What is exposure? Metering and exposure compensation

The purpose of this article is to let people realize that it is easy to understand and to control exposure

When a beginner photographer hears the word "exposure", it scares him and he tries to do everything to avoid to hear and to see it again. But you should not be afraid. Exposure will be your best friend if you learn how to properly use it.

 

Let's start with a bit of theory. Exposure is the amount of light that passes through your lens to your camera’s sensor. Is that clear?

So, boring theory is finished.

 

As you understand, the initial amount of light will be completely different on a sunny day and at moonlit night. But camera gives you control over that amount to compensate the difference. To do so you have to change the values for aperture, for shutter speed and for ISO.

In other words, these settings: aperture, shutter speed and ISO, allow you to control the exposure. And they are not separated from each other, actually, they are connected. If you have the proper exposure, and then you decide to change the aperture value, you have to compensate for this change by changing the shutter speed value or ISO or both of them.

To make it more visual let’s see the example: you have a proper exposure using f/4 (aperture), 1/25sec (shutter speed) and ISO 100. Then you decide to change the value for aperture, because you want to have larger depth of field. So, if you change the aperture to f/6.3 you need to change the shutter speed to 1/10sec to have a proper exposure again or leave the shutter speed the same 1/25sec but raise the ISO value to 200.

In the real life, if you are not shooting in completely manual mode, your camera will do these changes for you automatically. In case you are shooting in aperture priority mode A (Av for Canon) – camera will change the shutter speed for you to keep the proper exposure, if you are shooting in shutter speed priority mode S (Tv for Canon) – camera will change the aperture appropriately. But you can affect the camera’s changes by changing the ISO value in both cases, in case you do not agree with these changes.

 

Once we understand what the exposure is and how to control it, I would like to introduce two new terms – overexposure and underexposure. You can guess based on the names, that if we have a proper exposure then overexposure means that more amount of light passes to the camera’s sensor and as a result you will have lighter picture. Accordingly, underexposure means that less light goes to the sensor and you will have darker image at the end. Pay your attention to the left and to the right images in the illustration for this article at the top - where do you see overexposure and where is underexposure? 

 

Exposure compensation

Exposure compensation

As we learned, camera will make a decision what is a proper exposure and will correct all parameters to keep it. But that decision could be wrong. Using exposure compensation you can affect it. So, the exposure compensation is a feature on almost all modern cameras that allows you to make a picture lighter or darker than the camera’s recommended exposure.

If you think that your picture is dark you have to increase exposure compensation. Otherwise, if your picture is light you have to decrease it. Increasing exposure compensation means to shift it to a plus side: e.g. +0.3, +0.7, +1.0 etc. Decreasing it means to shift it to a minus side: e.g. -0.3, -0.7, - 1.0 etc.

Based on that your shooting process usually is the next: take a picture, look on the result, make an exposure compensation adjustments, take a picture again, look on the result, if your picture looks still not correct, make an exposure compensation adjustments again and take a picture, look on the result, etc.

 

Understanding metering exposure

Knowing how your camera meters light is critical for achieving consistent and accurate exposures. Metering is the brains behind how your camera determines the shutter speed and aperture, based on lighting conditions and ISO speed. Metering options often include matrix, center-weighted and spot metering.

All in-camera light meters have a fundamental flaw: they can only measure reflected light. This means the best they can do is guess how much light is actually hitting the subject. And that’s actually why in-camera meter can make the mistakes, because in the real-world, objects have completely different reflectance values. For the same lighting conditions two different objects should have the same exposure, but if one of them is white glossy plastic cube and another one is black cloth cube – camera will select the different exposure values. And, of course, it is incorrect. But we already know how to correct that using exposure compensation.

In order to have an exposure as accurate as possible and expose a greater range of subject lighting and reflectance combinations, most cameras have several metering options.

 

Matrix metering

Matrix metering

Matrix metering (or Evaluation metering for Canon cameras) measures the brightness of your subject at many places across the whole or almost the whole frame. Modern cameras can use up to more than 1000 different measuring points. After the camera collects all values, complex program logic determines an exposure from all these values, as well balanced as possible. Nikon, for example, also compares measured image data to a database (that is stored in your camera) that contains data of thousands of pictures for exposure calculation.

You should use this mode for most of your photography, because it will generally do a pretty good job in determining the correct exposure.

I actually use Matrix metering for most of my photography needs, including landscape and portrait photography.

 

Center-Weighted metering

Center-Weighted metering

Center-Weighted metering is close to Matrix metering because it determines an average brightness across the whole image but important difference is that it gives more weight to an area in the center.

Based on that, you can use this mode when you want the camera to prioritize the middle of the frame. It works great for close-up portraits and relatively large subjects that are in the middle of the frame. For example, if you were taking a headshot of a person with the sun behind him, then this mode will help you to expose the face of the person correctly, even though the background would probably get heavily overexposed.

I personally use this mode only rarely.

 

Spot metering

Spot metering

Spot metering measures the brightness of a subject only in a very small part of the image that is your focus point and ignores all the rest. This metering option is a good alternative for subjects with high contrast and if you want to focus exposure really on a selected part while the rest can disappear in white over- or black underexposure.

I personally use this mode in case the object occupies a small area of the frame and I need to make sure that I expose it properly, whether the background is bright or dark. Because the light is evaluated where I place my focus point, I could get an accurate exposure on the object even when it is in the corner of the frame. Spot metering works great for back-lit subjects, for example, when you were taking a picture of a person with the sun behind him but he occupies a small part of the frame. Another good example of using spot metering is when photographing a bright object that takes only small portion of the frame over the dark background, for example, the Moon. That way, we are only looking at the light level coming from the object and nothing else.

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