What is ISO?

Question

and when should i put it high...when should i put it low??

Answer 1

Film speed describes a film's threshold sensitivity to light. The international standard for rating film speed is the ISO scale which combines both the ASA speed and the DIN speed in the format ASA/DIN. Using ISO convention film with an ASA speed of 400 would be labeled 400/27°. ASA is by far the more popular of the available standards, especially with newer equipment, and is often used interchangeably with the term ISO, although DIN retains popularity in Germany. The prevalence of ASA is reflected in film packaging which normally boldly states the ASA speed of the film on the box, with the full ISO speed printed in smaller type on the reverse or base. A fourth naming standard is the GOST developed by the Russian standards authority. See the film speed article for a table of conversions between ASA, DIN, and GOST film speeds.

Common film speeds include ISO 25, 50, 64, 100, 160, 200, 400, 800, 1600, and 3200. Consumer print films are usually in the ISO 100 to ISO 800 range. Some films, like Kodak's Technical Pan, are not ISO rated and therefore careful examination of the film's properties must be made by the photographer before exposure and development. ISO 25 film is very "slow", as it requires much more exposure to produce a usable image than "fast" ISO 800 film. Films of ISO 800 and greater are thus better suited to low-light situations and action shots (where the short exposure time limits the total light received). The benefit of slower films is that it usually has finer grain and better colour rendition than fast film. Professional photographers usually seek these qualities, and therefore require a tripod to stabilize the camera for a longer exposure. Grain size refers to the size of the silver crystals in the emulsion. The smaller the crystals, the finer the detail in the photo and the slower the film.

A film with a particular ISO rating can be pushed to behave like a film with a higher ISO. In order to do this, the film must be developed for a longer amount of time or at a higher temperature than usual. This procedure is usually only performed by photographers who do their own development or professional-level photofinishers. More rarely, a film can be pulled to behave like a "slower" film.

Answer 2

ISO, in the photographic sense, is a standard measure of a film's light sensitivity, otherwise referred to as its "film speed." And actually these days, what you refer to is a digital imaging ISO, which is different than film ISO. The term ISO--It's really a shortened reference to the International Organization for Standardization's film speed (or digital still camera exposure index) standards, which are actually documents maintained by that organization.

It's referred to as "film speed" because a film that is more sensitive to light would require less exposure time to get an image (or alternatively, a smaller aperture). Thus, a higher sensitivity (ISO rating) is considered a faster film speed. Faster films were useful to shorten the exposure time necessary to catch low-light shots. But a downside to those film stock was that the grain of the emulsion was rather noticable in development--literally, bigger fatter crystals vs. slower film emulsions. You would get grainy pictures.

The digital photography equivalent ISO standard is actually an entirely separate set of ISO standards. But it serves the same purpose--a method for assigning and reporting ISO standard speed ratings, ISO standard output sensitivity values, and recommended exposure index values, for digital still cameras. For a given photosensor array, ISO ratings (digital still camera "exposure indexes") are mapped out based on it's characteristics. Except photosensors are capable of operating at various ISO sensitivity ratings because their output signal can be amplified. At each film ISO equivalence, a photosensor array must be able to produce a specific signal-to-noise ratio with a particular amount of light for it to be identified as that particular ISO setting. So, when you set a digital camera to take pictures at an ISO1600 setting, you're telling it to amp up the signal so that it will produce image of a particular light intensity given a specific amount of incoming light.

But here's the key for you...just like film speed ISO, photosensor ISO looses image quality at higher sensitivities. The root cause isn't emulsion grain (as it is in film ISO's problems), but signal noise. You'll see mottled variations in the color of a blue sky, for example, when in real life, the sky is a smooth blue color. That's image noise, and it's significantly more noticable in higher ISO settings.

There's the crux of your answer.

When should you put it in high ISO?

As seldom as you possibly can.

Keep the ISO as low as possible to avoid unnecessary noise. This, of course, is only an issue when you're trying for low-light shots without flash. You'll find that in brightly lit scenes, or when using a flash, you can always stick to ISO100 or ISO80. But when the lights dim, and you have an artistic preference for not using flash, that's when you'll have to start pushing the ISO up. What happens is that the shutter speeds start to get so long that you can't keep the subject from blurring in the picture. That's when you know that you're going to have to push the ISO up. If your subject is inanimate (doesn't move), then you can avoid pushing ISO up by putting your camera on a tripod. But if your subject is moving, then you'll have no alternative but 1) push ISO up, or 2) use a flash. If you use a flash, you'll change the artistic impression of the shot. If you push the ISO up, you'll concede image quality. Both are a mixed bag. One thing you could do if you had the option, is switch to a camera/lens that is capable of wider aperture settings, letting in more light.