Why doesn’t exposure value increase with ISO in the way I expect?

Asked 1/10/2013

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2 answers

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I’m trying to understand exposure value (EV) and ISO. My current understanding was that lower EV means a brighter exposure, since using a wider aperture or longer shutter time lets in more light.

What confuses me is ISO. A higher ISO makes the camera more sensitive to light, so I expected that if ISO is included in an EV-related formula, increasing ISO would reduce the EV number needed for the same scene, not increase it.

I’ve seen a formula written as:

log2(N^2 / t) + log2(S / 100)

where N is f-number, t is shutter time, and S is ISO.

Why is the ISO term sometimes shown as being added? Is that formula being interpreted incorrectly, or am I misunderstanding what EV represents?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

13y ago

2 Answers

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I think you got it backwards, EV increases when brightness increases.

See the wikipedia article for details

Originally by user2481. Source · Licensed CC BY-SA 4.0

user2481

13y ago

0

AI Answer

Generated from our catalog & community — verify before relying on it.

The main issue is that EV is often being read backwards.

EV is primarily a way to describe scene brightness or equivalent exposure settings, not “how bright the final photo looks.” A lower EV means dimmer light, so you need wider apertures, slower shutter speeds, or higher ISO to get a normal exposure.

For a given scene brightness, increasing ISO means you need less actual light at the sensor. So if ISO is included in the relationship, the ISO adjustment works opposite to aperture/shutter exposure time. In other words, higher ISO reduces the light required for the same final image brightness.

That’s why your intuition is basically correct: if a formula is written so that increasing ISO makes EV increase directly, it’s likely being rearranged or interpreted incorrectly. One of the community answers notes that the sign is effectively reversed in the version you quoted.

Also, EV does not mean “more exposed image” in the everyday sense. Two images made at different ISO settings can look equally bright even though the sensor received different amounts of light or used different sensitivity.

UniqueBot

AI

13y ago

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