Nikon Gamma Controls - v0.1 Beta Test

Overview

Nikon Gamma Controls bring interactive adjustment of the camera's gamma curve to video filming on DSLR's that support Nikon's custom Picture Control profiles. It is comprised of a set of calibrated gamma curves applied to Nikon's built-in Picture Profiles: Neutral, Portrait, Standard, Landscape, Vivid, and Monochrome. Unlike previous custom picture profiles such as Technicolor's Cinestyle, which provides a single, non-adjustable gamma curve that attempts to cover all types of shooting conditions, Nikon Gamma Controls enable the user to select a standardized gamma curve that best suits the illumination of each shot. And since Nikon Gamma Controls are calibrated to industry-standard grading tools, the selected gamma curve can be freely and accurately fine-tuned in a wide range of video editors without loss of image detail.

Gamma Control - Exposure's Hidden Dimension

One of the first challenges that confront videographers is gauging the proper exposure for each shot. While modern DSLR's provide a variety of illumination meters and exposure adjustments, high-contrast scenes can easily exceed the camera's limited dynamic range. In these situations it is often necessary to deliberately underexpose the darker areas of a scene in order to preserve highlight details in the brightest areas. This can result in starkly-lit video images that require significant grading to boost the visibility of shadow details. Unfortunately, the darkest areas in videos compressed with 8-bit H.264 encoders are recorded with noticeably degraded image quality, and this imposes a practical limit on the amount of enhancement that can be applied in post-production.

What is actually needed in these cases are controls that enable the user to set not only the exposure level of the highlights, but to independently control exposure of darker areas as well. When recording video, Nikon cameras convert the RAW sensor data into Rec. 709 video data and compress the scene's dynamic range into a format designed for viewing on consumer televisions. The core mathematical formula used in this conversion is known as the video gamma curve, and it determines the relative brightness of dark and midrange tones compared to the highlights. Broadcast engineers fine-tuned the Rec. 709 gamma curve for television viewing under subdued room lighting - a standard that is not well-optimized for image capture, particularly not for high-contrast lighting situations. For optimal image quality, the camera's gamma curve should ideally be adjusted specifically for each scene.

The Zen of Gamma Control

While there are many ways to adjust the perceived brightness of an image, the gamma function has a uniquely valuable property - it enables you to manipulate the proportional brightness of midrange tones while leaving the exposure of the brightest and darkest shades unchanged. That gives you the ability to protect highlight detail with a conservative exposure setting, while independently adjusting the overall brightness of the image with the gamma control. It is literally an extra dimension of exposure control, demonstrated in the video sequence below. The inset histograms measure the change in midrange brightness as gamma is varied from 1.0 to 0.4:

In practice, Nikon Gamma Controls are a set of seven gamma curves, in this case applied to Nikon's built-in Neutral Picture Profile. I have calibrated the curves with the base Neutral profile as gamma 1.0, decrementing with each step by 0.1 down to a gamma of 0.4. This provides a gamma adjustment range of 2.5:1, covering the full range of practical shooting conditions. The reason I chose this particular scaling is because it matches the calibration of the built-in gamma controls provided by a wide range of video editors. This calibration is so precise that you can grade each of the above clips back to gamma 1.0 simply by applying the same gamma setting that was used to record it. (In other words, to restore a video shot at gamma 0.5 back to the camera's default gamma 1.0 curve, simply grade it with a gamma curve set to 0.5. With a 32-bit video editor, this process is completely reversible - no image details are lost!)

Improving Image Quality with Gamma Control

If Nikon DSLR's recorded videos in uncompressed RAW format, there would be no practical advantage to using gamma control profiles. In addition to filming video, Nikon Gamma Controls can be used in JPEG still photography (e.g. to bring up shadow details in backlit photos). If you capture both JPEG and RAW image formats, however, you'll find that the RAW images are unaltered by Nikon Gamma Controls.

With video, however, the internal H.264 encoder burns the camera's gamma curve permanently into each recording, and it's here that Nikon Gamma Controls can visibly improve shadow detail quality. With a gamma of 0.5, for example, darker shades are recorded with about twice as much mathematical detail as that used in the camera's default gamma 1.0 picture profiles. An additional advantage is that boosting the recorded illumination of shadow details can give you the option of turning down the brightness when grading the video in post, instead of needing to dig image details out of the darkness. The example below shows how grading a video shot at the default gamma of 1.0 produces visibly coarser results than grading with gamma 0.7 or 0.5:

Nikon Gamma Controls Beta Test Info

This initial Beta Release of Nikon Gamma Controls has been thoroughly tested for accuracy and reliability on a Nikon D5100. In addition, I've verified proper operation on Nikon D5200 and D7000 cameras (and confirmed that Nikon D3100 and D3200 do not support custom Picture Controls). In addition, I've confirmed compatibility with the calibration of gamma controls provided in Adobe Premiere Pro and After Effects, versions CS5 - CS6.

