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Simple question really. When you double the ISO does it mean your shutter speed needs to be half of the original value to have the same brightness in the image?

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  • \$\begingroup\$ Typically if you increase the ISO in your camera the metering is adjusted so you get a correspondingly different shutter speed or aperture automatically. \$\endgroup\$
    – Mark Ransom
    Sep 9, 2019 at 1:03

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Assuming you want the same image brightness¹, if you increase the ISO setting by one stop and leave the aperture value the same, you need to reduce the exposure time by one stop.

Reducing exposure time by one stop can be described as:

  • Halving exposure time (e.g. reducing exposure time from 1/250 second to 1/500 second)
  • Doubling "shutter speed" (e.g. often referred to as increasing "shutter speed" from 1/250 to 1/500 - even though 1/250 is a larger number than 1/500)

The ambiguity of what "increasing" and "decreasing" shutter speed means is why I tend to avoid the term and use exposure time instead.

¹ The image projected by the lens onto the sensor will not be brighter or darker. At the same aperture it will be the same brightness regardless of the exposure time or the ISO setting. But the resulting photograph will vary in brightness based on how long the image is allowed to be projected onto the sensor and how much the collected light is then amplified. When either is halved and the other is doubled, if all other factors are equal the resulting photograph will have the same brightness.

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    \$\begingroup\$ Actually it is logical to call exposure time from 1/250 to 1/500 an increase of speed, because speed and time are the inverse of each other. It would remove a lot of confusion that exposure time would be noted in normal ISO format, like 4 ms resp 2 ms, but I"m sure there is a historic reason for this. \$\endgroup\$
    – Michel Keijzers
    Sep 7, 2019 at 23:35
  • \$\begingroup\$ @MichelKeijzers When the terminology was hatched the "speed" of an exposure was more about by how "fast" the film was. In other words, it was about how much time the film needed to reach proper exposure. Distance and time are the inverse of each other. Speed is the ratio between the two. \$\endgroup\$
    – Michael C
    Sep 8, 2019 at 1:49
  • \$\begingroup\$ While we're at it, your note about "increasing" also applies to "doubling" — double 1/250 is of course 1/125 \$\endgroup\$
    – mattdm
    Sep 8, 2019 at 8:19
  • \$\begingroup\$ I guess it's arguable that "speeds" like 1/500th seconds really include an implied "per height of frame" \$\endgroup\$
    – mattdm
    Sep 8, 2019 at 8:24
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    \$\begingroup\$ @mattdm Not always. Iris shutters, for example. \$\endgroup\$
    – Michael C
    Sep 8, 2019 at 10:34

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