Cinematography

The Practical Differences Between Film and Digital Sensors

The single best piece of advice that I can offer with regard to the difference between exposing film and exposing digital is this: It’s OK and sometimes desirable to slightly overexpose film, but it is not OK or advisable to overexpose digital unless that overexposure is exactly what you want!

Cinematographers have widely varying methodologies for determining an exposure: some simple; some convoluted. Film negative has enormous exposure latitude and is very forgiving of a wide variety of exposure techniques.

Modern digital cinema cameras are now achieving wide latitude too. The difference between digital cameras with a video heritage and limited ability to capture detail above a perfect reflecting diffuser, and digital cinema cameras with a filmic heritage (such as the ALEXA, the F65, and the Epic with a Dragon sensor), is quickly widening.

Spectra light meter.

Digital is more like reversal (or auto positive) film stocks. There can sometimes be very little margin for error in exposure, especially at the high end of the tonal scale, and once a photosite reaches full light-well capacity (or clip), it cannot continue to record any more detail. When using film negative, one can always “print down” a bit to save some detail in excessively bright areas, and this can frequently save an overexposed area in the frame. The same overexposure in digital traditionally could not be corrected. The attempt to “print down” over exposed highlights usually does not result in any detail regained, but rather, a similar amount of highlight detail, just rendered at a lower gray level.

Digital exposure tools.

The real distinction to be understood here is the difference between HD broadcast-type cameras that render internally and produce a display-ready output, which is like shooting on slide film, and digital cinema cameras not rendering internally and producing output in log or raw format that needs an additional step to be display ready, which is more like shooting on negative film.

Overexposed sky with clipped exposure in clouds.

The same image color corrected to lower levels. Note: Notice how the clipped areas do not gain any detail by “printing down” the image.

A good rule of thumb in digital work has traditionally been that at the bright end of the exposure range, digital cameras are not WYSIWYG (what you see is what you get) but rather WYSIAYG (what you see is all you get). When a sensor reaches 100% photon saturation, it is said to be at “clip,” and any picture detail that arrives above that “clip” level is lost (overexposed) forever!

Film grains as photographed by an electron microscope.

Film has a random pattern of different sized grains that (depending on the sensitivity of the particular film stock) can achieve very high resolution.

Film scanned at various resolutions.

Scanning film at resolutions higher than 4K (such as up to 10K) yields very well-resolved grain.

Digital sensors have an exact grid of discrete pixels or photosites, and yield a picture that some cinematographers feel is too clean and is not as satisfying as the organic look of film grain.

Silver halide film grains compared to a sensor grid pattern.

How film emulsion and digital sensors differ.

Their resolution is very predictable and can be calculated as the result of several factors: sensor type, CCD versus CMOS; Bayer pattern sensor versus 3-chip sensor de-Bayer algorithm; pixel or photosite count; optical low pass filter; and recording workflow all figure into the resolution equation (as we will soon learn).

As a result, digital reproduces real world images in a more isotropic way, uniform in all orientations, where film renders images in a more anisotropic way, systemically different for every different orientation or size of the original object.

Isotropic and anisotropic renderings of an object

Sensor Size Has an Effect on the Image We See

The desire to work with a sensor the size of a 35mm film frame is a strong force driving development in this area, and many of the issues of digital cinema revolve around the very basic differences between sensor technologies.

A typical digital sensor.

At the moment, a great deal of effort is being expended on the development of single chip sensors for digital cinema because there is a great demand to use existing cinema lenses to acquire images on digital cameras. Gathering the image using a single chip camera with a sensor the size of a 35mm film frame means that we can use existing cinema lenses without many of the problems digital camera builders encountered designing 3-chip CCD cameras around the inherent chromatic aberration of prismatic color separation.

Generally, single sensor cameras render images with the same depth of field and image characteristics that we have become accustomed to in our filmic experience.

There is a huge supply of good 35mm film lenses in the world.

Excerpt from Digital Cinematography: Fundamentals, Tools, Techniques, and Workflows by David Stump © 2014 Taylor and Francis Group. All Rights Reserved.

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