Gamma in Camera: Types of Response Curve

In principle, increasing the gamma primarily has the effect of boosting the mids.  But that simple explanation does not cover everything:

  • My experiences:
    • In cameras, selecting different gamma curves also affects the general levels, typically reducing them (not simply boosting the mids).
      • Doubtless because in a camera, the “focus” is on extending the latitude of the overall recording, to avoid blown-out highlights, as opposed to increasing the brightness of scene shadows etc.
    • In Sony XDCAM EX, the precise behaviour of the gamma has been empirically determined.
    • In Sony Vegas (version 9 at least), the 8-bits levels-space used is 0..255, as opposed to the broadcast-legal range of 16..235 adopted by most other Non Linear Editor (NLE) applications.  Thus, the lowest level generated by a typical camera, namely 16, constitutes a “mid” and as such does indeed get boosted by Vegas’s own Gamma effect.
      • A workaround might be, in Vegas, to apply Media FX to expand the levels range of the footage to 0..255, do all editing at that range then for the final delivery render, compress (linearly) down to 16..235.

In a linear space, a typical camera response curve is an S-curve where the bottom curve of the “S” is relatively short and the top curve of the “S” is long and drawn-out.  So the camera is sensitive to low-light and then saturates early-on in high light.  Presumably, if all we did was to alter gamma, then the bottom-part of the “S” would shorten (the response would “pick-up” earlier) and the top part of the S-curve would draw-out even more to the left, becoming even longer, tending to saturate earlier-on (with respect to light levels).  However in practice a typical camera will at the same time reduce levels prior to the gamma curve.  ???GET FORUM OPINIONS ON XDCAM EX DATA FLOW???.  The overall result is a reduced gradient.  This essentially extends the levels-range recorded while at the same time reduces the contrast range, giving a washed-out appearance.  Meanwhile, in log-space, which is meaningful because that’s how the eye perceives brightness, the overall effect of such gamma is to make the (log) curve more linear.  Presumably then the ultimate gamma curve would be logarithmic.  I found something called S-log, which sounds a bit like that, at

  • <<Its Sony’s version of shooting RAW>>
  • <<S-Log isn’t a very aggressive log curve. … most colorists just work with it as is without applying a LUT … (which can be done) with levels and gamma filters in any NLE.  A true Log encoded file, like a log DPX, or LogC from an Alexa is a bit more extreme and you’re probably better off finding a LUT to decode.>>
  • << if/when you shoot S-Log 8-bit, and you need to make it look normal or high-contrast … Then you’d be in trouble stretching that flat 8-bit image out.>>

Then at

  • <<the uncorrected image is so flat and washed out that it can make judging the optimum exposure difficult and crews using S-Log will often use traditional light meters to set the exposure rather than a monitor or rely on zebras and known references such as grey cards. For on set monitoring with S-Log you need to apply a LUT (look Up Table) to the cameras output. A LUT is in effect a reverse gamma curve that cancels out the S-Log curve so that the image you see on the monitor is closer to a standard gamma image or your desired final pictures. The problem with this though is that the monitor is now no longer showing the full contrast range being captured and recorded so accurate exposure assessment can be tricky as you may want to bias your exposure range towards light or dark depending on how you will grade the final production. >>
  • <<In addition because you absolutely must adjust the image in post production quite heavily to get an acceptable and pleasing image it is vital that the recording method is up to the job. Highly compressed 8 bit codecs are not good enough for S-Log. That’s why S-Log is normally recorded using 10 bit 4:4:4 with very low compression ratios. Any compression artefacts can become exaggerated when the image is manipulated and pushed and pulled in the grade to give a pleasing image. You could use 4:2:2 10 bit at a push, but the chroma sub sampling may lead to banding in highly saturated areas, really Hypergammas and Cinegammas are better suited to 4:2:2 and S-Log is best reserved for 4:4:4.4. >>

  • << The LOG mode … captures what the camera is capable of discerning. Because the maximum range of sensor data is being recorded at all times, there is more range to create the desired look in post. In a REC709 video gamma (in contrast), an image may have a bright light source overexpose to white and dark shadow areas record as black. The same image recorded in LOG may have considerable detail on both ends of the exposure range, which in later color correction can be exploited, if so desired. >>
  • << When footage is transferred with video gamma, it is meant for display (perhaps with minor adjustment applied later). When footage is transferred using the LOG CINEON curve, no artistic interpretation of the footage happens during the transfer – the goal is to preserve the full range of possibilities for later adjustment. This footage will look very flat and dull when displayed directly on a monitor. >>
  • <<  Most Digital Cinema cameras have a mode of recording or transcoding to a LOG curve. For example, Sony has S-Log (in the F35, F23, SRW-9000 and the PMW-F3), ARRI has LOG-C, RED has REDLOG, and Panasonic has FILMREC (which, while not technically a LOG curve, serves the same purpose). >>

  • << In a perfect world you would control your lighting so that you could use standard gamma 3 (ITU 709 standard HD gamma) with no knee. Everything would be linear and nothing blown out. This would equate to a roughly 7 stop range. This nice linear signal would grade very well and give you a fantastic result. Careful use of graduated filters or studio lighting might still allow you to do this, but the real world is rarely restricted to a 7 stop brightness range. So we must use the knee or Cinegamma to prevent our highlights from looking ugly. >>
  • << If you are committed to a workflow that will include grading, then Cinegammas are best. If you use them be very careful with your exposure, you don’t want to overexpose, especially where faces are involved. getting the exposure just right with cinegammas is harder than with standard gammas. If anything err on the side of caution and come down 1/2 a stop. >>
  • << If your workflow might not include grading then stick to the standard gammas. They are a little more tolerant of slight over exposure because skin and foliage won’t get compressed until it gets up to the 80% mark (depending on your knee setting). Plus the image looks nicer straight out of the camera as the cameras gamma should be a close match to the monitors gamma. >>
    • Great practical advice including the need to avoid fleshtones getting into the flesh region of the response curve


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