Cinematography-Theory-And-Practice

Figure 10.16. Differences betweena high contrast and a low contrastfilm.what crushed together. This is, in fact, what gives film the “filmlook” that video has never been able to achieve (High Def comes alot closer than previous systems but still has trouble with the highlights).It is a way of compressing very contrasty scenes so that they“fit” onto the film negative.The Log E AxisLet’s think about the log E axis (horizontal) for a moment. It is notjust an abstract scale of exposure units. Remember that it representsthe various luminances of the scene. All scenes are different, and thusall scenes have different luminance ratios. What we are really plottingon the horizontal axis is the range of luminances in the scene,from the darkest to the lightest.In 1890, the German physiologist E. H. Weber discovered thatchanges in any physical sensation (sound, brightness, heat) becomeless noticeable as the stimulus increases. The change in level of stimulusthat will produce a noticeable difference is proportional to theoverall level: if three units of light create a perception of brightnessthat is just noticeably brighter than two units, then the smallestperceptible increase from 20 units of light will require 30 units. Toproduce a scale of steps that appear to be uniform, it is necessary tomultiply each step by a constant factor. In fact, the perception ofbrightness is logarithmic.What Is a Log?First a bit of mathematics. ( Just a little, so don’t worry). An understandingof logarithms is useful both in film and in video, wherelog files are an important concept. Logarithms are a simple way ofexpressing large changes in any numbering system. If, for example,we wanted to make a chart of something that increases by multiplyingby 10: 1, 10, 100, 1000, 10,000, 100,000, we very quickly reachto make a graph that could handle both ends of the range.In log base 10, the most common system, the log of a number representsthe number of times 1 must be multiplied by 10 to producethe number. 1 must be multiplied by 10 once to make 10, so the logof 10 is 1. To arrive at 100, you multiply 1 by 10 twice, so the log of100 is 2. The log of a number is the exponent of 10: 10 2 = 100, thelog of 100 is 2. 10 4 is 10,000, so the log of 10,000 is 4. This meansthat we can chart very large changes in quantity with a fairly smallrange of numbers. Logs are used throughout lighting, photography,and video.exposure193