The truth about video gain and how to use it properly
Video gain is something that is misunderstood by most young camera operators. The ability of prosumer cameras to operate in an “automatic” mode has fueled this fire. In auto mode, cameras have a tendency to add gain in order to achieve a proper exposure. Camera manufacturers are trying to appeal to a mass audience and often, that mass audience doesn’t want to take the time to light a scene properly. I mean, when was the last time you saw a four-year-old’s birthday party with a set of Arri lights cranking in the background? (Besides my four-year-old’s party, I mean.)
The ability of a camera to go into “manual” mode is where the true magic can happen. If you’re serious about videography and film making, this is where you need to be. In manual, the operator has the ability to make any scene look perfect. This is often why professional photographs look 1000x better than most amateur photographs. It takes some knowledge of the camera, but if you’re serious about your craft, learning how to use the equipment properly is a necessity.
For the purpose of this article, we’re talking about exposure. In my opinion there are only two ways that professional videographers should properly expose a scene:
- Open the iris
- Add more light
Adding gain should never be considered when you have those two options available to you. When both of those options are exhausted, however, gain is the next step. Before you decide to touch that gain switch though, it would be a good idea to understand what it is that you’re doing to the image.
What is video gain?
Gain is an electronic amplification of the video signal. This means that the signal is boosted electronically, adding more voltage to the pixels on your imager (CCD or CMOS) causing them to get amplify their intensity and therefore brighten your image. This voltage increase is measured in decibels (dB) and is calculated using this equation:
Gain (in dB) = 20 x LOG(Voltage Out / Voltage In)
Now, I’ll be honest here – I’ve been doing this professionally since 1997 and I have never used this equation, so don’t freak out just yet. Allow me, or rather Bob Diaz, to put it into layman’s terms.
A +6dB Gain is equal to a 2x increase in the signal. A +12dB Gain is equal to 2 x 2 or 4x increase in the signal. A +18dB is a 2 x 2 x 2 or 8x increase in the signal. When we express Gain as dB, every +6dB increase represents another doubling the signal. Another way to think of it as F Stops.
+6dB = Adds 1 F Stop of light
+12dB = Adds 2 F Stops of light
+18dB = Adds 3 F Stops of light
If +6dB is like adding an F Stop, +3dB is like adding 1/2 an F Stop of light and +2dB is like adding 1/3 an F Stop. Information supplied by Bob Diaz via DVXuser.com
Wow! So this means that you can bump the gain up to +18dB and it would be like adding 3 F Stops of light? Well, not exactly. In theory, you are gaining 1 F Stop of light with each +6dB of gain but there is always a catch. Remember that gain is an amplification to the overall signal and when you amplify something – no matter how slight – you introduce something new to the signal. In our case, this “something new” happens to be noise. Even the best cameras (the one’s that cost more than your car) introduce noise when gain is added. Here is an example of what I’m talking about:
The image on the left is clear of any noise issues – we consider that a “clean” image. The image on the right, however, has had +12 dB of gain added. As you can see, there is a significant amount of noise added to the overall image. Take a look at the detail below:
The noise is prominent in the darker areas of the image, but you can also see the effect in some of the midtones too. The effect is more dynamic when the image is being played back, but you get the idea. The left side is clean, the right side isn’t. Start color correcting or adding effects to the gained up image and you will get some nasty results. This is why we, as professionals, try to avoid gain. We want our editors to receive the best possible footage from us – so that they can junk it up to their liking. “So why is gain even there?” you might ask. Good question.
When to use gain
Gain is mostly a preference. Some people think that gain is acceptable in small amounts. Some think that gain is always unacceptable. The best idea is to get to know the camera that you’re using. Test it out with a darkened scene and record video at +6dB, +9dB, +12dB and even +18dB. Check it using a good monitor and don’t forget to calibrate the monitor properly. Use a waveform/vectorscope monitor if you have one and see how your image looks at each level of gain.
Simply get an idea of how gain is affecting the image, and make sure you’re comfortable with the image after applying gain. In most cases, I’m happy with adding +6dB to a dark scene. I don’t see too much of an increase in noise with +6dB but I still try to avoid adding gain when adding a light is just as easy. However, it all comes down to the person in charge. This is either your DP, the Director, the Producer or the Client. Make sure that they are comfortable with the image before you shoot all day on +6dB of gain.
When in doubt – add more light and use less gain. It will never hurt you.
Real world uses for gain
If we’re talking real world situations for using video gain, the one that stands out the most is news gathering. Shooting for news is a different animal, it requires a different skill set than shooting video in a controlled environment. There is plenty of bad video on the air, but good video journalism stands out and good news videographers (or photogs, as they are often called) are worth their weight in gold. They have to be ready for anything – and this includes dark scenes with no possible way of adding light. This is where gain comes into play. Photogs are trained to “get the shot” at any cost. So when they are outside on a dark night with no lights around, they will “gain up” to expose the image. In this case, they only have one chance to get the image – they must use any means possible to capture that image.
Wedding videography is another area that gain is used. I have a few friends that make a great living shooting wedding videos and they consistently shoot at +6dB or greater. Most churches don’t allow lights during the ceremony and most receptions are in very large (and very dark) banquet halls. It is simply impossible to light the entire place so videographers choose to use an on camera light and +6dB or more of gain to capture the event.
When you capture footage with a great deal of gain, it gives you and/or the editor less options in post. An image with a great amount of gain has less of a chance to handle color correction properly – so now you less control over fixing any color issues. Adding effects to an image with gain has a tendency to exaggerate the noise that gain causes. With these factors in mind, just make sure that you know exactly what you are about to do when you touch that gain switch. Remember, if a scene is dark – follow these steps in order:
- Turn off the camera’s automatic gain control – you want to think for the camera, don’t let it think for you (this should be the first thing that you do at the beginning of the day anyway)
- Open the iris if possible
- Add another light source
- If all else fails and you need to add gain, make sure that you have a proper color balance – you might not get the chance to correct it later
It all comes back to being comfortable with your equipment. You should be your worst critic in everything that you do and you should always try to better yourself and produce the highest quality images possible.