Part 1 Lesson 6: METERING

How did you get on in the last lesson shooting on Manual mode? Congratulations if you managed to find Manual mode and worked out how to change the settings independently. 10/10 if you had a go at the homework and got a couple of photos that were broadly what you were aiming for.

If you found it utterly confusing, and are feeling dispirited, you'll be pleased to know that the worst is over. I can't say this too often:

You don't need to shoot on Manual mode all of the time (this blog post lists a few times you do need to).

You simply need to know:

1. There are 3 settings.

2. They each control the light.

3. They each also have a creative effect (or consequence, for ISO).

Throughout the year, when you have time to concentrate, keep coming back to these first 5 lessons. Try shooting on aperture and shutter priority. Remember to look at the shots you take on auto, and think about whether you could do better if you take control. And at least once a month, have a go on fully manual again (write it in your diary, or the AYWMC planner if you use it - maybe pick the first Saturday of the month or the last day of the month).

This week is the last of the technical lessons. Stay with me for one more bumper email, and then you can look forward to the next module which is a creative one: Composition. If you find the talk of histograms (below) baffling, feel free to skip that part and just make sure you understand how your own camera meters.

Think back to the first lesson, when you used auto mode and took a photo of something white and another of something black, and they both came out grey.

In the next 3 lessons you learnt how to take control of the 3 settings the camera uses to control the light.

In this lesson you'll learn: "How do I know what the right exposure is?".

You know the camera gets it wrong (it turns everything grey). And you know you can take control. But the final piece of the puzzle is - by exactly how much has the camera got it wrong?

You are not always taking photographs of something that is conveniently all black or all white. Most of the time you are shooting a mix of colours and tones. And it is very difficult to convert a scene into black and white in your head, and then average out all the tones to see how far off the camera is going to be. In the image below for example, the greens and reds of the leaves combine with the sky to create a very difficult image to estimate the correct exposure for. And in fact the camera's auto exposure for this shot underexposed by 3 entire stops.

histogram 1.png

The good news is that you don't need to constantly convert to black and white in your head and make tone estimates. On the back of every camera there is a display that tells you exactly what the camera is seeing. You can then decide if you and the camera are seeing the same thing, or if the camera is going to get it wrong and expose too dark or too light. The display is called the histogram. It's simply a bar chart of tones, with black tones on the left and white tones on the right, and will look something like this:

histogram 2.png

Once you know how to interpret the histogram, you can use it to check what the camera is seeing. Then you can either accept that exposure; or override it using any of the priority modes, manual mode, or a great little trick called "exposure compensation" (more on that below).

What is the histogram?

The histogram is just a bar chart showing light values. Imagine a bar chart showing how many apples, pears and oranges a shop sells in a week. If it sells 10 apples, 20 pears and 30 oranges, the bar chart will look like this:

histogram 3.png

Now imagine you are looking piece of paper which has 1 black stripe, 2 grey stripes, and 3 white stripes. The bar chart would look like this:

histogram 4.png

And that is all the histogram does. The camera assigns a tone value (from black to white via all the greys) to every single pixel in your image. It then plots them on a bar chart to give you your histogram. It counts all the pure black pixels and gives them a bar all the way to the left, and does the same for every shade of grey all the way to pure white over on the right:

histogram 5.png

When it comes to an actual photograph, the camera just counts all the pixels, sees how many of each tone there are, and then plots them all on a bar chart:

histogram 6.png

In this image the histogram is showing that there are no black and darker tones at all. Does that look right? It's hard to tell in the reproduction, but that is accurate. In the original image you can still see plenty of detail in the shadows which means they are not completely black.

And at the right hand side, the white bar chart has started to "climb the wall" - it finishes with plenty of white tones showing. This is a sign that the highlights have blown out, and lost all detail - you should be able to see areas of pure white somewhere in the image. Is that correct? Yes - some of the clouds are pure white.

Using the histogram to check exposure

If the histogram in this example had looked like the following diagram, then you would know that the exposure the camera had picked was wrong. Because you can look at the histogram, see that the blacks are touching on the left, and then look at the scene and realise that there are no pure blacks. The histogram is also showing no pure whites - but you know there should be. You would deduce the camera has got the exposure wrong and you can correct it.

histogram 7.png

Remember:

1. The camera's auto modes sometimes get the exposure wrong

2. Your eye/brain also sometimes gets things wrong

3. The histogram never gets anything wrong

4. You can see whether the exposure is correct by checking if the histogram looks like it should - you can't rely on your eyes to do this because they often get it wrong

5. If the histogram is not how it should be (or if you can tell just by looking at the shot that it is under or over exposed), you can compensate for the camera's mistake by changing aperture, shutter speed or ISO


What is metering?

The way that the camera interprets the tones of an image varies depending on which metering mode it is using.

 

Chapter 6 from Workbook 1 has all this information about histograms and metering, and explains the very very useful trick of using Exposure Compensation to quickly fix exposure errors without shooting fully manual. You can download a pdf of Ch 6 to print at home here:

 

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:

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.

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.

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.

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:

exp comp.png

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.


THIS WEEK'S PROJECT

Spend a day where you review the histogram for every photo you take. If it looks wrong, then use either Manual mode or exposure compensation to correct the exposure.

Most cameras allow you to put the histogram on as a preview in the viewfinder, but some don't (including most of the entry-level Nikons). If your camera doesn't have a histogram preview, you will need to use the post-capture histogram. If your camera does have a histogram preview, you can check the histogram before you take the shot and it saves you having to retake.

1. Start by taking a photograph of a piece of white paper again (not on Manual mode - use an Auto mode like in the first lesson). The camera will assume it is grey, so the histogram should look something like this:

histogram 8.png

2. You know the paper is white, so you know the histogram should look like this:

histogram 9.png

3. Use exposure compensation or Manual mode to increase the exposure until the histogram moves all the way to the right. You can watch it move in the viewfinder, if you have it switched on.

4. Now try other subjects, indoors and out, and just pay attention to what the histogram is doing compared to what you think it should be doing.


RECAP

1. Read this lesson. Download the pdf.

2. Remember that you don't need to understand histograms at this stage of the course. Like shooting on Manual mode, it's enough at this stage that you simply know that they exist.

3. Work out how to show the histogram on your camera. On most cameras you there is an option to have it showing in the viewfinder before you take the photograph (on some cameras you need to have Live View on to see the histogram pre-capture). You can see it changing as you move the camera. On a few cameras (some Nikons) you can only see it once you've taken the photograph.

4. Have a go at the homework.

5. Take a deep breath and look forward to the catchup week next week.

6. If you want to stretch yourself a bit further after making it this far, the workbooks have extra projects to try at the end of every module. More information about the workbooks.

7. If you want to share in the app, find the “Homework” section via the left hand menu and post in a thread. If you are working along at the same pace as an official start group (eg. July 2019), find the thread for your start group. If you are behind, you can post in the current week’s thread but just say which homework you were working on. If you are dipping in and out but not working at an official pace, find any thread that is currently on the Exposure topic and post there.

8. This is the last time I’ll remind you about the 50% discount option on the first 4 modules’ video lessons. Click here to claim the discount.