Image Quality and ISO Rate

In analog photography, the ISO rate defines the light sensitivity of films. Film sensitivity is usually indicated in ISO, DIN and ASA. More sensitive films have a higher rate of film grain which can result in visibly poorer image quality. Digital sensors also have different degrees of sensitivity.

The sensitivity of a CCD or a CMOS sensor is highly dependent on the size of the individual pixel. With the size of the sensor remaining the same and with higher resolution, the technical challenge for the design engineers is becoming increasingly more challenging. As the individual pixels are less sensitive to light on a smaller surface, the input signal must be increased respectively. This increases the useful information signal as well as the interfering signal (the unwanted dark current) and results in an unbalanced ratio between the useful information and the interfering signal, which can be seen in the picture as image noise. Manufacturers counter this problem with more or less efficient methods of noise suppression, usually at the expense of picture details.

A change in ISO rates also has influence on a camera’s image noise, as this increases the input signal at various rates. The higher the ISO rate, the more the input signal is increased and the camera’s image noise is increased. Therefore, ISO rates should not be increased unnecessarily in order to achieve maximum image quality. Low ISO rates can increase the risk of blurring. To try and counter this, try to stabilise the camera (camera should be placed somewhere, mounted on a tripod…). If this is not possible, the ISO rate must be increased. Nearly all of the digital cameras nowadays have an ISO automatic function which reduces the risk of blurring in at the expense of image quality. The camera assumes that it is being hand-held while the image is being captured. If the camera is e.g. mounted on a tripod, the ISO rate may unnecessarily increase. If this is the case, the ISO rate should be manually reduced.

 

Summary

  • ISO rate indicates light sensitivity
  • Low rates => max. image quality
  • Issues with low ISO rates
    • Risk of blurring
  • Issues with high ISO rates  
    • Brightness and/or image noise
  • Compromise between image quality and risk of blurring 

The following images illustrate the connection between image quality, ISO rates and sensor size.

Image 1 (compact camera, 8 megapixel, ISO rate 100)
100 ISO Bildqualität Kompaktkamera


Image 2 (compact camera, 8 megapixel, ISO rate 1600) – Regensburger Dom - Germany
1600 ISO Bildqualität Kompaktkamera


Image 3 (single-lens reflex camera, 8 megapixel, ISO rate 100)
Spiegelreflexkamera, 8 Megapixel, ISO-Wert 100


Image 4 (single-lens reflex camera, 8 megapixel, ISO rate 1600)
Spiegelreflexkamera, 8 Megapixel, ISO-Wert 1600

 

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