You have carefully read the articles on exposure , and now you are beginning to understand its mechanisms, and to work on it when you take a photo.
That said, in certain somewhat difficult situations (notably in backlight or low light), you still have difficulty obtaining a correct exposure unless you shoot in full Manual mode , which is not always the most practical. . And then you saw this “measurement mode” thing in the manual and on your device , but you don’t really know what it’s for, or how to use it. Follow the leader.
Let’s see how it works in your camera when you use a semi-automatic mode like aperture priority mode (Av or A) or shutter speed priority mode (Tv or S).
If, for example, you select shutter speed priority mode and set it to 1/250th, the camera will itself determine the aperture and ISO sensitivity necessary to obtain a correct exposure in the photo.
Obviously, it does not determine its parameters by magic , but by making measurements . Basically, the camera measures the brightness of the image and determines at which settings it will be exposed correctly. But how does it measure this brightness?
By default, the device measures brightness across the entire image . But you have the possibility to modify this behavior thanks to the brightness measurement mode . Let’s see the different modes generally offered by modern SLRs (but also by certain bridges and compacts).
The evaluative / matrix / multiple measurement mode
The name changes a lot depending on the brand: we talk about evaluative measurement at Canon, matrix at Nikon, or even multiple at Panasonic Lumix.
That said, it’s the same principle: it’s the default mode used by your device, the one that measures the brightness over the entire image .
It allows you to obtain an image that is correctly exposed in all its parts : no place is over-exposed (or “burned”) nor under-exposed (or “blocked”), within the limits of the technical capabilities of the equipment. sure, especially its dynamic range .
To understand clearly, I deliberately took a contrasting scene that you can reproduce at home : it’s just a figurine in front of a window.
The exterior is very bright (it’s broad daylight), but the table and the figurine are quite dark. The evaluative/matrix metering does what it can to avoid overexposing the sky: we have an image exposed for the average of the tones .
There’s a reason this is the default mode: it works in most situations , and most often you won’t need to change it.
But sometimes, this mode works quite poorly with the situation, for example if the image is highly contrasted , like here: backlit scene, concert photo where the singer is lit by a single spot and the rest immersed in the shadow, etc.
Spot/point measurement mode
This mode measures brightness only in the central part of the image , that is, the circle you see in the center of your viewfinder. This represents approximately 4 to 5% of the total image , or a very small circle.
It allows you to obtain a correct exposure of the subject when the rest of the image is lit very differently, as in the case of backlighting .
Be careful, in this case the bright part of the image (the sky for example) will be clearly overexposed . You have to know it, and play with it. It is generally not used to create silhouette effects, on the contrary, unless we measure the brightness on the sun for example: in this case the sun/sky will be well exposed, and the rest of the image very dark and therefore in silhouette.
Let’s go back to our example: on my device, I can select precisely where I want to do the spot measurement (we’ll talk about this more below). I focused on mini Groot’s head. The camera therefore makes the exposure so that this point is well exposed. And it is: mini Groot is in mid-tones, neither too light nor too dark .
But as a result, the rest of the image is obviously much clearer ! (which isn’t necessarily always very serious, even if here it’s a little ugly) .
Note that on some devices, notably Canon SLRs, there is also a fairly similar metering mode, selective metering, which is basically the same thing but with a wider circle, of the order of 9 to 10% of the ‘picture .
Center-weighted metering mode
This somewhat barbaric term actually means that the brightness is measured primarily in the center , but that the rest of the image is also taken into consideration.
In other words, the device measures light over the entire image, but gives more weight to measurements made in the center than to those made at the periphery. Here the diagram is very useful to understand how the brightness is evaluated:
This mode is a bit of an intermediary between the evaluative metering mode and the spot metering mode. It can be used in more or less the same situations , especially when you don’t really know what point to meter the light at and you just know that you want to correctly expose the center (wider center than in a spot metering).
Here I used this mode on the same scene. You see that we have an even darker scene than in the first one. I think the camera simply detected the midtones well (the reflection of the sky on the wood of the table), but did not take into account the shadows around it (which should not have been part of the area taken into account ). As a result, he reduced the exposure a little further.
As for using these different modes, I refer you to the manual of your device, I will not do it for all brands obviously 😉
PLEASE NOTE : the measurement mode setting is retainedwhen you turn off your device: remember to check it when you take a new series of photos 😉
Where is brightness actually measured?
I ask this question because I have always spoken of the “ center ” of the image. But as you know , we don’t always (or even often) want to place our subject in the center. So, how to do ? Hold on a little, it’s not necessarily very simple 😉
On many SLRs (notably Canon), the spot measurement will be done at the center of the image , regardless of the collimator used to focus .
On certain hybrids and also certain SLRs (notably Nikons), the spot measurement will however be done at the place where the focus is made , which is often more logical.
So if you use the spot metering mode on a SLR, the metering will often be done in the center of the image, even if your subject is placed elsewhere . How to get around this problem? By using the exposure lock button , often represented by an asterisk * , or by the acronym AE-L (like “auto-exposure lock”, which translates to “automatic exposure lock”) .
How does this button work? It’s simple. You place the subject you want to expose with spot metering in the center of the image. You press the * button . A * then appears in the viewfinder.
The exposure is then memorized , and all you have to do is focus and frame your image as you wish, and of course trigger. It’s a bit like pressing the shutter button halfway, but instead of locking the focus , it locks the brightness metering (and therefore the exposure).
There you go, you now have the keys to using these different measurement modes to their full potential. That said, they’re not easy to master (especially the spot metering mode), and you’ll need some practice before you intuitively use them to their full potential. But this tool is often essential in difficult lighting situations, and knowing it will allow you to better manage them.
Speaking of practice, know that in my flagship training Become an Accomplished Photographer, I approach in detail and step by step this subject of measuring light (among others), as I would have liked to have it explained to me when I started photography. If you want a comprehensive, structured method to thoroughly master your device, it might be worth taking a look 😉
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