Why using negative gain can be bad, unless you have an F3.

One way to reduce the noise in a video camera image is to reduce the cameras gain. We all know that increasing the gain to lets say +6db will increase noise and generally the reverse holds true when you reduce the gain, the noise typically reduces and this may be helpful if you are going to do a lot of effects work, or just want a clean image.

However in most cases negative gain reduces dynamic range as it will artificially clip your low key parts of the image. The brightness range is restricted by the cameras DSP (Digital Signal Processor), it only has so many bits of data to sample the sensors output, from dark to light. If you reduce the gain the highlight handling doesn’t change (DSP bit depth limited) but your entire image get shifted down in brightness and as a result you will clip off some of your shadow and dark information, so your overall dynamic range is also reduced. Dynamic range is not just highlight handling, it is the entire range from dark to light. 3db is half a stop (6db = 1 stop) so -3db gain reduces the dynamic range by half a stop.

So for cameras like the EX1 and EX3 or even PMW-500/PDW-700 using negative gain can be a bad thing to do. You need to be aware that there is a trade off of noise against dynamic range and need to be sure that the small noise benefit are worth the sacrifice of some latitude. The problem is further compounded on the EX1 and EX3 where the use of -3db gain also reduces the peak white recording level by 3db and with Cinegamma 2 this will prevent you from actually reaching peak white (unless you change the gamma curve gain).
Interestingly the PMW-F3 has an excess of dynamic range for the normal gammas and cinegammas and the processing appears to take advantage of this to keep the images very clean. When you shoot with the standard gammas and cinegammas on the F3 the cameras base ISO (sensitivity) is 400 asa at 25p. This gives a very clean, ultra low noise image with 11.5 stops of dynamic range. When you switch the camera to S-Log, which gives a greater dynamic range (approx 13 stops by my estimation) the base ISO increases to 800 asa. ¬†Looking at some of my S-Log test footage a clear increase in under exposure latitude can be seen when you use S-Log. I suspect that the “0db” point in the F3 is actually 800 asa as used by S-Log, where maximising dynamic range and using the full sensor range is the priority. Meanwhile with standard gammas, which are limited to 11.5 stops anyway, you can reduce the gain by 6db (1 stop) sacrificing one stop of underexposure but still have the full 11.5 stops but with 6db less noise.

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12 Responses to Why using negative gain can be bad, unless you have an F3.

  1. darraghsinnott says:

    Alister,
    thanks again for the insights – this is really helping me get to grips with the s-log feature on the F3.
    Can I just double check with you though – does one actually need a dual-link 10bit RGB recording device in order to get the benefit of the extra dynamic range in “s-log mode”?
    Your recent posts suggest to me that a single link 10bit YUV 4:2:2 signal would also carry the extra dynamic range (which would be fantastic if correct) and if so – why would one bother with an RGB 4:4:4 recorder – especially if the F3 is natively a 4:2:2 bayer sensor camera as you pointed out before in your article “when is …4:4:4 not really 4:4:4”?

    many thanks
    Darragh.

    • alisterchapman says:

      You definitely do NOT need 4:4:4 to record S-Log. 4:2:2 will do a very good job with S-Log and should be more than sufficient for the majority of projects.

  2. Lokendra Parajuli says:

    We already have an AJA Ki Pro Mini that we use to record 10 bit ProRes 422 (HQ) from our F3. We want to upgrade the F3 for S-Log ($3,300!).

    Can we record S-Log to 10 bit ProRes 422 (HQ) through SDI (or one of the dual link outlet!)?

    The $3,300 investment is big for us so we want to be sure before spending it. Any suggestion??

    Thanks in advance!

  3. alisterchapman says:

    Yes, you can record S-Log to ProRes via a single HDSDi Connection. You would use SDI port 1 (The left hand dual link BNC) to feed 4:2:2 S-Log to the Ki-Pro mini. Use the highest quality setting that you can (HQ preferably).

  4. Alister,
    I’m shooting wildlife exclusively with an EX3 and have used -3db since day one but you are saying that its costing me dynamic range… would you suggest 0db as a better option…

    Ray.

    • alisterchapman says:

      Yes you are loosing half a stop of dynamic range using -3db. If DR is critical then you should use 0db.

  5. projekt2501 says:

    That guy (http://blog.abelcine.com/2011/08/04/f3-s-log-part-1-on-the-charts/) suggest to use the sdi bnc for 422 s-log to kiPro, with the 1.5g 444 and s-log selected for the dual link. is there a 422 s-log available that can be recorded by KiPro over Dual Link?

    • alisterchapman says:

      You can get S-Log 422 out from the monitor HDSDi, but then you can’t use the look up tables. There is a planned firmware update due out some time in early 2012 (maybe January) that will allow you to get 422 S-Log out of the A/B outputs, then you will be able to use the LUT’s.

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