What is ISO and how does it affect my photos

What is the ISO setting on my camera? What does it represent? What should I be aware of with regards to ISO when taking photographs

Answers 4

  • Changing the ISO setting in digital camera's is like changing the gain (amplification) of your photo sensors. With a low ISO setting (e.g. 100) you don't amplify the signal from the sensors. But using a high ISO setting (e.g. 1600) results in an amplification of the signals registered by the sensors. A high ISO setting usually results in more noise in your images so try to keep the ISO as low as possible.

    This article explains various sources of image noise: http://en.wikipedia.org/wiki/Image_noise

  • The ISO number represents the sensitivity of the film material. You need to configure the right ISO number in order to obtain the fight brightness of your photos because the camera will adjust the time of exposure, size of opening and/or flash in the appropriate way.

    Newer cameras are able to read the ISO number from the label on the film role.

    See also here and here.

  • The ISO number (sometimes ASA) is a standardised measurement of how sensitive the film (or digital sensor) is.

    Higher ISO film tended to have more grain; and higher ISO digital shots exhibit more noise - a similar cause, but the visual appearance is different.

    Digital ISO noise is related to the size of each pixel, as the noise is per-pixel (so the more pixels you have, the less obvious noise is when viewed the same size). One analogy I've used in the past to demonstrate this is to ask several people to time with a stopwatch how long it takes a car to drive around a car park, and then to time how long a person takes to do the same journey - because the person is slower, the margin of error is a smaller in proportion to the overall figure, even though different people will give timings to within a few seconds of each other.

  • The name "ISO" is the official logo of the International Organization for Standards. The French version of the name, Organisation internationale de normalisation, arranges the four words in a different order for proper syntax in french. The letters "ISO" correspond to neither arrangement.

    From Wikipedia:

    The three official languages of the ISO are English, French, and Russian.[3] The organization's logos in two of its official languages, English and French, include the word ISO, and it is usually referred to by this short-form name. The organization states that ISO is not an acronym or initialism for the organization's full name in any official language.[citation needed] Recognizing that its initials would be different in different languages, the organization adopted ISO, based on the Greek word isos (????, meaning equal), as the universal short form of its name.[4] However, one of the founding delegates, Willy Kuert, recollected the original naming question with the comment: "I recently read that the name ISO was chosen because 'iso' is a Greek term meaning 'equal'. There was no mention of that in London!"[5]

    The ISO has written many technical standards, technical reports, technical specifications, etc. Each of these is assigned a number by the ISO. Three standards that apply to the sensitivity of photographic film are ISO 6, ISO 2240, and ISO 5800. Over time, a film's speed was referred to as its "ISO" because the number used to describe the film's speed was in compliance with the ISO standards cited above.

    With digital cameras, "ISO" has continued to be used as a way of expressing a digital camera's sensitivity to light at various amplification levels of the analog electrical signals coming from pixel sites on the camera's sensor. In theory, an ISO setting of 400 on your digital camera should result in an exposure equivalent to an exposure on ISO 400 film. Film sensitivities varied slightly from one film manufacturer to the next. A film that had an actual value of, for example, 388 based on the ISO standards would be marketed as "400 speed". Likewise, most digital cameras vary slightly at different ISO settings from the exact standard. At least one company, DxO, publishes test results for many cameras. If you go to the link and select the "measurements" tab you can see that the actual ISO can vary by as much as 1/2 a stop for the three entry level camera bodies I selected.

    The primary thing regarding ISO you should be aware of when taking pictures is that the higher ISO number you select, the "noisier" your image will be. Noise is an electrical signal from a pixel that was caused by anything other than light falling on it. When the signal from a sensor is amplified to increase ISO this noise is amplified as well. As your camera (or processing software on your computer) processes the signals from your sensor, certain measures are applied to smooth out the noise. Most cameras have settings that allow you to select how much noise reduction you want applied to the images you shoot. The downside to heavy use of noise reduction is that it also reduces the sharpness of the image at the pixel-to-pixel level. Because of this, it is best practice to shoot with the lowest ISO number that allows you to select the aperture and shutter speed combinations you desire. On the other hand, a blurry image due to a too slow shutter speed can't be fixed in processing. A noisy image that stopped the motion of your subject can be dealt with to a degree. The larger you intend to display the image, the more noise becomes an issue.

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