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Before we look at metering modes in more depth let’s remind ourselves that the reason cameras get exposure wrong is because they measure reflected light, and give incorrect exposure readings if the subject is lighter or darker than average.
I discussed this in more depth in Exposure Lesson #2: Why Cameras Get Exposure Wrong.
If you’re struggling with exposure it could be because you don’t fully understand the way the metering mode you’re using works. But it’s more likely to be that you don’t understand how your camera’s meter works.
Most digital SLRs have the following exposure modes.
Center-weighted average metering
Center-weighted average metering works on the basis that your subject is most likely to be located in the center of the frame. The camera measures exposure from the center of the viewfinder.
This diagram shows how it works. The gray circle shows the area metered by the camera.
This is the sort of photo that center-weighted average metering is suitable for. The subject is central and there are no large bright areas likely to influence the camera’s meter into under-exposure.
If the subject is off-center, you can measure exposure by pointing the center of the viewfinder at the subject. Hold the shutter button half-way down to lock in the exposure settings, and reframe.
For example, if you’re shooting a scene that contains a bright sky, you need to point the camera down to exclude it, hold the shutter button half-way to set exposure, then recompose.
This is the type of photo where the bright sky would create a problem with center-weighted average metering.
This metering mode has been around a long time. If you own an old film camera it may be the only metering mode it has. Center-weighted average metering is predictable and easy to use once you understand how it works.
In spot metering mode the camera takes an exposure reading from a circle in the center of your viewfinder.
This diagram shows the metered area. There may be a circle in the center of the viewfinder to show the area covered by the spot meter.
Using spot metering takes practice. If your camera is behaving strangely in regards to exposure, it’s worth checking that you haven’t accidentally set it to spot metering mode.
Remember in the last lesson we learnt that cameras measure reflected light, and that the camera is expecting the tones within the area that it meters to average out to mid-gray?
If you point the spot metering circle at a tone that is lighter or darker than mid-gray, the camera will give you an incorrect exposure reading.
For example, in a photo like this you would different exposure reading if you pointed the spot metering circle at the white sign, than you would if you pointed it at the red wall.
Here are three ways you can use spot metering.
1. Point the spot meter at something in the scene that is naturally mid-gray, such as grass.
2. Take the exposure reading from an 18% gray card placed in the scene.
3. Take an exposure reading from a subject that’s heavily backlit, or from a bright subject positioned against a dark background (such as a performer in a theatre or a singer at a concert).
As you can see, spot metering is a lot of work! It makes sense to use it only when you really have to.
Center-weighted and spot metering both take an exposure reading from the center of the frame. The obvious flaw with this is that most photographers prefer to place the subject off-center. The process of metering with the subject in the center of the frame, then recomposing, slows you down. A better metering mode is required.
That brings us to evaluative metering*, the most advanced metering mode on your digital camera. It has been developed by camera manufacturers to make it easier to measure exposure with off-center subjects, like this portrait.
The camera divides the viewfinder up into zones. It compares readings from each zone to come up with a suggested exposure setting.
As cameras get more advanced manufacturers pack more zones into their metering systems. The Canon EOS 80D has 63 metering zones, the Fujifilm X-T2 has 256 and the Sony A7 has an incredible 1,200 zones. I’m not sure that more zones equals better exposure, but it just shows how sophisticated cameras are getting.
The camera weights the exposure reading towards the active autofocus point (or points) as they are likely to indicate the location of the main subject. It also takes into account readings from nearby zones and analyzes the contrast of the scene to come up with a suggest exposure setting.
Each camera manufacturer uses a slightly different process in their evaluative metering modes. While they don’t release precise details of how their systems work, there’s normally a guide in the instruction manual. It’s well worth a read so you understand how it works on your camera.
Evaluative metering really comes into its own when you are using an automatic flash to light the subject. Modern cameras are capable of analyzing the flash to subject distance and balancing that with ambient light levels (as in the portrait below). It would take you much longer to work out all that stuff yourself.
* Note: Everybody uses a different name for this metering mode. Canon and Sigma use the term Evaluative metering. Nikon uses matrix metering, Sony and Pentax use multi segment metering, Fujifilm uses multi metering and Olympus uses digital ESP metering.
Which metering mode should you use?
To be honest with you, it doesn’t matter. I haven’t used a metering mode other than evaluative metering (on my Canon EOS cameras) or multi metering (on my Fujifilm cameras) for years. As far as I’m concerned the others are redundant.
Evaluative metering is supposed to take into account the contrast of the scene as well as ambient light levels. But in practice it’s not much more accurate than center-weighted average metering.
It still gets exposure wrong if the scene doesn’t average out to 18% gray.
The only exception is if you are using automatic flash. Evaluative metering systems are far superior for this.
Spot metering may also come in useful in some situations.
My advice is that you take the simple approach to exposure. Set your camera to evaluative metering and forget about it.
Pay attention to your histogram and learn your camera reacts in different situations.
For example, I know that if I’m photographing a low contrast subject in the shade, then my camera tends to underexpose the subject by up to a stop.
With practice and experience you’ll soon learn what to expect from your camera.
This is the third in a series of lessons about exposure for digital cameras. I’ll add links to new lessons below as I publish them.
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