Film Compression

The electronic sensor and the film emulsion capture light using different methods.

The sensor measures light in the same way that a measuring glass meters the volume of water. After capturing a threshold amount of light, the sensor overflows like a measuring cup, and cannot perceive any more information with further exposure.

Highlights clipping occurs abruptly. Similarly, a glass can overflow from an extra drop of water.

The photographic emulsion works differently. The sensitive layer has considerable thickness and volume, which allows the silver halides to absorb light like a sponge and not overflow in an instant.

Finally, after capturing a lot of light, the emulsion also ‘saturates’ and after reaching a certain limit, responds less and less to further exposure. However, this process is much smoother than with the electronic sensor, and much more light is required for highlights clipping.

Thus, the emulsion reacts more smoothly to an increase in the amount of light near the clipping threshold. Practically speaking, it means that the loss of detail in the highlights occurs much later when shooting on film than when shooting on a digital camera.

Such behaviour of the emulsion affects the overall appearance of the image. On film the highlights exposure rises gradually up to the maximum film density (Dmax) and further exposure leads to almost no increase in the density of the negative. Meanwhile, the shadows and midtones are exposed further, and as a result they are ‘lifted’ against the highlights, and it makes the whole image look low contrast. This is how the luminance range is distributed on film in the case of overexposure.

This tendency is especially evident in the case of overexposed color negative film.

Note that even a heavily overexposed film (up to +6 stops) does not ruin the highlights, despite making the image flatter (compressed in tonal range). This is nearly impossible with a digital camera, where even +3 stops usually result in inevitable clipping of the highlights.

We call this film feature the highlights compression.

To emulate the film-like compressed tonal range, we invented the  Film Compression tool in Dehancer. It lets you fine-tune the redistribution of highlights towards the midtones. The midtones are also shifted slightly, while the shadows and mid-shadows remain unaffected.

The image processed in this way looks more analogue and becomes more flexible for further manipulation with exposure, contrast, film / print profiles, etc.

Let’s look at the  Film Compression tool in detail.

Impact
This parameter determines the degree of compression. The higher the Impact value, the more the highlights are pushed towards the midtones.

White Point
The White Point parameter defines the ‘film clipping threshold’, i.e. the point in the tonal range after which further exposure does not affect the photographic medium.

The white point directly affects contrast because it determines the steepness of the transition to the clipping area. As the white point gets closer to the midtones, the more contrast the image appears.
Use the White Point parameter to control the position of the white point within the compressed luminance range.

By default, White Point = 100. This means that the white point stays at its initial position.

If necessary, the White Point can be lowered, thereby increasing the overall contrast of the compressed range. The minimum possible value is 50. The lower the White Point is, the more likely clipping will occur in the highlights.

Alternatively, the white point value can be increased. In this case, the overall contrast of the compressed range is reduced. The maximum possible value is 120. The higher the White Point is, the more flat and grayed the highlights appear.

Tonal Range
This parameter represents the width of the tonal range affected by Film Compression tool. A minimum value = 0 means no compression. A maximum value = 100 means that the compression affects the wide range from the brightest highlights almost all the way down to the deepest shadows.

Color Density
Different films reproduce color differently as they get closer to the highlights. Negative films tend to noticeably loose saturation in the highlights. Slides, on the other hand, remain more vibrant, even though the clipping occurs earlier.

The Color Density parameter controls the color intensity of the compressed range. Color Density = 0 produces the lowest saturation in the highlights, which is more typical for negative films. Color Density = 100 provides maximum saturation, and the image looks more like positive films.

Also, you can watch our 15-minute video about how Film Compression tools works in Dehancer.