ISO Film Speed

ISO film speed is determined by the International Organization for Standard-ization (the ISO), a network of national standards institutes from 161 countries. It is based in Geneva in Switzerland but is non-governmental in its organisation.

The function of the ISO is to set standards in commerce and industry. One of these standards is the ISO film speed rating for the sensitivity of photographic (emulsion based) film

According to the ISO, 800 speed film is more sensitive than 400 speed film, and 400 speed film is more sensitive than 200 speed film and so on. The more sensitive the film, the faster it is said to be and the less light you need in order to take a picture.

Without getting too technical, the speed of the film is calculated from the optical density (the absorbance of light by a material such as film) and the exposure time using a mathematical equation. But more importantly is how to choose the right ISO film speed when you are shooting so read on below.

Using Film Speed Effectively

The size of the grains of silver halide in film emulsion determine the film speed. Bigger grains means greater light sensitivity and vice versa. An ISO film speed of 50 (that is, a slow film speed) is great for potraiture as the grains of silver halide are smaller and therefore the result will be literally more fine grained. Fast film speeds of 800 or more have bigger grains and are good for shooting when the light is poor or if you want to freeze something in fast motion. However, the result will look coarse grained. Also bear in mind, that if you underexpose your shot the result will be even more grainy than if you overexpose it.

In normal day-to-day use, a film speed 100 or 200 is adequate, usually when shooting out of doors when there is a fair amount of sunlight available or at least bright sky with some cloud. 400 and 800 speed film are more suitable for indoors particularly when you don't want to use flash.

Film versus Digital

Of course the main problem with using film is that if you want to alter the ISO rating you actually have to change the film itself or have an interchangeable back. With a digital SLR you can change the ISO settings frequently, sometimes for each individual shot. Digital cameras have ISO settings which go beyond 800, up to 1600 or 3200 or even more which means you don't need to fire the flash. At 1600 ISO you can probably take a shot hand-held at about 1/30 second indoors in poor light.

Another drawback with film is that with higher ISO settings you get more 'grain' (called 'noise' if you are using a digital camera). In the above photo you can see an example of 'grain' on a slide film that has an ISO setting of 1000. Nowadays this is considered undesirable and indeed if you are photographing a sunset or a flower in close-up that is probably true. But some people still value grainy images as they have a more photojournalistic or documentary feel and grain can create an interesting mood in the picture.

If you want the least possible grain or noise in your images go for the lowest possible ISO setting such as 100 or even 50. Just make sure you have a tripod as the shutter speed will need to correspondingly slower. Kodak film ISO goes down as low as 6. So far as I know there isn't anything lower than that!

If you are about to buy a new camera and want one with the lowest possible amount of digital noise, go for a Canon as they are renowned for their lack of noise, particularly the EOS range. I have used Canon EOS cameras for many years now and because the level of digital noise is so low, I have become used to shooting without flash most of the time. If you really want to buy a superb camera go for the Canon EOS 5D Mark II as digital noise is absolutely minimal.

After the Event

If you do have a great shot that you took with an ISO setting of 800 or more and it has too much 'grain' you can manipulate it in Photoshop once it is scanned. I use Noise Ninja which is a plug-in for Photoshop. After you have downloaded the programme you can get a profile for your camera from the Noise Ninja website. Install that after you've installed Noise Ninja and then whenever you open an image in Photoshop, Noise Ninja will detect automatically the ISO film speed or setting that you used and will apply their custom-made filter. It is easy to use, inexpensive to buy and the quality of the results will astonish you.