Color Matching Lenses for Video
Traditionally, cine lenses have been color matched. Careful selection of glass and coatings results in consistent color and images that intercut flawlessly after the film is processed and edited together. Consistent color is one of the many features of cine lens sets. Among the others are T-stop markings, matching barrel size, fixed front diameter, smooth aperture, consistent focus and aperture ring sizes, consistent out of focus rendering, consistent contrast, etc. Lots of consistency there. No similar consistency is expected from photographic lenses.
Unfortunately for most video enthusiasts, cine lenses are both quite expensive and usually manufactured to fit cinema mounts like Arri PL or Panavision PV. Only recently lens sets like the Zeiss compact primes have started to appear for hybrid and photo mounts. Based on the latest generation of Zeiss SLR photo lenses, the Zeiss compact primes have been reworked to offer a significant degree of consistency, plus interchangeable mounts. But while not as expensive as bigger cinema lenses, Zeiss compact primes are not exactly cheap.
This leaves most large sensor video shooters in the photographic lenses camp. Photographic lenses present a lot of challenges for video, but they have a significant advantage: price. Selecting photo lenses for video is an art in itself (and possibly a topic for another article), but here we will focus on lens color and color matching. First, a quick word on a related subject.
White balance and lens color
One advantage of digital cameras over film is the ability to easily tweak white balance. Color film stocks are balanced for some specific color temperature, usually 5500K for daylight and 3200K for tungsten light. Color balance can be adjusted further during post-production, either chemically, through printer lights manipulation, or in DI.
Color temperature in Kelvin is not very useful for judging the color difference between illuminants. For example, the perceived change from 2400K to 2500K is about the same as the change from 10000K to 12000K. The difference of the reciprocals of the color temperatures is better related to perceived color changes. That’s why the mired (micro reciprocal degree) concept comes handy.
Mireds = 1000000 / (CT in Kelvin)
Incidentally, light color conversion gels often describe the resultant color shift in mireds.
Digital sensors are also optimized for specific spectral response (usually biased towards daylight). But by applying gain on individual color channels the signal can be white balanced in-camera to pretty much any desired color temperature. Furthermore, while film is usually only balanced on the orange-blue axis and expects green-magenta neutrality, digital video can also be white balanced on the green-magenta axis. For RAW video the white balancing decisions can be deferred to post. For video transformed to some working color space for recording the white balanced is baked during RAW conversion in-camera.
In-camera white balance means that lens color is less critical for digital video compared to film. The camera can be manually white balanced with a grey card after every lens change and this through-the-lens balancing will lead to neutral rendering. And this is how a lot of videographers work. But this practice deprives the videographer from a very nice tool: white balance can’t be used for creative purposes. Typical creative white balance uses include colder than neutral balance for winter or overcast feel and warmer than neutral balance for night indoor scenes. Of course, this can be done in post. But too much post-processing is bad for low precision video. Color matched lens sets, on the other hand, allow such creative choices to be dialed in when the sequence starts as subsequent lens changes do not introduce color deviations.
Photo lenses and color
Ideally, a lens should be completely neutral in color rendering. In reality it is not quite so. Lens color rendering is dependent on coatings and glass: both can cause tints. But if two lenses use the same glass and coatings, they will almost certainly render color in a very similar way. “Almost”, because tinted glass or coatings will generally lead to heavier tints (tint stacking) in lenses constructed from more glass elements.
Early coated lenses often exhibit yellowish tints. One reason is that warm coatings render skin lighter and skies darker in black and white, resulting in a favorable tonal separation. Earlier Leica lenses are like this. Some lenses from the 50′s to the 70′s used thorium dioxide to increase the refractive index of the glass. Thorium radioactivity leads to brownish tints over time. Well-known examples include some Pentax Super Takumar and various Kodak lenses.
