manual exposure procedure with zebra patterns and lighting

[Luke McMillian]

Hi all,

I've aquired a Panasonic GS400 now, a step up from the lower end mini dv cam I was using before. The GS400 has the ability to show zebra patterns and colour bars and manually adjust exposure and shutter speed etc.

I'm wondering about the steps of lighting a scene and adjusting the exposure? Should i be setting up my lighting so it looks good to the eye, and then adjusting the exposure on the camcorder according to the zebra pattern keep adjusting it until no zebra patterns are shown?

Or is it acceptable to blast the scene with light, and then decrease the exposure to compensate, what is better for getting quality video results? Is it better to just get a decent well lit image and then darken and play around with contrast in post production? i understand there asthetic choices that depend on the type of project and story etc, but I'm just wondering about getting the best quality of image i can.

Also I've heard the use of ND filters will allow me to use more film style type lighting, because i find camcorders can very quickly get into over exposed areas even with lights far away. Should I always be using ND filters, my cam has internal ones. What about grey cards, I've heard people white balancing to those before they set up a scene? Also i have a circular polarizer, i know it decreases the light coming into the cam a bit.

What are my options?

[Marcus Marchesseault]

ND filters aren't selective about what they block. Everything gets darkened evenly, so the highlights aren't reduced while shadows are left visible. It is almost the same effect as closing down the iris. Polarizers reduce light, but they cut down polarized reflected light more than non-polarized light. They will cut down reflections from water and glass and the surface of plants. It helps bring out the blue in water, the green in plants, and can allow you to see through the glare on glass.

What you should concentrate on with exposure is to ensure that people's faces are exposed properly. Everything else is relatively unimportant. Assuming your zebra bars are 100ire, you should see them on the highlight side (direction light is emanating) of white or shiny objects. A white shirt will have some zebras on the shoulder facing the light. Silverware will have zebras in their highlights. Where you should not see zebra bars is on the shadow side of anything. It is no big deal to lose detail on the bright side of a white object, but overexposure so that detail is being lost on a large area will not look good. The only exception is backlight. People standing in shade with the bright sky in their background may very well be surrounded by zebra bars. Try to avoid camera angles that cause this effect.

White balance is done after the lighting is in place. White balance to a white card. Grey might work, but that is used more for exposure. A grey card can be used as an anchor reference point to keep the light levels relative between your shots.

[Luke McMillian]

Hey is there any websites you'd recomend to learn how do all of this properly. Using a grey card, and keeping the light levels relative between shots? So for exposing you should zoom into the area you want properly exposed, like someones face, and then zoom back and out and if the sky is full of zebra patterns put an appropriate nd filter on there? How do you know what to put on?

[Glenn Chan]

Some of my information on the "correct" exposure:
http://www.glennchan.info/video/exposure/exposure.htm

There really isn't too much of a correct exposure technically, since you will either get clipping + noise in dark areas, or clipping in highlights. Any detail that gets clipped can't be brought back in post... i.e. anything that is zebra-ing.

A good rule of thumb is to have a little bit of zebras in your shots on specular reflections (i.e. a glint of light off a shiny surface) or any little highlights in the shot. Do expose so that you can see faces well.

Or is it acceptable to blast the scene with light, and then decrease the exposure to compensate, what is better for getting quality video results?

Quantity of light usually isn't important. Too little light would be a low light situation where everything becomes noisy and grainy. Too much light forces the iris down, which means you get deeper depth of field (not necessarily a bad thing, although some people like a shallower depth of field for aethetics or to selectively focus on things). Higher than that, things will overexpose or you have to use shutter. Even higher, the CCDs get damaged (this would be really extreme however).

Quality of light is usually what you're concerned about. This is mostly aesthetic.
Good books on this would be ross lowell's matters of light and depth, and jackman's lighting for digital video (this last book is much more pragmatic and is less focused on art).