Cinematography

The Hot Spots You Want to Avoid

One problem you need to always be watching for are hot spots in the picture. These are often reflections on shiny surfaces, which must be “killed” because they will generally produce a region of clipped white in the final video. Camera operators and DPs with a film background will already be used to watching for hot spots; but since overexposure is more obvious in video, the vigilance must be increased. In fact, in the past this was a problem area for many film Dps shooting in a digital format for the first time.

Dulling reflective surfaces from over exposure in film

Photo by Vancouver Film School

This is because film is more forgiving of overexposed areas; the gentle roll off in the gamma curve of film means that as an area approaches over exposure, there is a soft edge to the overexposed area.

In digital video, however, the overexposed area will rapidly hit the digital “ceiling” of 255, or 110 IRE. Because there are no more “bits” available to record data above that, the area will be recorded as a solid area of overlimit white with no detail. Any detail that might have been visible in that clipped area is permanently gone. This area of clipped white may be only marginally noticeable on straight playback, but may be dramatically aggravated if the video clip is broadcast and run through automatic proc amp correction. Either the proc amp will turn down the overall gain to make the hot spots legal, thus making the rest of the scene too dark; or more commonly, a limiting proc amp will simply “plane off” any values above 100 IRE, resulting in a much larger and noticeable area of clipped white with hard edges. In addition, the MPEG* compression used in digital cable and satellite tends to “posterize” the image anyway and that compression will aggravate the hard edges of a clipped area.

Obviously, there will be times that you will need to shoot with overexposed areas that are corrected in post. But an important fact to bear in mind is that the “headroom” is only 10%. Most broadcasters will allow tiny hot spots, known in video engineering terminology as limited excursions. But many networks will reject video that has too many areas of clipped overlimit white. PBS in particular is notorious for this.

Hot spots are most often caused by reflections of lights on glossy surfaces. They are easily spotted when using 100 IRE zebra display or when monitoring the signal on a waveform monitor. A little experience will allow the DP to spot them with the eye; often it’s simply a matter of watching details like a hawk.

Offenders can range from eyeglasses and jewelry to tabletops and picture frames. With set pieces or items in a room where an interview is being conducted, the simplest solution is often to simply remove the offending piece. A minor change of camera or lighting angle can often fix the problem. In a Film has an S- shaped gamma curve (left) that gradually rolls off response as the film nears full exposure. This is kinder and more forgiving for hot spots than the flat gamma curve of video (right). Pro cameras often have a “knee” adjustment (parallel to film “shoulder”) that creates a truncated response at the top end of exposure. However, this is usually less gradual than the gentle curve of film gamma. Several higher-end HD cameras now offer adjustable gamma curves that can closely mimic that of film. situation where the object really can’t be moved, dulling spray is available that will create a temporary matte finish and kill the reflection. Dulling spray is specially formulated to be easily cleaned off of most surfaces; however, you should be very hesitant to use it on a piece of high value. In other words, when interviewing the Queen of England, it is not advisable to squirt dulling spray on her eighteenth-century sterling tea set! Find another solution to that particular problem. The most common uses for dulling sprays are chrome fixtures, glass in picture frames, or mirrors. In lots of cases, simply covering the offending object with a small piece of black gaffer tape will be sufficient.

Glossy tabletops can also be a problem. Since you are probably using some sort of backlight or kicker on the person sitting at the table, it’s very likely that some of that light will fall on the table and bounce directly into the camera. A diffused highlight is not a problem; it’s an actual direct reflection of the light source itself that must be avoided. Minor changes in the camera angle or lighting angle can often solve this problem. Don’t be a slave to precise continuity of the angle of the backlight. It’s often necessary to “cheat” angles between shots to fix problems like this. As long as you don’t alter the general sense of the lighting too much, the audience will never notice.

Jewelry can be a particular issue since it moves with the body. An occasional glint off a ring as the subject gestures is not really something to worry about; but a brooch that causes a repeating flash, flash, flash as the subject moves must be dealt with. On a dramatic set with actors, you simply ask wardrobe to find something better; but in an interview, the subject may not be happy about removing great-grandmother’s brooch. A little diplomacy is called for if you want to be invited back!

Ask the subject about removing the offending piece (don’t refer to it that way; something along the lines of “that stunning brooch” is more appropriate!). If they are hesitant, figure out another way around the problem. Flagging the key to keep the key light off the shiny piece works; if it’s a simple piece that would be easy to clean, dulling spray will work.

Excerpted from Lighting for Digital Video and Television- Third Edition by John Jackman ,© 2010 Elsevier Inc. All rights Reserved.

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