The resolving power of analog image carriers

You are here: Nature Science Photography – Visual acuity – Image sharpness II

Silver film can resolve details that lie within the size range of its fundamental constituents – the silver particles. In black-and-white film, this size range is between 0.2 µm and 2.0 µm. With color film, instead of silver particles, we are dealing with dye clouds between 10 µm and 25 µm in size. Both dimensions are extremely small, smaller even than the wavelength range of visible light (380 µm – 750 µm) and thus consequently invisible to us. For this reason alone, it is not very useful to look at this level to determine the resolving power. At least as important, however, is the fact that most films have several superimposed layers and that we perceive the silver particles (or dye clouds) arranged behind and next to each other in them after development as a quasi agglomerated mass in the overall thickness of the emulsion. We refer to this agglomeration as grain, and it’s crucial to understand that the film’s perceived graininess is not a result of actual particles present but solely a perception. Slides are considered grainless because they no longer contain silver particles when developed and the dye clouds have blurred edges. The actual resolving power of the film is affected by the size of the basic film particles and how much they seem to „clump together“ (the grain is anywhere from 10 to 30 µm).

1 micrometer (µm) = 1 millionth of a meter or 1 thousandth of a millimeter

After we have worked out the physical limitation, we turn to the determination of the resolving power. We express it in a contrast transfer function (MTF), just as we do in optics. In order to determine it without the influence of a lens, the test pattern is exposed directly to the film tested by means of a glass plate with etched patterns and evaluated after its development. Unlike a lens, the film, assuming its absolutely flat position in the image plane, does not show any decrease in resolution towards the edges. Therefore, we structure the MTF diagram as described in the first optics case: The horizontal axis indicates the number of resolved line pairs, and the vertical axis shows their brightness difference (contrast) as values between 1 and 100%. The only difference is that since color films have at least three color layers, some manufacturers give curves for each layer (red, green and blue) and others combine them into a single one. In the first case, the green function is crucial as our visual system is particularly sensitive to this middle part of the spectrum. However, the data sheets in both cases provide the resolving power for subjects with a very low contrast ratio of 1.6:1 and those with a very high contrast ratio of 1000:1. In the case of the Kodak Ektachrome E100VS (Figure 42 – MTF Kodak E100VS), these are 80 Lp/mm at a contrast ratio of 1.6:1 and 160 Lp/mm at a contrast ratio of 1000:1. In the wild, we rarely encounter such extremes, and the values for realistic subjects with a contrast ratio of, say, 10:1 are in the range of 100 Lp/mm.

Regardless of whether the resolving power of an MTF is specified in lines or line pairs per millimeter, the value always refers to line pairs because a single black line is useless without the separating white one.

Photographic paper used to make prints by the wet process is much less sensitive than silver film. Therefore, the absence of a light protective layer favors scattering more than the size of the silver particles or grain in determining its resolving power. In addition, a number of other factors come into play. The type of paper base (plastic-coated or baryte) plays just as much a role as the surface (high-gloss, glossy, matte or textured) and the type of drying. For an average pearlescent RC paper that has been air-dried, a resolving power of a good 75 lp/mm is usually specified, so that the paper is ultimately not a limiting factor in the image chain.

Next The resolving power of digital image carriers

Main Visual acuity

Previous The resolving power of optics

If you found this post useful and want to support the continuation of my writing without intrusive advertising, please consider supporting. Your assistance goes towards helping make the content on this website even better. If you’d like to make a one-time ‘tip’ and buy me a coffee, I have a Ko-Fi page. Your support means a lot. Thank you!