the sensor starts with more pixels than HD or 2K. The AMIRA will expose all of the available photosites (about 3.4K sensor pixels) in what they call the “open gate” method. This image is lightly cropped to 3.2K and then scaled by a 1.2 factor, which results in UltraHD 4K recording on the same hardware. Pretty neat trick and judging by ARRI’s image quality, I’ll bet it will look very good. Doubling down on this technique, the ALEXA XT models will also be able to record ProRes media at this 3.2K size. In the case of the ALEXA, the designers have opted to leave the upscaling to post, rather than to do it in-camera.
To conclude, if you are working in 4K today, then by all means continue to do so. It’s a great medium with a lot of creative benefits. If you aren’t working in 4K, then don’t sweat it. You won’t be left behind for awhile and there are plenty of techniques to get you to the same end goal as much of the 4K production that’s going on.
Click these thumbnails for full resolution images.
©2014 Oliver Peters
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