Metering and Exposure Compensation

What is metering?

When the camera measures how much light is falling on the scene, this is called metering. In the old days we used to use a light meter. We carefully picked a mid-grey part of the landscape, or took a reading off whatever we wanted to be correctly exposed (usually someone's face), and then translated the meter's suggested settings to suit our creative priorities. 

This is still the most accurate way to get the right exposure.

But 21st century cameras pretty much do away with the need to use light meters, unless you're working in a studio. Now you can rely on the camera's built-in light meter, plus the instant histogram feedback you get. 

How does the camera meter?

Just as it's critical that you know what the camera is programmed to do when it comes to fixing exposure (it turns everything mid-grey), so you also need to know what it has been programmed to do when it measures the light in first place (metering). There are 3 kinds of metering that the camera can use, and you can switch between them (check your manual). In each one, the camera is programmed to do something different. Your camera may have a different name for each kind of metering, but the principles are the same:

evaluative metering.png

1. Matrix, or evaluative metering

This is usually the default setting. The camera takes a broad reading from everywhere in the frame. Useful for beginners, but if the background is light (like the sky), and your subject is just in the middle, the light background will skew the exposure and your subject will not be correctly exposed.

centre weighted metering.png

2. Centre-weighted metering

The camera gives priority to whatever is in the middle of the frame. This is a better all-purpose setting to use, if you generally want what is in the middle of the frame to be correctly exposed.

spot metering.png

3: Spot metering

You pick one very small area of the frame to take a reading from, usually one of the small rectangles that you can see in the viewfinder that the camera uses to show where it has focussed. This is the most accurate way of getting a correct exposure for a single part of the image.


Download chapter 6 from Book 1 here - it has all the information about histograms, metering and exposure compensation. It also has a short quiz so you can test your understanding of histograms:


How to correct exposure mistakes quickly: Exposure Compensation

The camera will either have under exposed or over exposed; those are the only 2 things to fix. You either switch to Manual mode and compensate with more/less aperture, shutter speed or ISO, or you can use the exposure compensation function. This function lets you dial in between 1/3 and 2 or more full stops of compensation (depending on your camera) without having to fiddle about with aperture, shutter speed or ISO settings. 

This section is optional, but if you're interested, find out how to use exposure compensation from your camera manual. It is probably a dial on the top of your camera. In the viewfinder, look for a scale that looks a bit like this:

exposure compensation

The scale shows you what's happening when you move the dial. The indicator underneath will move left or right as you dial in plus or minus exposure compensation. So if the histogram showed a slightly underexposed image, you could dial in +1 stop of exposure compensation to let more light in, and the camera will increase the aperture by 1 stop (or the shutter speed, or the ISO - it depends on your camera and the mode you are using). Check your manual to see exactly what your camera will change. You have less control over the exact settings than if you are on Manual mode, but this is a very quick and easy way to get the correct exposure without having to look away from the viewfinder.


AYWMC students can claim a 50% discount on the Get Off Auto module whilst it is being taught in the email lessons - until the end of lesson 6


The Workbooks are available on Amazon

 

Take a look at the workbooks if you haven't yet. The "Look Inside" feature is enabled on Amazon so you can flick through before you buy. There's also a short video on this page that gives you a guided tour:

This is the first book I’ve found which really explains digital photography from first principles (& I have tried many). Originally I signed up for the weekly emails, thinking I’d save money and not buy the book, but the emails blew me away and I realised I needed it all in one place to easily refer to. This book is a must for anyone wanting to learn or improve their digital photography skills. My photos are so much better now and it’s made photography even more fun!
— Elaine Daniels, Amazon review

The Exposure Triangle

Understanding Stops

What are stops?

Do you remember the list of aperture f-stops? And shutter speeds and ISOs? f4, f5.6, f8; 1/60th, 1/125th, 1/250th; 100, 200, 400?

The gap between these measures of aperture, shutter speed and ISO is important, and it's called a "stop". 

From f4 to f5.6 is 1 stop. From 1/60th to 1/125th is 1 stop. From ISO 400 to ISO 800 is 1 stop. Each stop lets in the same amount of light. So 1 stop on the aperture scale corresponds to 1 stop on the shutter speed scale. Knowing this allows you to compensate accurately. 

If the camera picked f5.6 but you want f11, that's 2 full stops smaller. f5.6 > f 8 > f11. So, to keep the same exposure, you would need to change the shutter speed by 2 full stops larger, eg. 1/500th < 1/250th < 1/125th. Or the ISO by 2 full stops larger; ISO 100 < ISO 200 < ISO 400. Or you could change the shutter speed by 1 full stop and the ISO by 1 full stop. 

Full stops, half stops and third stops

The benchmark stops are the ones mentioned so far in the book. f4, f5.6, f8, f11, f16. 1/60th, 1/125th, 1/250th, 1/500th. ISO 100, 200, 400, 800. But you may find that your camera picks an aperture like f7.1, or a shutter speed of 1/160th. That's because there are stops that fall exactly half way between the benchmark full stops, and there are also stops that fall exactly a third and two thirds of the way between the benchmark full stops. Your camera will be set to show either full plus half, or full plus third stops. You can change between them.

It's not critical that you remember all the half stops and third stops, but you should be familiar with the benchmark full stops for each of the 3 settings:

Double the light / half the light

As you move between stops, remember that each stop lets in twice as much light as the one before it (rather than just +1 stop). This diagram shows the concept for apertures, so you can see the huge difference between each end of the scale:

aperture stops diagram

Stops and the exposure triangle explained a bit more...

This is an excerpt from the video lesson that accompanies AYWMC. It explains stops and the exposure triangle in a more visual way, which some people find easier to understand than just reading about it (click the volume icon to hear the commentary):

AYWMC students can claim a 50% discount on the Get Off Auto module whilst it is being taught in the email lessons - until the end of lesson 6


Download chapter 5 from Book 1 here - it has all the information about stops, and the quiz to test yourself:

The test is intermediate level. Give it a go, but don't be discouraged if you find it hard. You'll be able to come back to it throughout the year and find it getting easier and easier.


The Workbooks are available on Amazon

 

Take a look at the workbooks if you haven't yet. The "Look Inside" feature is enabled on Amazon so you can flick through before you buy. There's also a short video on this page that gives you a guided tour:

This is the first book I’ve found which really explains digital photography from first principles (& I have tried many). Originally I signed up for the weekly emails, thinking I’d save money and not buy the book, but the emails blew me away and I realised I needed it all in one place to easily refer to. This book is a must for anyone wanting to learn or improve their digital photography skills. My photos are so much better now and it’s made photography even more fun!
— Elaine Daniels, Amazon review