ISO and ASA

Apologies first that this will be a rather dry article, but we are often asked about ISO and indeed what film speed actually means: why do we need to know? To casual users of digital cameras the sensitivity of the camera sensor is seldom an issue since the results are instant so it can be seen immediately if the photograph has ‘come out properly’ or if it is too dark, too light. The film user really needs to know how sensitive the film is, because an incorrect setting on the camera will result in a poor quality image or none at all.

History
A quick look back in history will show that early photographers made their own film and chemicals, mixing ingredients from raw products and then coating glass or metal with a light sensitive gloop. Judging the correct exposure was a matter of experience or guess work. When Kodak industrialised the process it became essential to offer guidance to photographers to help them calculate the correct exposure and thus film ‘speed’ was born.

Rules of Thumb

  • High ISO
  • High film speed numbers = more sensitive to light
  • A high film speed = 400 ISO or greater
  • High film speeds = more grain

 

Benefits of high film speed

  • Faster shutter speeds, helping to freeze fast moving subjects OR
  • Smaller aperture so increasing depth of field
  • Useful in low light situations
  • When using flash, it places less demand on the battery

 

Low ISO

  • Low film speed = less sensitive
  • Lower film speed = 50 ISO or less
  • Low film speed = finer grain

 

Benefits

  • Finer grain allows for bigger enlargements, showing less grain
  • Useful in very bright situations
  • These films tend to have a ‘character’

 

What is the difference between ASA and ISO
Not a lot: there is a rather dull mathematical logic to it but 100 ASA = 100 ISO

What do the numbers mean?
The list above suggests 100 ISO is the 'normal' speed for film. In the 1980's the fine grain quality of 100 ASA meant that 35mm film could be happliy enlarged up to 12"x18" without intrusive grain, but modern films such as Fuji Superia have improved significantly, so that 200 ISO is now excellent when enlarged.

A 400 ISO film is twice as fast as 200 and four times as fast as 100 and so on. On a film camera the controls are arranged with the same 'doubling sequence' - 1/60th is twice as long as 1/125th and four times longer than 1/250th. The aperture mechanism follows the same logic: f4 is twice the size of f5.6 and four times bigger than f8.
By using this pattern of doubling shutter speed / aperture / ISO all the variables can be kept in a logical pattern. It is regrettable and baffling why digital camera manufacturers have moved away from this accepted format so that we now have f7; f6.8 or 1/45th and 1/300th and numerous random options.

What is DIN, GOST, EI?
DIN is a logarithmic scale; GOST is an arithmetic scale similar to ASA / ISO but with Russian numbers and is no longer used. If you enjoy maths then have a look at DIN, but for the rest of us use ISO.
EI is used by photographers ( who are good at maths ) to calculate correct exposure if they are intending to push-process the film.

What our eyes can see v. What the camera records

Both film and digital cameras can record a limited range of brightness and frequently the scenes we wish to photograph contain areas that are either too bright or dark to record as we might hope. Our eyes ( and brain ) can general cope much better than either film or digital cameras in these situations. Our brain cheats by imagining missing detail or by filling in the blanks when necessary.

Digital cameras are poor at recording highlight detail: the shine on a bald head or the detail in a bride's dress on a sunny day are often rendered pure white instead of showing detail. Digitals' are much better than film in low light conditions but even here struggle a little, resulting in intrusive speckling or 'noise' in shadow areas.

Film is described as analogue and although there are limitations to its ability to accurately record the full range of tones on a sunny day, it does not result in digital ‘cut-off’ : analogue rules are stretchy. At the lab we see better results from film than digital when printing contrasty scenes of a sunny day. Digital technology is continually improving and no doubt in time it will find a solution to this short fall.

In Summary
When the auto button is switched off, the list of options can seem incomprehensible. There is a pattern and the key with film speed and exposures generally is the 'doubling' concept outlined above. Keep that in mind and you can begin to make sense of the camera controls.


The Darkroom UK Ltd
www.the-darkroom.co.uk
01242 239031