Image sensor size and lens resolution
Albert Chen 26th April 2012
First, congratulations to RED for their 6K dragon sensor. However with the advent of the 6K Dragon sensor measuring 15.8 X 30mm, it renders all Super 35mm lenses unusable for this sensor size. The required image circle of 33.906mm (diameter) cannot be met by S35 lenses which covers only 31.1mm image circle.
Even with the possibility of downsizing (image crop) in the menu to 4K, it is now at the approx resolution limit of the presently available S35mm lenses the likes of Carl Zeiss UP and MP lenses, Cooke’s 5i’s and Zoom lenses from Angenieux, Fujinon’s Aluras and the new Canon CN-E lenses to name a few.
Clearly the 6k sensor with a pixel size of 5 microns requires at least 200L/P resolution which means these lenses must be maintained at their highest quality. However, not all lenses are made the same. Some are incapable of 200L/P and some may be able to but due to high production costs, are measured, calibrated and tested to 100L/P.
It is no secret that there’s a race to increase sensor resolution. However, one wonders how many pixels can be packed into a small package like the S35 sensor size- 24 X 18mm approx 648 sq.mm. or approx 31.1mm diagonally which lenses must provide in their image circle which is another important issue. All S35mm lenses are not similar in their provision of image circle size – even the new ones. Case in point- the 2 new Canon CN-E lenses provide image circle of 27.5mm that would cover a sensor size of 24 X 13.5mm (1.78:1) 35 Academy.
Confused? Well, what’s the point here? It seems there’s a misunderstanding about camera and lens resolution and camera manufacturers have much to bear for this misconception. While there are lower resolution lenses made from yesteryears’ technology, which in their heyday was heralded as “best in the world” resolving 2K very well, these lenses are not really usable in a 4K camera selected to 2K. reason is that the sensor still maintains 4K resolution. How is that so? Well, normally one would expect the camera manufacturer to select alternate pixels (2K spread over the whole 4K sensor) when switched to 2K in a 4K camera, which has another very important issue, that of sensor coverage maintaining the 31.1mm image circle in the case of S35 full academy giving a true angle of coverage without image crop, it is unfortunately not so. When selected to 2K in a 4K camera, it instead crops the sensor by half loosing the outer image fields. This renders the lens angle narrower than it is. The solution is to put on a wider lens. However, this is where the mistake begins. With 2K selected, some operators would install a 2K resolution lens. What’s wrong with that? Well, the sensor is 4K resolution even when it’s selected to 2K (4K pixel depth) as stated above.
Camera manufacturers should correct this for 3 reasons. 1) When selected to 2K in a 4Kcamera, alternate pixels should be selected maintaining image acquisition size at S35. 2) When alternate pixels are used, then 2K lenses can be used. 3) The life of the pixels are maintained when they are universally used and not only those in the center. Now if the camera is smart enough to alternately select odd then even pixels at 2K then the life of the sensor would be even.
On another note, we have seen 4K as the maximum amount of pixels packed into the S35 full academy size 24.9 x 18.7 (4 perf) which allows the use of S35mm lenses covering 31.14 image circle but at 5K the sensor has gone slightly larger. Why? Well, there are a number of limitations like how small can you make a pixel, how many tracks do you need for each pixel and the cross talk from the close proximity of the pixels, tracks, etc. another subject, I reckon. And so now with the 6K Dragon having grown to 15.8 X 30mm, it is only a matter of time when processors are improved that an 8K sensor possibly at 24 X 36mm (8 perf) will emerge. The data storage will be huge but not as huge as the expenditure owners will have to foot out for new lenses. But wait, last time I’ve heard, there were a huge amount of these full 35mm lenses available but were rendered antiquated when film was phased out. Would Canon, Nikon, Carl Zeiss, Leitz, etc not wish to capitalize this market? Well, expect newly packaged full 35mm lenses to shortly appear in the market with new more sturdy mechanical housing with various mounts to match. As a matter of fact, Carl Zeiss is already on the band wagon ahead of the others with their Compact Primes. Remember? They are the only ones suitable to use on the Canon 5D?
While that is yet to be envisaged, Lens manufacturers are now coming up with a slightly larger image circle catching up with the 5K sensor (Millennium X) like in the RED Epic. Angenieux with their debut of their 2.7X zoom the 45- 120mm now offers a S35+ size covering 31.4mm image circle. Looks like lens manufacturers are playing catch up with camera sensor sizes. Why don’t they all come to the table and discuss what sensor size would be appropriate and building lenses to suite. At 24 X36mm full 35mm, lens manufacturers can produce glass resolving 200L/P at a moderate cost as opposed to staying at S35 Full Academy and trying to squeeze out 400L/P at astronomical costs. Now if camera and lens manufacturers wish to maintain S35 size, then they may have to adopt the 3 CCD chip system with the optical block and lenses manufactured for high resolution but have their chromatic aberration corrected via the optical lock. The CAC technology can also be adopted in this case.
Present owners of S35 lenses should press lens manufacturers to produce replacement relay lenses (rear group) that would enlarge the image circle to cover the new sensor sizes. However, not all lenses can be adapted and the modification may render a lower resolution from the fact that the original build is only for S35 resolution. At least the viability of the lens is extended for a period.