Neutralizing Wide Foot-Candle Denisty
Foot-candles are the standard light measurement used in the photography and film industries. One foot-candle is defined as the one foot radius uniform sphere of light that is created from one candle burning. Most people incorrectly associate wattage with a light's brightness, when in fact wattage is a unit that measures the amount of electricity being used. The amount of foot-candles in your frame will have a direct impact on the image's exposure. You can adjust the camera's aperture and shutter speed to compensate for the foot-candles, but sometimes that isn't always practical and you need to use other tools to help you attain your vision. One of these tools is Neutral Density.
Neutral density is a colorless filter that reduces the intensity of all wavelengths of light equally. This can allow you to reduce the image's brightness without sacrificing any of the color. You would use ND when you wish to have the brightness reduced without compromising your shutter speed or aperture.
Step 1: Using Neutral Density Filters
Neutral Density (ND) will have the widest effect over the foot-candle levels in your image, if it's used as a filter. Using an ND filter will affect everything in your frame. The filter is placed in front of the lens on the camera.
ND filters come in various intensities that are measured in filter optical density. ND.3 is the lightest neutral density. The heaviest neutral density is ND3.9. Their effect on the image can be measured in their f-stop reduction. For example, ND.3 reduces the f-stop by one stop, while ND 3.9 reduces it by 13 stops.
Step 2: Using Neutral Density Gels
Sometimes the foot-candle levels in your image have a huge contrast between the bright and dark areas. A neutral density filter on the camera will not help the image's quality because it affects everything instead of certain areas. In these situations, you can use ND gels to help balance the foot-candle levels.
As classic example is if you're shooting inside a room that has windows in the middle of the day. The natural daylight is very powerful and will blow out your exposure. You can have the room properly exposed by the windows will be white. You can add ND gels over the window so to bring down the day light's intensity. This would make the scenery outside of the window as bright as the light in the room. If you're interviewing a high power business executive, then this would be a great way to capture the view outside of his office.
Gels come in rolls that measure 4 feet by 25 feet. Simply cut the gel to the size of the window's glass. Take a spray bottle full of water and get the window wet with a light mist. The water will get the gel to naturally stick to the window. Once it is in place, take a squeegee and smooth out the gel so it looks clear like glass.
You can also place ND gels over lights to bring down their intensity without affecting their color.