The two characteristics of Image Structure are grain and sharpness. There are two ways of evaluating these characteristics –subjectively (by looking at them) and objectively (by measuring them with a suitable instrument). It’s rather like the way we look at the tones and contrast of an image, and measure them with a densitometer.
We have been talking about grain and sharpness with reference to tabular crystals and conventional silver halide crystals. These are rather vague terms, but can be defined more precisely using metrics such as graininess and RMS granularity and sharpness, limiting resolution and MTF or Modulation Transfer Function.
- RMS Granularity
- Visual Sharpness
- Limiting Resolution
- MTF – Modulation Transfer Function
Starting with “grain”, the term “graininess” is the subjective impression of a random dot-like pattern in an image, by the eye and brain. It is thus just a relative term and uses units such as JNDs or “Just Noticeable Differences”. Image scientists use an objective measurement called granularity, which is measured using an instrument called a microdensitometer.
Here are photomicrographs of a black-and-white emulsion before and after development. You can see the randomness of the developed image that we call grain.
We measure granularity with an instrument called a microdensitometer, which has a very small aperture, usually 48 microns in diameter.
If we scan a processed film sample with a uniform density area, we record the fluctuating density as a variable voltage called “root-mean-square” or RMS voltage.
RMS voltage is converted to a metric known as RMS granularity.
RMS granularity data is provided by film manufacturers for their products.
RMS granularity is usually measured for each step on a gray-scale, and plotted on the same chart as the characteristic curve or curves. The symbol sD is used (“Sigma-D”). In this example of one layer of a camera color negative film, you can see how the granularity profile corresponds to the different grain sizes used in the emulsion layers.
The “bump” in granularity in the toe of the curve corresponds to the larger grains in the fast component of the emulsion. This is one of the reasons to avoid underexposing color negative films.
Here are the granularity and characteristic curves for all three layers of a color negative film. If you compare the curves given in data sheets for different films, you will see the faster films are grainier. When film manufacturers introduce improved films, they often achieve lower granularity by employing new emulsion techniques. One of the most notable improvements was achieved by the introduction of tabular crystals.
Turning to sharpness, we also have subjective and objective measurements. Subjectively we talk about sharpness or “resolution”.
The objective measurement of sharpness is called “MTF” or Modulation Transfer Function.
MTF is measured by imaging a special MTF test target onto the film. This consists of a progression of finer and finer bars of black and white. Their density profiles are sinusoidal, not square waves.
When the film is developed the large bars are very clear, but they become less sharp as they become finer, until they merge into a uniform blur.
If we scan these patterns with a microdensitometer, we find that the high and low densities become closer to each other until there is no fluctuation. After a little mathematics we get a series of numbers called Modulation Transfer Factors which, when plotted against the dimensions of the bars or “spatial frequency” in cycles per millimeter (also known as line pairs per mm), produce the complete Modulation Transfer Function curve.
Here are the MTF curves for two B&W films. What conclusions can we draw from them, particularly which film is the sharper, of course!
Well, it looks as though film B is the sharpest because its MTF response holds up better at the higher spatial frequencies. This is a microfilm, where very fine detail is essential. However it would look rather unsharp to the human eye.
Film A is a camera film. It would look much sharper on the screen/photo paper because of its excellent MTF response (actually greater than 100%) at lower frequencies around 10-20 cycles per mm, where the human eye is particularly sensitive to sharpness information.
As with density plots, color films have three MTF curves, one for each imaging layer. The MTF chart is given in the data sheet for every film.
The layer order in a film affects its MTF curves. As light is scattered as it descends through the emulsion layers, and normally the top layer is the sharpest and the bottom layer the least sharp.
We find this is true for the color negative film on the left –the yellow (blue curve) on top is sharpest and the cyan (red curve) at the bottom is the least sharp.
In color print film, with its layer order magenta, cyan, and yellow from the top, the magenta (green curve) is sharpest and the yellow (blue curve) the least sharp.
So far, I have already written several blogs about film structure and characteristics:
- Film structures or layer orders
- The functions of the layers
- Characteristics of the light-sensitive silver halide crystals
- Spectral sensitivities
- Problems in color reproduction
- Color correction using masking with colored couplers
- Image structure –grain and sharpness
I hope this will help everyone to have a basic understanding about photographic film.