Anamorphic Lenses in 8K – An Interview
Every now and then someone will say something that ‘re-frames’ a problem or a topic (or a dispute!) in a way that brings a different and, sometimes, better understanding of it. This week Scott Dewald, a designer of anamorphic lenses that can be used with 8K digital sensors, reframed the idea of that kind of optics in a way that really changed how I think about them.
So what is an Anamorphic Lens?
Anamorphic lenses were adopted by the movie industry many decades ago to solve a technology mis-match. (although, according to one source, they were developed originally to allow French tank crews to see a wider field of view in WW2!) Film frames are, generally, relatively ‘square’ in their aspect ratio. 35mm film in the days of silent movies used a format of 4:3 aspect ratio (24.89 mm × 18.67 mm (0.980 in × 0.735 in)), although when audio was added on the film, the size dropped to 22 mm × 16 mm (0.866 in × 0.630 in) or 1.375:1 aspect ratio. That’s the kind of ratio that we use for digital displays.
Back when cinema content was always captured on film, movie makers wanted to produce very wide aspect ratio content that was more immersive by covering more of the horizontal field of view of the audience. There are about ten wide screen formats from ‘the old days’, Dewald explained. One even used three projectors running at the same time. Vistavision was probably the most popular wide-screen non-anamorphic format (horizontal 8 perf) and Panavision is one of the companies that continues to develop anamorphic lenses, although there are a number around the world.
(If you are interested in the topic of film aspect ratios, there’s a good article on Wikipedia – it’s amazing how many different formats have been used. However, most movies these days are in 1.85:1 or 2.39:1 2.39:1 is not very different from the 21:9 format – 2.33:1 – used for widescreen desktop monitors and some widescreen TVs).
A Big 35mm Supply & Technology Chain
The movie business was based on 35mm film and it was not feasible to change the whole system of film production, cameras, processing and projection to offer alternate formats. So, the idea developed of using special lenses that would compress the wide format horizontally at the time of capture. Matching projection lenses stretch the image out again at the time of projection. The 1.375:1 ‘Academy’ format could be stretched out to 2:1 and much more. That’s what anamorphic (literally ‘no shape’) lenses did. If you put them on the camera and the projector, you could see wide aspect ratios, while maintaining the standard size of the camera, film and projection supply chains. Digital camera sensor formats have tended to be based on 35mm film sizes with a ‘full-frame’ imaging area of 36mm x 24mm. That makes it easier to re-use existing lenses and optical designs.
It’s important to be able to match the capture lenses with the projection systems. If you project at, say, 2:1, but the capture system is at 1.8: 1, the actors will tend to look fatter and that is something they are unlikely to be happy about.
Now, with the development of digital sensors and no need for physical film stock, you might think that to get wide shots, you just need to make that shape of sensor, but it turns out that if you have something approaching a ‘square’ sensor, it is much more efficient to get the data out of the sensor – and that is a limiting factor that is significant for sensor designers who are under pressure to provide more resolution, more bits per pixel and more frames per second.
Re-Framing the Issue
That’s how this writer thought of anamorphic lenses – they just squeezed up the image one way, and expanded it the other. However Dewald re-framed that for me when he pointed out that what that means is that the focal length of the optical system has to be different in the vertical plane than in the horizontal. Now, I’m not an optical engineer, but I am interested enough in camera optics to realize that this is a non-trivial challenge.
Dewald has been designing new anamorphic lenses for the Atlas Lens Co company for several years now and they are good enough to be used in cameras with 8K sensors which is why we at the 8K Association were interested. But why do we need new ones?
Complex Optics with Critical Alignment
Dewald explained that the system used by market leaders such as Panavision is based on complex optics, using double prisms or cylindrical optical elements that need to be extremely accurately aligned. The need for accuracy is such that often lenses have to be, effectively, re-built after being used on a project, where cameras might be subject to all sorts of ‘non-ideal’ handling. That adds a lot of cost. Furthermore, the complexity makes the lenses physically bulky and heavy which goes against the trend for lighter and smaller camera systems in movie making.
