Factors we must take into account to meet requirement 0: Factor 1 – Stray light

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In photography, we call stray light that diffuse part of light caused by reflection, which appears in the image as a uniform veil. It increases the brightness at each point and therefore reduces the contrast. In the imaging chain, it plays an important role at two points: in image formation and in image viewing. In the case of image formation, the following factors are relevant:

The lens design

In the lens, the incident light is inevitably scattered by the optical surfaces and the diaphragm blades. Therefore, the basic rule is to minimize the number of elements in the optics, vaporize them with an anti-reflective coating, and reduce the amount of scattered light. Mechanical components should be black and have a matte surface.

The camera design

Inside the camera housing, stray light is created by the single and multiple reflections on the shutter and the numerous other surfaces of the existing components. Here, too, the matte black finish of the mechanical parts goes a long way to counteract this. And of course, the camera interior should be absolutely light-tight so that no unfocused light reduces the contrast.

Even with meticulous design of the optics and camera housing, the stray light still remains, preventing a direct correlation between subject brightnesses and tonal values. Lens mounts, present filters, or the image carrier itself will always scatter a small portion of the focused light, resulting in reduced contrast. Because of this, stray light cannot be completely eliminated and is an indispensable part of every projected image.

The subject characteristics

Large brightnesses produce a lot of stray light. For this reason, subjects with high contrast ratios or large bright parts of the image also produce a lot of stray light. Examples are snowfields, beach scenes or high-key shots (bright in bright). All of these settings tend to produce a high flare factor in photography.

Lighting

Backlit situations, where the light source is in the field of view, introduce more stray light than subjects illuminated from the front or side.

Dust and similar particles

Dust and dirt particles represent reflective surfaces that inevitably produce stray light. For this reason, and to avoid mechanical damage, all surfaces and internal parts of the acquisition unit should be as dust-free as possible.

The measure of stray light can be expressed in figures with the stray light factor. We determine it by dividing the subject’s contrast ratio by the image’s contrast ratio.

Formula 10

For our above mentioned example with the subject contrast of 160:1 and the image contrast of 80:1, this results in a stray light factor of

Formula 11

This value is realistic. Numerous tests have demonstrated that the average value for current cameras and optics is approximately 2.5. This is based on normal subject conditions, because, as we have already seen, reflection properties and lighting conditions also play a role. Depending on how they interact, the flare factor can drop to a low 1.5 or rise to 6 or 8.

The stray light component in image viewing is caused by the following relationship: Density values are determined objectively with a densitometer. This instrument has a photocell with which it measures the light reflected perpendicularly from the surface of the object to be measured. – Only this part of the light reflected at an angle of 90° is included in the measurement result (figure 23). However, this measurement process does not reflect the reality of our visual experience. Under normal viewing conditions, the image surface not only receives directional light from a single direction, but also receives relevant components from all other directions, which diffusely reflect from the surface without passing through the image layer. Regardless of where we view the image from, we also perceive this diffuse light component, which becomes noticeable as a whitish veil and reduces the perceivable maximum density or contrast (figure 23 B).

The portion of light diffusely reflected from the surface of the image carrier (paper or plastic) is composed of three main components:

• A part that is reflected by the surface of the emulsion
• A part reflected by the starch and silver grains of the emulsion
• A part reflected from the surface of the paper or plastic support

Stronger exposure virtually eliminates the latter reflection source by increasing the amount of silver in the layer. However, this does not affect the other two types of reflection, which limit the maximum perceived density. Thus, like light scattering during exposure, they mainly affect the dark areas of the image.

The degree of impairment depends essentially on the nature of the surface. If the surface is glossy, it reflects the light almost only directly, i.e., it is reflected back at the same angle at which it falls. This means that very little stray light reaches the eye of the observer, and the contrast reduction is low. Glossy black-and-white papers still have silver grains in their layer after development, which scatter the light in the range of 1%. For this reason, they usually have a maximum density of around 2.1. Glossy color papers have a very thin emulsion layer, which after development contains only dye clouds instead of silver. This results in minimal light reflection, enabling these papers to achieve a maximum density of approximately 2.5.

Due to their special nature, to which starch grains are often added to achieve the effect, matte surfaces reflect light almost only diffusely. Thus, a good 4% of the light reaches the eye again. This, along with the extra reflection from the silver grains in the black-and-white area, results in a pretty strong loss of contrast and a maximum density of 1.3 to 1.6. The best inkjet papers currently available achieve maximum densities of 2.7 to 3.0 in conjunction with dye inks.

Applied to a subject with the average contrast value of 160:1, one additional uniformly distributed unit of stray light means that the brightness ratio changes to 161:2. This cuts down to around 80:1 and represents a considerable reduction. Relative to the two ends of the tonal spectrum, the stray light has a different effect in percentage terms: In the shadows, the previously described increase in brightness by one unit can double the value, while in relation to the highlights it represents only a fraction of the original value. – The stray light produced during the exposure therefore has a disproportionately strong effect on the dark areas of the image. For this reason, the contrast boost required for compensation must be of different magnitude in the highlights (0.3), midtones (1.25) and shadows (2.5). This results in a curve as shown in figure 24, which takes into account the average stray light values (camera and optics stray light component of 0.4 % of the image white, a stray light component of 9 % caused by the enlargement – although this is omitted when outputting to an inkjet printer or laser imagesetter – and a stray light component of 2.7 % occurring during viewing).

Next Factors we must take into account to meet requirement 0: Factor 2 – Ambient brightness

Main Contrast

Previous Requirement 0 – Our expectations from the contrast reproduction of a photograph

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