Rating Your (Video) Camera's ASA

By B. Sean Fairburn, SOC
High Def DP & Camera Operator
Los Angeles

A good point of reference is very important when making lighting evaluations using a light meter, Spot or Incident. The meter will tell you how to expose the light, to properly expose an 18% Gray card. HD cameras using video style lenses will have a feature allowing you to AUTO expose the frame, but much like auto exposure meters in still cameras the camera cannot always decide for you what you want to properly expose. For this reason most professionals use Manual exposure, and with Cine style lenses auto exposure is not an option. Therefore proper exposure must be evaluated using a light meter or Waveform monitor.

Things to consider: The HD video camera is very playable and the look that is created is made up of many features, some of which affect the sensitivity of the camera. I recommend rating the camera initially then creating the look you intend to shoot and re-rating the camera to insure consistent performance. There are a number if ways to reach the same goal, each of which should be used to confirm or deny the other.

How do you rate the camera?

Necessary tools include:

• Camera with shooting lens
• Light meter (Spot and Incident)
• 18% Gray Card (new)
• Mac Beth or DSL chart with 11 step chip
• HD Waveform monitor
• HD Monitor 24 inch (Properly tweaked)
• Down converter and all monitors (HD or NTSC) going into the field
• Large even light source positioned close to camera (Flat light)
• A person possessing beauty and good skin tone (preferably leading lady)
• Common Sense and gut instinct on how things should look.

First set the camera up using No Gain, No ND, DCC or Auto Knee OFF in Tungsten light with no internal or external filter. Turn shutter OFF (even though you will be using the camera with it on). Frame up the Gray Card at full frame but not at the end of the lens and focus up and set the stop to f-4.

Second set your Meter to 500 ASA to start with and 24 FPS or 1/50th of a second (actually 1/48th but many meters cannot display it) Strike the light aim in on the Gray card and set the level of the light to f-4. Now look at your Waveform monitor and HD monitor you should have a flat fuzzy line. Here is where deviation can occur the question is what do you want to call proper exposure. Generally proper exposure will fall at 42 Units. Now open or close the stop until the gray card is at the level you want it 40, 42, 45, or whatever you want to call proper exposure. Now look at the f-stop and adjust the ASA so that the f-stop matches from meter to lens. You have just rated the Camera to properly expose light consistently.

How to test your work

Pan over to the gray scale and look at the crossover point the highlights and the black chips on the chart Black should be at the bottom Crossover in the middle and Highlights at the top. Pan over to the Beautiful person. Does your subject look like you want? If not, put the Exposure where it looks good. Look at the stop once again make a note of it in relation to the Gray card level. Now pan back to the chip and the gray and see how they hold up. Make note of any deviations and adjust your ASA so the f-stop matches the stop on the lens.

Proper shutter speed should be set at whatever you want to call normal. In most cases when shooting 24P "Normal is equal to 180° shutter angle, or 1/48th" So turn ON the shutter and decide what shutter speed = shutter angle do you want to shoot at. Now at 180° or 1/48th from a 24 frame base that is a 1 stop light loss or in other words if your ASA was 500 it is now 250. So that becomes your Baseline ASA. I would build a Gaff Tape Label with a Sharpie and label the side of the camera as a reminder of what you are working with.

Your baseline may look like this:
250 ASA Base Tungsten, 180° shutter or 1/48th Sec, No Gain, No ND

Change your Look

I recommend re-rating the camera if you modify the look much, then you can hone in on exactly where you want to place "Proper" exposure. Shoot exposure test 3 stop over 3-5 stop under and reevaluate your look and your ASA rating. This will help you become more familiar with the HD medium and its limitations as well as new areas of control you now have.

Also insure that the look you decide on with your Eye, Meter, Waveform, and HD monitor is being duplicated well through Down convert and into the other monitors as well.

 

Credits: I was first shown how to rate a video Camera by Harry Mathias, Director of Photography and Author of "Electronic Cinematography" during my first Workshop in Maine.  This article was reprinted with permission. © Copyright 2002