What does AF sensitivity of -0.5 EV or -3 EV mean?
Asked 1/1/2014
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Camera autofocus specs sometimes list low-light sensitivity values such as -0.5 EV or -3 EV. For example, a Canon 6D is rated to focus at -3 EV while a 600D is rated around -0.5 EV. What is EV referenced to in this context, and how should these AF sensitivity numbers be interpreted in practical terms?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Exposure Value is actually an absolute scale, where EV 0 corresponds to an nominally-exposed photo having an exposure time of 1 second at aperture f/1.0 (or of course equivalents, like 2 seconds at f/1.4).
You can see by that baseline that -3EV is pretty amazing — it corresponds to a minute-long exposure at f/2.8.
We're used to seeing EV in the context of EV compensation, where the camera is told to expose differently from what its meter tells it to, and that's expressed in relative terms (plus or minus some number EV), but the basic scale is actually fixed. More on this a What is the EV scale? and What is exposure compensation?
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
12y ago
0
Generated from our catalog & community — verify before relying on it.
EV here is on an absolute exposure-value scale, not just a relative compensation setting. A common reference point is EV 0, which corresponds to a nominal exposure of 1 second at f/1.0, or any equivalent exposure.
So when a camera is rated to autofocus at -3 EV, it means the AF system can still operate in very dim light—3 stops darker than EV 0. That’s why the number is meaningful: it refers to a fixed brightness level, not just a comparison between cameras.
In practical terms, more negative AF sensitivity means better low-light autofocus capability. A camera rated at -3 EV can focus in much darker conditions than one rated at -0.5 EV.
As a rough illustration from the community answer, -3 EV is extremely dark—about the level where an equivalent exposure might be around 1 minute at f/2.8.
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