black and white tone reproduction 4
During film emulsion manufacture (and it must be said here that, regretfully, there is becoming less and less of this about) a light sensitive solution consisting of Silver Halide Grains is sprayed onto an acetate base, obviously in complete darkness. The quantity of grains distributed on a 36x24mm section is in the region of 500,000,000. Pixel people eat your heart out. The trouble is a lot of them get wasted or at best under utilized!
Granularity is the scientific term used in measuring the graininess in an image. Using micro-densitometers (which practising professionals don’t own either but I’m sure Kodak have a few) comparison graphs of grain size can be made but not much practical value can be gleaned from this for us to use.
The term graininess describes what we see in the print and is the light sensitive silver halide grains, atomic in size, theoretically evenly distributed within the emulsion. This emulsion is not actually flat but has a depth of a few microns.
Within this, individual grains are stacked up virtually on top of each other, waiting to devour their quanta of light. It stands to reason that the luckier ones at and near the surface receive rather more pure light than those below receiving a more diffused illumination.
During development the grains have a habit of clumping together, lumpily, in various ways depending on whether they are in what will become shadow areas, receiving the least light so only the surface grains respond, and few of them. The rest get dissolved by fixation and ultimately washed out leaving graininess very low in these areas.
In the areas soon to become highlights the intensity of light is sufficient to penetrate down to the lower levels where most of the grains respond uniformly due to light diffusion so graininess here is low also.
The grains in the middle area respond to the mid-tones of the image and, being more numerous, become more noticeable in the print, particularly sky-blue mid-grey tones and other large evenly toned areas. The spaces between clumps are devoid of image.
It’s clumps of grain that we see not individual ones, being too small, and the amount of these is fast being caught up by pixels.
Certain combinations of exposure and development, or even choice of Developer can inhibit or encourage the clumping of grain. A fast film, underexposed then overdeveloped, will be exceptionally grainy. Slower emulsions, correctly or slightly overexposed then under-developed will restrict clumping.
Obviously smaller formats such as 35mm. Require greater enlargement amplifying the graininess and destroying fine detail.
One way to alleviate this is to use a highly diluted developer with minimum agitation. This works itself to exhaustion on the surface of the film so is unable to affect the grains down below. Needless to say image quality suffers, as in the graph above, tonal range is reduced. Everything is a trade off.
With this theory in mind certain specialised developer solutions where created known as high acutance developers. Acting only on the surface grains, that is, lodging all the image information - highlights, mid-tones and shadows - amongst the top few stacked grains so virtually unaffecting the lower ones that get washed out after fixing.
Fewer grains and less diffusion produce the appearance of finer grain with increased sharpness in the final print. The theory works very well in practice, these products are excellent at doing what they do, the downside is the loss of the lower level, mid-tone grains hence tonal quality is sacrificed again. Another trade off.
The industry is now in a crisis. Digital Imaging is sweeping the world and I cannot envisage any of the few manufacturers that remain, pouring any more money into Silver Halide R&D.
Perhaps what we’ve got now is as good as it will get. Purists lament!
That’s the end of the curve, next, some photos.
Text & graphics C steve rostron 2007
Un-developed Silver Halide Distribution (Cross section of emulsion depth)
Grain Clumping during Development
Green curve shows effect of highly diluted developer solution.
Tonal compression is severe
Surface acting developer
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