The wide adoption of color film led to manufacturers developing coatings with a more neutral color rendition. Nevertheless, color varies not only among lenses from different manufacturers, but also in a series of lenses from the same manufacturer. It is common sense, though, that lenses produced by the same manufacturer close in time have higher chances of being made from the same glass and with the same coatings, seeing as both of these usually don’t change very often. The Zeiss Contax/Yashica mount lenses are good examples of consistent color in lenses stretching production over a significant period of time.
The following image shows a grey card rendered by various lenses. The white balance is set for the first lens and left as is for the others. Consequently, any tints manifested are relative to the first lens. Note how the Leicas are very close to each other (they were manufactured in the same year). Same with the two Contax Zeiss lenses (an early AEJ Planar and a somewhat later MMJ Distagon). The Leicas are noticeably colder than the Contax Zeiss lenses (a difference of around 20 mireds). They also have a green tint.
Color matching photo lenses in camera
Differences in color rendition between lenses can be easily quantified. One method appropriate for DSLR cameras takes advantage of their ability to shoot RAW still images. This involves shooting a RAW image of a grey card under a consistent light. Manually white balancing the image shot with each lens in Lightroom, Camera Raw or a similar RAW development software and comparing the resulting color temperature and magenta/green tint values will then reveal the relative differences in color rendition. Note that color temperature needs to be converted to mireds to yield a translatable result that can be used for color matching.
Some digital cameras have options for white balance shifts (or correction). For example, on a Canon DSLR camera white balance can be corrected from -45 to +45 mireds on an amber-blue axis in 5-mireds steps. There is also a similar -9 to +9 step correction on the magenta-green axis. While originally intended for correction under non-full spectrum light sources exhibiting color spikes, these options can also be used for in-camera lens color matching. For example, the Zeiss Contax/Yashica lenses from the above test can be decently color matched to the Leica R lenses by ticking the WB shift 4 steps towards blue and 4 steps towards green.
The Magic Lantern firmware for Canon DSLR cameras offers another way to measure color differences. Its point-and-white-balance feature allows the white balance to be set by pointing the camera to a neutral surface (ideally, a grey card). The calculated color temperature and WB shifts are immediately displayed for reference. This method has the advantage of yielding information directly in camera specific white balance terms. For better precision you should do this test under daylight because Canon DSLRs only set WB color temperature in 100K increments.
Once the relative differences in color rendition are measured against a known base setting, creative manual white balance can be used even with photo lenses. White balance can be preset for specific effects and left untouched for the whole sequence. Any lens change will only require dialing the respective WB shift values. Note that this will only help match color. Differences in contrast or out-of-focus rendering will still be present if they did exist in the first place.
This entry was posted by cpc on June 25, 2012 at 2:39 pm, and is filed under Uncategorized. Follow any responses to this post through RSS 2.0.You can leave a response or trackback from your own site.
I know very little about lenses but after reading this I understand a little more of the differences between photo & cine lenses. Thank you.
Let me see if I understand, once I balance/color match my lenses, where do I change white balance for creative purposes. Is it best done in camera before its rendered or is post ok? Is one better than the other?
For reference I’m using a Canon 60D & canon lenses.
“Selecting photo lenses for video is an art in itself (and possibly a topic for another article)”. I haven’t seen anyone else do this so its a big … yes please!
With heavily compressed video (as in DSLRs) it is best to have creative WB baked in camera. Less tweaking in post means less possibilities to break the image. With high quality compressed video you can take much more liberties while shooting. And with RAW video there is no WB burnt in the actual data, so WB is always applied in post.
That said, you can certainly shoot for neutral image (perfect white in camera) with any camera, and tweak WB in post. A lot of people actually shoot this way. But this generally means you will be amplifying/pushing some of the color channels and may lead to noise/banding in these particular channels. If you are baking WB in-camera, the camera firmware pushes the channels for you working with higher precision data before compression.
(and I am certainly not Roger ;)
Sounds like it might be worth trying to get the ‘look’ in camera so only very little is needed in post. Thanks cpc
Just a little note regarding temperatures in general: Even though you might say “degrees Celsius” or “degrees Fahrenheit” the SI-unit for temperature is just called “Kelvin”, not “degrees Kelvin”.
Good point. :)