The new Atlas “Mercury” lenses are said to be 80% of the diameter of other anamorphic lenses, and with 50% of the length because they exploit a moving element within the cylindrical optics of the lens. The patent-pending configuration also minimises ‘focus breathing’ – where the size and magnification of the image changes slightly as the focus is adjusted, and “anamorphic mumps”, where the anamorphic “squeeze” ratio changes as the focus distance changes. This can be a key consideration for cinema lenses.
The idea for a new optical path came from one of the Atlas executives ‘playing with some glass at home’. He realised that the effects he was getting could be really useful and the company set about the process of developing the optics, with Dewald’s help. Dewald told us that he had spent some time modelling the optics mathematically and that solutions to the complex equations (where practical) could lead to different optical configurations.
Atlas is very close to receiving a patent for the optical system, which should be published ‘any time now’ (and this article was a bit delayed as we were hoping to say that the patent had been granted).
All Colors is Tricky
Now, getting the lenses to work well for all colours when the focal length is different in the two planes is very tricky, so there is a tendency to concentrate most effort on the green, where the human photopic response is most sensitive. (Dewald was very involved with the development of the DCI P3 color gamut when he worked at TI during the introduction of digital cinema systems and our conversation wandered off at that point to the challenges of finding a good gamut when the only practical light source at the time was Xenon lamps).
The new lenses are suitable for 8K sensors, but Dewald told us that for some creators, the lenses are ‘too sharp’. The same has been said about some of the top quality digital prime lenses from other makers. For this reason, Atlas has not done a lot of work to make the lenses ‘super sharp’ in the corners, They have concentrated their efforts on the region where most of the story-telling takes place – the center of the frame. The lenses have been optimized for the P3 gamut but also to take account of the human visual system in the way that it perceives colour. They have also been designed to give attractive aberrations (or ‘bokeh’ in the jargon. There’s a nice graphic on the B&H website that allows you to wipe between regular and anamorphic bokeh – editor).
Anamorphic Has a Particular ‘Look’
The writer was intrigued by this and checked out a series of videos by Panavision that emphasised the particular ‘looks’ that came from the use of anamorphic lenses. In particular Panavision highlights the directional, rather than rounded, bokeh and lens breathing that tends to just be visible in one plane. There is also a softer depth of field focus roll-off. By using a lens that has a longer focal length than if you took an image from a wide angle lens and then simply cropped the top and bottom off, you get a shallower depth of field with a smoother roll-off of focus. (Red, the camera maker, has a nice demo of this on its explanation of anamorphic lenses.)
To this editor, most of these ‘features’ seemed to be making the best of the aberrations in the process! However, some cinematographers obviously like the effects and Dewald pointed out that in Hollywood, lenses are often ‘tuned’ in particular ways to meet the desire for particular ‘looks’ for different content. The decisions are often ‘artistic’ rather than being aimed at technical excellence.
The Cinema Experience
Dewald is of the opinion that part of the ‘cinema experience’ actually comes from less than ideal image display – for example with just 24fps and with low levels of illumination, where the visual system works differently. Some of the artifacts help to reduce the realism, but this can help with a ‘suspension of disbelief’ and he said that users actually tend to prefer some reduction in sharpness towards the edge of the frame. This helps creatives to focus more of the attention of the viewer on the main action in the center.
The early specifications for digital cinema projection lenses were very demanding in accuracy at the pixel level. That was a challenge, but Minolta was able to solve the problem, he said. The reality was that creators were less worried about this extreme accuracy as film suffers from many factors that can reduce the effective resolution of the image anyway and they were used to that.
As well as the idea that a slightly softer image helps the immersion of cinema, Dewald said that part of the appeal of the anamorphic format was a kind of nostalgia for the look of movies that came out when wide formats were very new.
One of the big challenges of designing these lenses is the process of manufacture, in particular lens grinding and polishing, which is basically unchanged for centuries except for some automation and improved measurement. If you could make big lenses using additive processes such a 3D printing, the challenges would be very different, he said.
Overall then, one of the advantages of using 8K is the improved ‘smoothness’ of images that has recently been reported in articles, rather than the hyper-resolution of extreme resolution.