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EX1 Gammas

Those cine gammas I like? They're not for everyone.

By Adam Wilt | February 17, 2008

When I reviewed the Sony PMW-EX1 camcorder, I raved about its cine gamma settings, derived from the hypergamma curves in the F23, F900/R, and various XDCAM HD 'corders. The email I've been getting indicates that my enthusiasm isn't universal. Let's explore the issue a bit further.

Capturing a pleasing yet wide-ranging tonal scale on video is a bit of a challenge. Traditional video gamma settings render a "linear" look, in which the perceived contrast on screen closely matches the perceived contrast of the same scene in real life (OK, that's an oversimplification, but bear with me here). That works fine, up to a point: the point where the video hard-clips at a brightness, in the digital domain, of about 109%.



Video cameras typically ameliorate the harsh clipping with a knee, a flattening of the contrast above a certain point. The knee does two things by flattening the contrast curve: it allows the capture of somewhat brighter values before the video hard-clips at 109%, and it lessens the nastiness of that crash by allowing more of a "hard handing" instead of a "hard crash" when scene brightness approaches the clipping level.



Sony's Cine gamma (or Hyper gamma) curves try to emulate the more gradual, nonlinear "shoulder" of film, so the gamma curve starts flattening out earlier, and rounds off instead of having a distinct knee point. There's no hard crash, nor even a hard landing, rather the bright parts of the scene softly roll into featureless white overexposure at the clipping level.



Here's the problem: the cine gamma round-off and the associated smooth desaturation set in at a comparatively low level, say, around 80%. But you'll often have skintones at or above that level; skintone highlights will often reach up into the mid-90s.

With a normal knee, the entire skintone range (short of specular highlights and glare) is usually within the linear part of the tonal scale, so faces retain a warm, saturated, natural look. Overexposed highlights intruding into the knee area will show strong tonal compression, but if your knee point is set up in the high 90s or so, the affected area will be fairly small. Also, most knees don't do much with saturation; what typically happens is that overexposed areas start clipping in one or more colors, so skin typically goes a bit yellow just before blowing out completely. It's a look we're all very familiar with.

If you use Sony's cine gammas on those same faces, and keep the same levels, more bright areas of the face (or other skin areas) will show some tonal compression and some desaturation. As the compression sets in a bit earlier, more of the face will be flattened and desaturated: by comparison to the "standard" look, skintone highlights aren't as vibrant and contrasty and colorful. Faces can look a bit wan and washed out, without that ruddy, saturated,"glow of good health".

(I should mention here that most of the Cine gammas depress midtones without changing exposure, so that skin tones will often be low enough to avoid the compressed region. But for video use, you'll either need to pull those midtones up in post or increase the exposure to put the skintones back where we're accustomed to seeing them. The former can't be counted on unless you are the editor, and the latter just brings the faces up into the compression zone again.)

In isolation, I find that the cine gamma look is a bit more "true to life"; it handles overexposure more gracefully, and IMHO looks a little bit more like the way film handles highlights. But when compared to the standard gamma, yes, faces don't look as good, at least as long as you avoid overexposure.

And let's face it, faces are important. If you're shooting people and pretty much putting their skin tones throughout the upper half of the tonal scale, you may very well prefer using the linear standard gammas with a sharp knee affecting only the very brightest bits of the picture.

I'll be doing more detailed side-by-side tests with the various gammas with a bunch of folks in two weeks, so I'll have some good examples to post, as well as more discussion of the tradeoffs of the different gammas and knees. But for now, just be aware that the cine gammas I was so effusive about aren't necessarily right for all subject and all situations. For example, you might use cine gammas to keep a naturalistic sky when shooting establishing shots outdoors with uncontrolled, contrasty light, but switch to a standard gamma just for the close-ups, where the attention is focused on the face.

And as always, don't take my word for it, or anyone else's: test it for yourself and form your own opinion—and post that opinion here, so we can all learn from it.

