(This article is a supporting part of our ongoing testing of low-light camera metering reliability)
Exposure Values (EVs) have two common usages. First, they can be used to quantify the brightness of a scene. Higher EV numbers indicate a brighter scene. Second, they can describe differences in exposure settings (for example exposure compensation). In both definitions, a change in EV of 1 indicates that the light level has doubled or halved. Below are the approximate EV ratings of an assortment of scenes:
Photographic light meters typically meter a scene and report its EV plus the exposure settings appropriate to capture it. The table below shows what settings correspond to different EV situations.
To use the above chart, suppose you want to know what shutter speed properly exposes a 5 EV scene at f/4 and ISO 400. Look down the ISO 400 column to the row with “5”, then follow that row right to the f/4 column. The appropriate shutter speed is ⅛ second. If this is still unclear, the link gives several more examples.
Below is a graph of the ambient brightness in Exposure Values for moonlit landscape conditions. You can also use this moonlight exposure calculator to check exposure for a specific aperture and ISO and moon phase.
EVs and Your Camera’s Histogram: Different Dynamic Ranges Result In Different Histograms For The Same Light
Due to cameras having different dynamic ranges, however, that lone histogram spike will fall in a different place for one camera versus another.
To demonstrate this, I set a Canon M5 and Sony A7m3 to the “correct” EV0 settings of ISO 100, f/2.8, 8 seconds, then inserted them into the sphere with it set to EV0. The same 50mm lens at 2.8 was mounted via an adapter to both.
As you can see, the Sony (left) reports that the scene is ⅓ stop underexposed, but its histogram is perfectly centered. The Canon (right) reports that the scene is 1 stop overexposed, but its histogram is slightly left of center. EV0 is about where it should be (middle of histogram), but the two cameras have different ideas of where in their dynamic range it should go. Canon and Sony have programmed their metering differently.
In summary, the exposure value of a scene describes its brightness, and it can be converted into camera settings that can capture the scene. However, because different models of cameras have different dynamic ranges, not all cameras will place the histogram bump in the same place, even with the same settings and lens.
Integrating Sphere & Camera Metering Test Project