(HD)TV Cameras

The relation of the input signal of a cathode ray tube monitor (CRT) and the output luminosity is not linear. Using a linear input signal, the image of a grey wedge with constantly increasing brightness would not appear with constantly increasing brightness on a monitor. In order to have the monitor output the image, so it appears visually pleasing and correct, the camera signal has to be adapted to this non-linearity. For this reason, the camera signal is pre-distorted with an exponent, (Gamma). This is also called Gammacorrection.

Negative Film Stock

The characteristic of negative stock can be described as an S-curve. The interesting part, in this case, is the upper range, the shoulder of the negative material. This shoulder makes highlights appear with a soft transition. As described under Clipping, the imagers in digital cameras cannot handle highlight as well, as the imagers have a linear characteristic that does not end in a soft shoulder. Instead of a soft transition, these cameras show overexposed highlights with a hard edge, at worst with a colored border.

Digital Motion Picture Cameras

For digital motion picture cameras, the ability to provide a 1-to-1 transmission to be used as TV signal has no priority. The characteristic curve of these cameras can therefore be used to optimize the output signals. This means that the signal from these cameras provides an ideal output of the captured dynamic range, rather than a pleasantly looking image.

On the ARRIFLEX D-21, the operator is presented with choice of different contrast characteristics/characteristic curves: EI 100, 200, 250, 320, 400, 500, 640, and 800, as well as LOG C and LOG F.

EI 100 – 800

These characteristic curves should be considered "what you see is what you get"-settings. Their purpose is a visually correct representation of the camera image on a monitor while retaining as much image information as possible. An identification like "3601" in front of the EI value follows the example of how film stock is identified. "36" means that middle grey in the scene is output at 36% signal level. "01" means the version number.

EI stands for exposure index. As described under Adjusting Sensitivity, the imager of a digital camera only has a fixed sensitivity. The adjustment is done by amplification of the signal after it was readout from the imager. The EI value represents the corresponding ISO sensitivity of negative stock. If the operator selects e.g. EI 400, he has to set his exposure meter to 400 ISO to get the correct reading. Using the label ISO would be wrong, as explained under Adjusting Sensitivity. The EI 800 curve delivers only about 3 stops dynamic range below middle grey from the scene and should only be used as an emergency solution. The operator may as well opt for the 500 characteristic and have the tonal balance adjusted in post to have more detailed control.

LOG C

This characteristic curve provides a signal output according to Cineon format specifications. Each stop of exposure is mapped in equal intervals of digital code values. This characteristic is intended for use in a DI workflow, as it delivers the same tonal steps representation as the scans from a film negative. LOG C delivers an EI of 200 (ISO equivalent). As the sensitivity is not adjustable for this characteristic, this step has to be done in post. For viewing directly on the set, a so-called LUT (look up table) is used. A LUT transforms the signal to adjust the sensitivity and to provide a correct monitor input.

LOG F

This characteristic curve provides a signal according to FilmStream specifications. It has been implemented for customers using HD workflows adapted to FilmStream signals. LOG F delivers an ISO equivalent/EI of 200. As LOG F also does not allow altering the EI the same procedure applies as for the LOG C curve.

FilmStream, Panalog and S-log

Manufacturers such as Thomson Grass Valley, and Panavision all provide curves optimizing their camera's output. Thomson's curve is called FilmStream (as mentioned above), Panavision's is called Panalog and Sony's is called S-log. The cameras deliver a fixed sensitivity with these curves. When underexposed, the signal has to be adjusted through LUTs or color correction. Next to the S-log curve (available since the F23 came out), Sony offers the users an individual adjustment of the characteristic curve. This can be done on a laptop and then transferred to the camera, or adjusted directly on the camera. While this possibility is praised by a few DPs, one has to consider that the user can change the characteristic to a degree where the material cannot be corrected in postproduction anymore, forcing them to shoot the scenes again. Incorrect exposure happens all the time. If the margin for corrections is reduced by the user-created curves, the initial enthusiasm, however, quickly fades away. With the F35, Sony also offers the possibility of adjusting the camera's sensitivity when shooting with S-log. This is basically in accordance with the adjustment described under EI 100 – 800.

Look Management

Probably the most innovative solution is look-management via metadata. The captured material is always captured with the optimized characteristic curve. The look, which is created by the DP directly on the camera or e.g. on his laptop using reference frames, is only displayed in the camera preview output by means of a LUT. The look-metadata is recorded together with the images (or embedded in the images) and can be read out in postproduction. This metadata delivers a first and very detailed information about the Look that is intended by the DP and can be used to reproduce the look using postproduction tools. The D-21, unfortunately, does not allow transmission of metadata along with the images or the use of custom looks (LUTs) on a preview output. Some recorders (S.two, Codex) do however allow to put a look (LUT) on the monitoring output of the recorded signal. These LUTs can also be stored along with the material.