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Comments

Simon Wyndham: | December, 16, 2008

The compression of skin tones is something that a lot of people do not consider with these cinegamma style settings. As well as this, regarding the rather ‘flat look’ of these settings Sony recently released a document stating that Cinegamma 2 should be used for straight video output.

This is something that I found rather odd because there is no way I could allow something shot with Cinegamma 2 to go out without bringing those mid tones up in post.

One solution is to use Cinegammas 3 and 4 which stretch out the mid tones more and have a higher ‘shoulder’ than gammas 1 and 2. These should provide a nice smooth highlight roll off without compressing the skin tone areas as much.

It would be interesting to see if your test results bear this out, Adam.

Adam Wilt: | December, 16, 2008

Simon: I simply haven’t found a situation for using CGs 1 or 2. Indeed, my default setting now is CG4, to the point that I find it painful to have to shoot in standard Rec.709 gamma. So I agree with you:CG2 is too dark in the midtones for direct use (for televisions, at least; it may be OK in a darkened cinema setting).

Simon Wyndham: | December, 17, 2008

The STD gamma is an odd one in the EX, and the 3xx series XDCAM cameras. The SD XDCAMs had similar menus and gammas to the first HDCAM 750’s. The STD gamma had an okay knee. Perhaps slightly low at 85%, but pretty good, and could be manipulated, as Alan Roberts showed, to produce a very wide contrast range ability.

Have you tested all of the STD gammas? I would assume that STD gamma 4 is the “BBC gamma” setting with an initial gain of 4.5ire? I’m just wondering what kind if usable range could be obtained with this gamma, or even STD gamma 3 if the black gamma, knees slope, and knee point were manipulated compared to Cine Gamma 1 and 2?

I too am pretty much locked onto Cinegamma 4. Though I do wonder if a better result could be had by manipulating from scratch via the STD gammas.

Adam Wilt: | December, 17, 2008

I did look at all of the Std gammas. I found that, as far as I could tell, all the gammas (with appropriate knee settings on the std gammas) were capable of the same dynamic range.

A problem on the EX series cams is that the knee fails to properly process highlights with saturated color (see: http://provideocoalition.com/index.php/awilt/story/review_sony_pmw_ex3_removable_lens_1_2_3_cmos_hd_camcorder/P2/). That pretty much discouraged me from using any of the std gammas in real work, except under very carefully controlled conditions. However, as I’ve fallen under the spell of cine gammas (and hyper gammas on other Sony cams), I don’t want to shoot with std gammas any more!

Simon Wyndham: | December, 17, 2008

Interesting. Did you play around with the knee sat function? Not questioning your testes, which I know will be very thorough. Just playing devils advocate. grin

Simon Wyndham: | December, 17, 2008

I meant to say “tests” in case any rather odd minds are reading this!

Adam Wilt: | December, 17, 2008

I tried everything, including knee sat. With appropriate manipulation of knee point and slope it was possible to minimize the effects of it, but only at the expense of having the knee do its job properly. I even asked the Sony engineers about it at NAB (having previously reported it to Sony after testing the EX1, and visiting Sony’s San Jose facility to demonstrate it in person: you should have seen the long faces; you’d have thought somebody had just died). They knew immediately what I was whingeing about. Apparently it is “very difficult” to fix, and it’s something more fundamental than can be solved by a firmware update.

Adam Wilt: | December, 17, 2008

My Sony contacts give me the following info on EX gammas:

Standard Gamma
STD1?3R9T1 (DVW-709 like)
STD2?SMPTE-240M
STD3?ITU709 gain 4.5x
STD4?BBC gain 5x (more black detail)

Hyper/ Cine Gamma
Cine-1 = HG4 of HDW-900R, F-23… (109% clip)
Cine-2 = HG2 of HDW-F900R, F-23… (100% clip)
Note: there is no HG3 equivalent curve on EX series cameras.

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