black and white tone reproduction 2
Gamma was a term used once but now fallen from favour as it only dealt with the straight line portion of the curve. Gamma is of more use to the Densitometrist (sic) than the photographer. A modern and aesthetically more useful measurement is Average Gradient.
Two points are plotted on the curve - say Point S, just inside the toe section, and Point H, just upon the shoulder - the curve sections we need for our pleasing print. Between these points we can calculate the Average Gradient of the slope at Point A, gleaning from this an image brightness range of 1.9 on the R log E scale. So, what can we do with this? Well.....
You see, the problem with Characteristic Curves and why most practising professionals don’t bother with them, (most pro’s, even large studios, don’t even own a Densitometer) is that they’re very difficult to relate to. That great picture you just took and can’t wait to see in print looks meaningless as a graph. Varying density or rather, tonal range is an obviously visual component of the image, like wise, the relative logarithm of exposure is harder to grasp than 125 at f11.
What’s more, graininess and sharpness, also visually obvious and vital characteristics in the print, do not show up on the curve anyway! So what use is it to the a photographer? Not a lot but then you can always impress your peers with the terminology!
It is, however, important to understand what is actually happening during exposure and development in order to alter these variables and get the images that we seek. So as not to impede the pursuit of the craft, the following simplified graphs show a more relatable aspect of the science and the exposure latitude of film emulsions.
Under-exposure with normal development places all the highlights on the straight line portion yielding good separation but lacking sparkle and, missing the darkest areas at the toe of the curve. Very little shadow detail being recorded.
Over-exposure and normal development, being the opposite, will record a lot of shadow detail but all the highlights will be ‘burnt out’ showing no tonal separation.
Also you’ll get increased graininess
Over-development with correct exposure increases overall contrast producing a negative beyond the brightness range of photographic paper. Unprintable in other words.
Increased graininess will be severe.
Under-development with correct exposure renders a ‘flat’ image, lacking both ‘depth’ and ‘sparkle’, that is - no pure whites and no solid blacks.
Probably the worst treatment you could do to a negative.
The four scenarios above really describe extreme over & under treatment of negatives but because we can do this we have scope for manipulation.
Slight deviations in any of these general directions gives us the creative freedom to manipulate the image according to the subject, how we see it and how we wish to interpret it to the viewer, pre-visualization in other words. Combinations such as slight under-exposure then slight over-development can yield startling results with deep shadows and lots of brilliant highlights, grainy too if that’s what you want.
Next, the Print.
text & graphics C steve rostron 2007
Most negative materials have an exposure latitude equal to about 5 f stops. At virtually any exposure - say point A, relating to Average Gradient, which we shall call 125 at f11, the film will record not just a single tone or brightness level but a range of tones embracing about two stops under-exposure, into the shadow areas, and about three stops over-exposure into the highlight areas.
The so called correct exposure should place this 5 stop latitude upon the straight line portion of the curve and include some toe and shoulder areas. The nature of the curve itself, as we shall see, is determined by time and temperature of development which is largely dictated by subject brightness range.
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