Sunday, August 22, 2010

Latitude and Grain

As well as the factors we have looked at, which are all indicated on the box the film comes in, there are other aspects of film you may want to consider.
Basically this is the amount by which you can over or under expose your film yet still get an acceptable result. Different types of film have differing degrees of latitude.
Standard colour negative film of 100/200 ASA , the stuff most people use, can yield a reasonable enprint (6x4) from a negative 1 stop under exposed to 3 stops over exposed.
Black and white negative film from 1 stop under to 2 stops over and transparency film from 1/2 stop under to 1/2 stop over.
At the extremes of film speed, very low or very high, latitude may be reduced.
As you can see colour negative film is very forgiving of exposure errors and will compensate for short comings in your exposure metering technique. On the other hand, if you would like to work with transparency material you must be able to expose accurately. Many people make the change from print film to transparency and are very disappointed with the results which are usually down to poor exposure technique which was not apparent using print film.
A films exposure latitude is not there to make up for your mistakes, although it helps, and you will always get the best results from accurate exposure.

Film consists of a layer or layers of emulsion containing light sensitive silver halide crystals coated onto a plastic base.

Coarse grain structure.
Coarse grain
After processing the image is made up of lots of clumps of silver from the developed crystals, dyes in the case of colour film, these are referred to as grain. A bit like a TV or monitor screen is made up of pixels so a photograph is made up of grain. Although, while your monitor may have thousands of pixels, even the lowest quality film will have millions of grains. For the photographer the issue is not so much the number of grains as the size. Grain size is directly related to film speed. Slow films have a finer grain structure than fast films. With a slow fine grained film the actual grain structure will not be visible in a print approx. 10x8 while with a fast film the grain may be clearly visible in a standard enprint.

Fine grain structure.
Fine grain
Finer grained slow films are able to resolve more detail than faster, coarse grained films. Grain in itself is not something to be feared. There will be times where you will have to use a fast film and accept the grain and there may other be times you will use grain for pictorial effect. Grain is simply a characteristic of film which you will have to take into account when choosing or using a film.

Over or Under Exposure.
This is just a quick look at the effect of under and over exposure on your photographs with different types of film. We are not talking about gross errors just what happens when you start to drift away from the correct exposure.

Black and White Negative.

  1. Over Exposure. Loss of highlight detail. Although the detail will be present in the negative special printing techniques will be required to render it correctly.
  2. Under Exposure. Loss of shadow detail. Unlike over exposure if their is insufficient exposure to record shadow detail nothing can be done to retrieve it.
Colour Negative.
  1. As above but with the added 'bonus' that under exposed shadows may have a colour cast.
With negative film you must avoid under exposure.

Transparency or Reversal Film
  1. Over Exposure. Loss of highlight detail. Bright areas will 'burn out' leaving just a clear section of film . When it's gone it's gone.
  2. Under Exposure. Shadows start to 'block up' and become solid black. Highlights and colours will become darker. Still viewable or printable using a bright light source.
With reversal/transparency film you must avoid over exposure.

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