Posts Tagged ‘gamma

31
Dec
10

About the Picture Profile and S-Curve of Canon video DSLR cameras

Hey Martin,

I have another question for you, since I’m just beginning to learn in the ins and outs of gamma curves:

I noticed in the original Marvel’s Cine Flat Profile 1.2, the gif of the gamma curve you posted seems to have a fairly pronounced toe, but not so much of a shoulder. This looks like to me that you’re pushing the apparent middle part of dynamic range a bit higher up. And I’ve noticed in my tests that the picture occasionally looks a tad dark if I expose for it correctly with a incident meter light meter (especially in low-contrast scenes.)

Should I be compensating +1/3 or +2/3 a stop for proper exposure with this profile? Or should I be exposing normally (with a light meter) and adjusting the picture to my preference in post? Or should I just not use a light meter and eyeball it instead?

Am I misunderstanding the gamma curve?

-Dave

Hello Dave.

What really is going on, is that we apply a curve on a curve. The sampling, filtering and further post-processing of the camera’s image-sensor is a very complex matter and we can’t imagine (and are not being told) what goes on “behind the scenes”. You can get an insight into the complexity by reading this wikipedia link on the Bayer Filter. After the camera eventually has rendered an electronic representation of a real world image, many extra circuits – both analog and digital – are put to work to turn it into anything useable and digital. With the extra handicap that the video image is further processed and compressed.

The picture profile settings (and a few extra image settings that can be accessed through the menus) are just the tip of the iceberg of the motherload of parameters and circuits that can be tweaked. Not only “simple” electronics are involved, but quite a lot of physics as well.
The eventual image (based on the selected preset profile, e.g. Neutral or Faithful) already has a quite complex curve applied to it electronically sometime during the processing described above. In fact, there already is something in place that resembles an S-curve! What we do by altering the curve, making that existing curve more pronounced. It also means, that when we pull it too much, relations between color channels are disturbed, resulting in “plastic skintones” and other weird and unexpected artifacts.

To get back to your first observation. The Canon D cameras all seem to underexpose when using an external light-meter, and this seems to have to do with the way the camera handles the ISO setting. Specially when you are using a flat profile, you can push it much more, maybe indeed to +2/3. The built in histogram display is your friend here! Try to Google on the subject and you will find many discussions regarding this issue. Some propose to lower the ISO setting on the meter.

You are right about the curve. It is S-shaped and you can see a typical picture of the Marvels cine profile below (many people have asked for this, so here it is!). Because it’s applied on top of an existing curve (we have no other option, we have no Raw), the actual resulting curve is hard to calculate/visualise without reliable measurement. Digital video cameras such as the Sony EX-1 can deliver a number of test signals to the post-processing electronics, so the effect of curves and painting can easily be analysed. The Canons don’t have this option. What one could do, is shooting calibrated multi-level grayscales and analyse the results. A graph can be drawn from those results, most probably resulting in a wobbly s-curve.

Marvels Cine 3.0 Panalog example curve

Click for enlargement

Also, changing the profile’s Contrast setting alters the curve – it is NOT linear. So, having said all this; using your eyes and the histogram is still your best option, and getting some experience with the profiles and the resulting footage.

I am currently using the new Magic Lantern firmware patch that gives me a spot-reading, so i can e.g. expose human skin on 75% and check highlights. This and watching the histogram gives me good results.

I wish you all a happy and prosperous 2011!

Martin Beek.

14
May
10

How to perform on-set in-camera (hyper-) gamma adjustment with HD cameras

How to perform on-set in-camera gamma adjustment
by Martin Beek, DP

Update may 14: this workflow does not work with the Sony EX1 & EX3 Cine Gammas; please use Std-3 or Std-4 gamma for these cameras.

In this article i want to explain the steps that i take to get the best exposure and gamma settings, before each shoot. Although it’s common practice for DPs using HD cinema cameras,  there is not much information to be found on the internet on this subject. Of course; we have seen many picture profiles published and there are many discussions on which gamma curve to use under specific conditions, but is usually stops there. Most people don’t take a moment to adjust the camera any more than the way it was set up out of the box; or resolve to using the picture profiles they find on the internet, without alteration. I have always found that each specific scene demands a different setting, sometimes only slightly, sometimes drastically.

Because we publish a number of picture profiles ourselves, i receive questions regarding gamma settings almost weekly – mainly from indie film makers looking for a way to add a more cinematic look to their pictures; exactly what i’ve done on Vrije Val (“Free fall”, IMDb: here) . Now that after two years the world seems to have settled on using one profile or the other , the questions i receive are nowadays more far more specific and are mainly about workflow and detailed gamma settings.