My understanding of Nikon's documentation is that all contemporary pro and prosumer DSLR's support custom Picture Controls using a standardized, cross-compatible set of built-in picture profiles. Detailed information on the Nikon Picture Control System is available here:

http://imaging.nikon.com/lineup/microsite/picturecontrol/catalog/PicCon.pdf

The reason I'm releasing this version as a Beta test is because there are additional compatibility issues I don't have the resources to investigate on my own. I would appreciate help in confirming support for Nikon Gamma Controls in the following areas:

• Support for custom Picture Controls across Nikon's DSLR models.
• Accurate gamma calibration on each of the built-in Picture Profiles.
• Calibration of gamma adjustment controls in prominent video editing suites.

With Nikon Gamma Controls v0.1 Beta, I've included the complete set of gamma controls for Nikon's built-in Neutral profile, with default user settings adjusted for video recording. The Neutral profile is well-suited for video and provides a solid baseline for evaluation. Once we have confirmed compatibility with a broad range of Nikon DSLR's and video editing applications, I will generate calibrated gamma control profiles for all six of Nikon's built-in picture profiles and include them in subsequent releases.

Download and Installation of Nikon Gamma Controls

I've packaged Nikon Gamma Controls in a zip file attached to this post. After downloading and unzipping it, you'll find a folder named NIKON, which contains a CUSTOMPC folder that contains a set of Nikon Picture Control profiles. Copy the entire NIKON folder structure to the root folder of a compatible memory card and insert it into your camera.

To install Nikon Gamma Controls, find the "Manage Picture Control" option in the SHOOTING MENU. Select the "Load/Save" option, and then the "Copy to Camera" option. You should then see a list of the Nikon Gamma Controls profiles, starting with one named "Sepia". I included this profile as a convenient placeholder to fill in the seventh slot in the stock "Set Picture Control" menu, at the bottom end of the list of built-in picture profiles.

If your camera works like the Nikon D5100, it will accept up to nine custom profiles, labeled C1-C9. If you load Sepia into C1, it will be displayed along with the built-in profiles on the first page of the Set Picture Control menu. You can then load the seven Neutral Gamma profiles into C2-C8 slots, and they will all fit neatly on the second page of the menu. When loading each profile, the camera will give you the option to revise the profile name. You need to take care at that point, and press the +Magnify button instead of the OK button, to accept the custom profile without changing its name.

Usage of Nikon Gamma Controls

On Nikon DSLR's, video is recorded in Live View mode, and prosumer models work in a quirky manner that can subtly foul up the use of Nikon Gamma Controls. The fundamental problem is that the camera is unable to electronically update the lens aperture after you enter Live View - the lens iris will stay fixed even when the displayed aperture appears to change. In order to reliably set the aperture, you must exit Live View mode to do so.

Once you've set your aperture (in either M or A-priority modes), and returned to Live View, you may want to lock down your exposure with the AE-Lock button. This will work properly as long as you do not exit Live View mode. But beware, because there's a hidden pitfall - if you snap a still photo in Live View mode, the camera will exit and return to Live View. That will silently invalidate the AE-Lock exposure settings and your video exposure will no longer be correct. To be absolutely sure you're shooting with your intended exposure, always engage AE-Lock just before pressing the movie record button.

Once those glitches are under control, you can experiment with the seven profiles in Nikon Gamma Controls. As long as you stay in Live View mode, you can interactively select different gamma settings from 1.0 to 0.4, and evaluate their visible effects on the LCD screen. You can also use gamma bracketing - repeating a shot with a range of gamma settings - without leaving Live View between each recording. This is how I recorded the sample videos and it was very quick and convenient to adjust gamma on-the-fly while composing each shot.

If you examine the user settings in each Neutral Gamma profile, you'll see that I've set Sharpening to 0, Saturation to -2, and Hue to 0. These settings differ from the defaults in the built-in Neutral profile - they're what I found most useful for shooting video. If you experiment with these user settings, be sure to update each of the seven Neutral Gamma profiles the same way. That will preserve your ability to fine-tune the gamma profiles in post, with calibrated grading and intercutting among all Nikon Gamma Controls profiles.