Film speed Totally Explained

Everything about Film Speed totally explained

Film speed is the measure of a photographic film's sensitivity to light. Film with lower sensitivity (lower ISO speed) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed) can shoot the same scene with a shorter exposure and is called a fast film. In the first approximation the amount of light energy which reaches the film determines the effect on the emulsion, so that if the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor so that the energy received is the same, the film will be exposed to the same density; this rule is called reciprocity, and the concept of a unique speed for an emulsion is possible because reciprocity holds. In practice this holds reasonably well for normal photographic films for the range of exposures usually used, say 1/1000 sec to 1 sec, but longer exposures, different for different films, are required outside these limits, a phenomenon known as reciprocity failure.

Technical information

ISO film speed scales

International standard ISO 5800:1987 from the International Organization for Standardization (ISO) defines both an arithmetic scale and a logarithmic scale for measuring color-negative film speed. Related standards ISO 6:1993 and ISO 2240:2003 define scales for speeds of black-and-white negative film and color reversal film.
   In the ISO arithmetic scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that's necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.
   Commonly, the logarithmic speed is omitted, and only the arithmetic speed is given (for example, “ISO 100”). In such cases, the quoted “ISO” speed is essentially the same as the older “ASA” speed. GOST (Russian: ГОСТ) is a pre-1987 arithmetic standard used in the former Eastern Bloc. It was almost, but not quite identical to the ASA standard. After 1987 the GOST scale was aligned to the ISO scale. GOST markings are only found on pre-1987 photographic equipment (film, cameras, lightmeters, etc.) of Eastern Bloc manufacture.
   The most common ISO film speeds are 25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°. Consumer film speeds are generally between 100/21° and 800/30°, inclusive.
   Conversion from the logarithmic speed S° to the arithmetic speed S is given by ť

S = 10^=200

. In this case, the camera should report ť ISO 200 (daylight),

as well as a user-adjustable SOS value. In all cases, the camera should indicate for the white balance setting for which the speed rating applies, such as daylight or tungsten (incandescent light).
Despite these detailed standard definitions, cameras typically don't clearly indicate whether the user "ISO" setting refers to the noise-based speed, saturation-based speed, or the specified output sensitivity, or even some made-up number for marketing purposes.
As should be clear from the above, a greater SOS setting for a given sensor comes with some loss of image quality, just like with analog film. However, this loss is visible as image noise rather than grain. The best digital cameras as of 2008 exhibit no perceptible noise at ISO 200 sensitivity, and some produce usable results up to ISO 25,600.

Further Information

Get more info on 'Film Speed'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://film_speed.totallyexplained.com">Film speed Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.