Magazine article American Cinematographer

# Deep Focus the Ubiquitous Ks

Magazine article American Cinematographer

# Deep Focus the Ubiquitous Ks

## Article excerpt

If you're a baseball pitcher, nothing would be more extraordinary than having 27 Ks on a single game's scorecard. If you're a cinematographer, however, you're bombarded by Ks on a constant basis, and rather than being welcome, they can be downright bewildering.

The discussion of Ks has become something of a hot-button topic. Certain exhibition venues and platforms demand a minimum number of Ks. Producers have a handful of questions for every K in consideration. And many cinematographers feel at best that they kind of understand Ks. In this environment, misunderstandings abound. So let's try to clear things up.

First off, there are many things that "K" can stand for. Let's take a look at them, one by one.

К can be the symbol for Kelvin, the unit of measurement used for the colortemperature scale that we deal with on a daily basis. Daylight is, roughly, 5,600K, and tungsten light is approximately 3,200K. These numbers are based on the scale created by William Thomson, the Right Honorable Lord Kelvin (1824-1907), who postulated that if you take a pure black body that absorbs all light and emits none, and you slowly apply heat, it will start to glow - first red, then orange, yellow, blue and, finally, white. The "Kelvin scale" starts at absolute zero (-273°C = OK), the point at which all molecular activity stops. When color film was invented, it was decided that tungsten and daylight would serve as the two target temperatures for "white" light. A note about daylight: 5,600K is the nominal average of sunlight and skylight at noon on a cloudless day; depending on the source, daylight is also sometimes expressed as 5,500K or 6,500K.

Next, К can refer to kilowatts, part of our nomenclature for a lamp's electrical power. Kilo is derived from the Greek word for "thousand." So, when we're talking about a 1К Fresnel, or a 4K PAR, or an 18K HMI, those numbers designate thousands of watts - kilowatts. That Fresnel is 1,000 watts, the PAR is 4,000, and the HMI is 18,000. Wattage is one of the four main properties of electricity - the other three being amps, volts and ohms - and it is a constant in a given fixture. A 1,000-watt fixture will always require 1,000 watts to produce its light. Watts are the product of amps and volts, and they are, in essence, the measurement of the work being done in an electrical system.

So far, this has been pretty straightforward. But here's where Ks get confusing. With regard to digital images, ? is a measurement of the number of pixels or photosites-and the term has been commandeered as marketing jargon, a yardstick by which to measure the prowess of a particular camera system, a rallying cry in the ongoing "war of the Ks" wherein each manufacturer vies to outdo the rest in delivering an everincreasing ? count. But what does any of it really mean?

First, let's talk about pixels in a display or projection system. In the digital arena, Ks no longer compute to a true value of 1,000 or even a standard base of 10. Instead, digital systems use binary values, or a base of 2, because we're dealing with digital bits. In binary systems, every bit can define only two levels of information: a value of 0 or 1. This means that 1 kilobit is not 1,000 bits, but rather 1,024 bits-2^. That's an important distinction, and one that's often overlooked.

So, with a digital image, 1K stands for 1,024 pixels or photosites. The measurement was originally used for digital film-scanning specifications and has carried over to describe other digital cinema systems: cameras, projectors and display formats.

In display/exhibition systems, 2K and 4K are our most common pixel counts - aside from 1080 HD displays. A big point to understand is that 2K and 4K are categories of image sizes. To allow for different aspect ratios, there are variations of exact pixel counts within each category. The Digital Cinema Initiatives is an organization that was formed in 2002 by the major motion-picture studios - Disney, Fox, MGM, Paramount, Sony, Universal and Warner Bros. …

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