Effect of Metering on Captured Image

Question

I've been using the advanced metering modes on my Canon EOS 7D (and also on an XSI I owned previously) for several years now. I had always assumed that the purpose of the different metering modes was to help guide the photographer toward choosing the right aperture and shutter speed to get the correct exposure.

I was talking with another amateur photographer recently and he suggested that the camera will capture the image differently, depending on the metering mode that is used. I can see how this might be true in a program mode since the photographer is letting the camera make some of the decisions.

So my question is, does the metering mode of a DSLR (Canon specifically) have any affect on the captured image, assuming the photographer is manually selecting aperture and shutter speeds themselves?

Answer 1

It depends how you choose to look at things. If you use the same settings (ISO, aperture and shutter speed), you will get the same image. The metering doesn't affect how the camera creates the image/interprets the sensor details, no -- at least not on current cameras.

But choosing a different metering mode may, in fact, guide you to make different exposure choices when you choose your aperture, shutter speed and ISO, and that will result in a different image. Spot and centre-weighted metering are based on rendering the metered area as a middle grey (18% used to be standard, but it's more varied these days by manufacturer and sensor characteristics). Knowing what's being metered and how you want to render it is up to you. Evaluative metering, at least in more recent cameras, takes readings from enough areas of the picture -- often with colour information included -- to pattern-match against common picture types and come to a more "intelligent" conclusion about exposure. It's not always right, or even nearly right, and it doesn't know what your intentions are, but it is often substantially different from the older, simpler, "dumb" metering options.

Answer 2

For the most part the metering mode will affect the resulting image when shooting in Manual (M) mode only if you take the reading provided by the meter and change your manually selected values based upon that reading. There are, however, a few scenarios with Canon cameras when the meter reading is used by the camera to select a value that affects the resulting image even when the user selects the shutter speed (Tv) and aperture value (Av) in M exposure mode.

The 1D X and 5D mark III allow the selection of Auto ISO in M exposure mode, in which case the metering mode could affect the meter reading that determines the ISO selected by the camera after you have selected the Tv and Av. With the latest 1D X firmware installed, you can even apply exposure compensation when using Auto ISO in M mode.

If you are using e-TTL flash while selecting the Tv and Av in Manual mode, all Canon DSLR cameras will adjust the power output of the flash based on the meter reading. Different metering modes could return different results and thus different flash power levels.

There are a few cameras, such as the Canon 1D X that falls within the scope of your question, that meter in more than one color, and the metering mode selected could have an effect on the White Balance if Auto WB is also selected and different areas of the scene are illuminated by different types of lighting.

Answer 3

Different metering modes change how the camera estimates the light in the scene. They are provided to help deal with hard to meter scenes. Average metering looks at the average brightness of the entire scene. Spot metering looks at a particular spot and ensures that it is properly exposed without providing much, if any, consideration to the rest of the image and Evaluative tries to figure out what the scene is based on what parts are bright and dark and judges accordingly. Other modes may be available and some of the modes I mentioned may not be depending on the camera model. Reading the manual is the best way to make sure you understand the metering modes available.

The final output of all metering though is simply an estimate of how bright the scene is and thus, what the exposure settings should be to deal with that brightness. The camera will only adjust things that it would adjust due to thinking the brightness is different. For most cameras, this is just shutter speed, ISO and aperture, so if you have them all locked down, then it would make no difference unless your camera bases more than those on the metered level (rare).