As a start, i have published the steps i take before each shot below – regarding the setting of exposure, blacks and gamma –  and they generally apply to each camera that offers black, gamma-level and black-gamma settings. Ranging from the Sony XDCam-EX series to the Panavision Genesis and everything in between. For the latter and other cameras that use a LUT and/or RAW workflow such as the RED, the basics of the following still apply, but then partly in a the postproduction stage.

The following workflow is pure technical and skips the whole leading cinematographic process of lighting the scene, defining scene contrast and composition.

  1. Black balance
    If you are using a camera that has a black balance function (on the sony ex-1 in one of the hidden service menus), perform a black balance. Most camera’s will close down the aperture; on others you have to place the lenscap on the lens.
  2. White balance
    A correct white-balance, or a faulty one in that respect, changes the camera’s optical block behaviour and should therefore be carried out with some care. Use a 50% grey card or dedicated white balance card.
  3. Set black pedestal (Black level / master black) – bottom of the curve
    Do this once and leave it alone for the rest of the camera’s life. Connect the camera to a hardware or software waveform monitor (eg. Adobe OnLocation). Set gain to zero, cap the lens. Dial down the Black level until it just rests on the bottom Zero line of the scope. Don’t stretch or crush the blacks – taking master black further down will influence the gamma curve and all other levels of the picture. So leave it at Zero and don’t touch it again. We will use the following detailed gamma controls to darken or brighten the picture.
  4. Set exposure (lens aperture) – top of the curve
    Highlights can not be easily fixed in post, so i always set exposure for highlights. You do this using the lens aperture (iris). Set highlights using the iris around 100-103 IRE. If you only have zebras or histogram available, let highlights come up to the level of being on the brink of burning out. If you are not concerned about highlight detail in a contrast-rich scene, it’s up to you to define “burned out” ;-). If you have to deal with bright skies, consider using a ND.6 grad filter.
  5. Set gamma level – middle part of the curve
    Some cameras offer a course and detailed control over the gamma curve. The Sony EX-1 for instance has a “Level” adjustment on the same menu-page as where you define the gamma type. Use this gamma level for this step.
    Since human flesh tones generally fall in the middle section of picture signal and gamma curve, we can use a human face or a dedicated grey card to adjust the 50-70% range. Use either a waveform monitor, zebras or histogram to monitor what you’re doing. Adjust the gamma level so that the skin exposure or test card falls in the middle range between 50 and 70%. Highlights on skin can easily go up to 70% but leave it at that max for this specific procedure. Bringing the gamma level up will increase the brightness of the middle range and lowering the gamma level will bring it down.
  6. Set black-gammal-level – lower part of the curve
    Instead of crushing blacks and doing other
    terrible things to the signal, we now continue to adjust the lower part of the curve – the shadows – using the Black Gamma setting.  With this we control how the lower part of the curve behaves and dialling it down makes shadows darker and reduces detail in these areas. Dialling it up makes shadows more grey and will show more information. This setting can thus be used to delicately control the contrast of the scene and to obscure noise and other unwanted information from the darker areas of the picture by lowering this value. On the contrary, e.g. during night shots, extra information can be extracted by raising the black gamma level.
  7. Use your eyes!
    The eyes can be found on either side of your nose and do not have any controls you can access through any of your menus. If you have a reasonable HD monitor that has at least it’s black and white levels calibrated, you should be able to see what your’e doing to the overall picture. Use some common sense too. If you have to increase or decrease any of the levels mentioned over one quarter of the total scale (e.g. -50 on a -99 <> +99 scale), you should consider adjusting the overall lighting and contrast of the scene – if that is something under your control. If you don’t have a reliable monitoring option, use the camera zebras and histogram to judge levels.

The above works best with a gamma curve that – initially – gives you the flattest look; e.g. the Cine Gammas. Choose a gamma that offers you the maximum latitude possible for your specific camera. Since you have full control over the behavior of the gamma curve with the controls mentioned above, there is usually no need to choose a different gamma curve; unless extreme conditions require so. AND DON’T CRUSH BLACKS! Listen to your old DP, please! 😉

Update may 14: for Sony EX1 & EX3 cameras, use Std gamma 3 0r 4, switch off auto-knee, set knee to 88, slope to +15 – to follow this workflow.

I want to conclude with the habitual apologies for my English – i am not a native English speaker.

Please feel free to comment on this post.

Martin Beek.




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