How does exposure compensation change shutter speed, aperture, or ISO?
Asked 8/15/2012
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2 answers
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Given an exposure such as 1/40 sec, f/2, ISO 1000, how do you calculate its exposure value (EV), then apply exposure compensation like +0.3 or +0.7 EV and work out the new shutter speed, aperture, or ISO? In other words, how does exposure compensation translate into changes in the exposure settings?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
6
The Exposure value can be calculated with the given Aperture and shutter speed using the following formula:
(src: wikipedia)
where,
N: Aperture value
t: Shutter Speed
The above EV is for ISO 100 called as EV100. EV For subsequent ISO values is calulated with help of EV100 using the following formula:
where,
EV100: EV from the first formula
S : required ISO value
The Exposure compensation works by adding or subtracting the required Exposure compensation value with the current EV to get the new EV.
This new EV can be reverse engineered with the above formulas to get the Aperture and Shutter speeds.
Applying the values you have given:
EV100 = 7.32 //At ISO 100
EV1000 = 7.32 + 3.32 = 10.64 //At ISO 1000
On applying Exposure Compensation of 0.33:
EV(new) = 10.62 +0.33 = 10.95
Now reverse engineer this value to get N and t accordingly.
Originally by user7079. Source · Licensed CC BY-SA 4.0
user7079
14y ago
0
Generated from our catalog & community — verify before relying on it.
Exposure compensation is measured in stops (EV). A change of +1 EV means one stop more exposure; -1 EV means one stop less. In practice, the camera applies that change by adjusting whichever setting is auto-controlled in the current mode.
For your example, 1/40 sec at f/2 and ISO 1000 is about EV 4 using the full EV formula that includes ISO. You can think of compensation as adding or subtracting fractions of a stop from that exposure.
Each 1/3 EV step is one third of a stop. Typical examples:
- shutter: 1/40 -> 1/50 is 1/3 stop less exposure; 1/40 -> 1/30 is 1/3 stop more exposure
- ISO: 1000 -> 1250 is 1/3 stop more; 1000 -> 800 is about 1/3 stop less
- aperture: one 1/3-stop change from f/2 is about f/2.2 or f/1.8 depending on direction
So if you apply +1/3 EV, the camera must increase exposure by 1/3 stop using shutter speed, aperture, ISO, or some combination, depending on mode and camera settings. For example, if only shutter changes, 1/40 becomes about 1/30. If only ISO changes, ISO 1000 becomes about ISO 1250.
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