Cinematography-Theory-And-Practice

Figure 10.1. (previous page) Thebasics of exposure: a source and alight meter. (Photo by author.)EXPOSURE: THE EASY WAYFrankly, exposure can get pretty technical, so it’s important to graspthe basic concepts first before we plunge into the world of H&Dcurves, the Zone System, and the mathematics of densitometry. Let’stake a look at exposure the easy way.This introduction is a bit simplified, but it will provide a workingunderstanding of exposure that is useful without being too technical.First of all, there is one notion that has to be put away right now.Some people think of exposure as nothing more than “it’s too dark”or “it’s too light.” There are many more crucial aspects to exposurethat are important to understand.What Do We Want Exposure to Do for Us?What is it we want from exposure? More precisely, what is “good”exposure and what is “bad” exposure. Let’s take a typical scene, anaverage one. It will have something in the frame that is very dark,almost completely black. It will also have something that is almostcompletely white, maybe a white lace tablecloth with sun falling onit. In between, it will have the whole range of dark to light values —the middle grays, some very dark grays, some very light grays and,some right in the middle — halfway between black and white.When we shoot this scene, we want it to be reproduced on film orvideo exactly as it appeared in real life — with the black areas beingreproduced as black in the finished product, the white areas reproducedas white, and the middle grays reproduced as middle grays.Now of course, there will be times when you want to deliberatelyunder or overexpose for artistic purposes, and that is fine. In this discussionwe are only talking about theoretically ideal exposure, butthat is what we are trying to do in the vast majority of cases anyway.So how do we do that? How do we get the film or video to exactlyreproduce the scene in front of it? Let’s look at the factors involved.The BucketLet’s talk about the recording medium itself. In film shooting it is theraw film stock; in video it is the sensor chip, which takes the lightthat falls on it and converts it to electronic signals. For our purposeshere, they are both the same: exposure principles apply equally toboth film and video, with only a few differences. They both do thesame job: recording and storing an image that is formed by patternsof light and shadow that are focused on them by the lens. For conveniencehere, we’ll refer to it as film but, it’s the same either way.Think of film as a bucket that needs to be filled with water. It canhold exactly a certain amount of water, no more, no less. If youdon’t put in enough water, it’s not filled up (underexposure). Toomuch and water it slops over the sides and creates a mess (overexposure).What we want to do is give that bucket the exact right amountof water, not too much, not too little — that is ideal exposure. Sohow do we control how much light reaches the film?Controlling ExposureWe have several ways of regulating how much light reaches the film.The first is the iris or aperture, which is nothing more than a lightcontrol valve inside the lens. Obviously, when the iris is closed downto a smaller opening (Figure 10.2), it lets less light through than whenit is opened up to a larger opening (Figure 10.3). How open or closedthe iris is set for is measured in f/stops (we’ll talk about that in moredetail later). Remember, the film or sensor wants only so much light,no more no less. If our scene in reality is in the bright sun, we cancinematography182