Lesson 5: Exposure, Buckets, and the Histogram

Chelsea London © 2019. Fujifilm X-T1 | 35.0 mm | ƒ/3.6 | 1/250 | ISO 200 - Histogram showing clipped shadows.

Introduction to Exposure

After getting an overview of what a camera is, how focal length works and what kind of image files we can work with we’re now going to look at one of the most fundamental concepts of any photograph: exposure.

In order to keep things (relatively) short, we will split this vast subject into smaller, easier to digest pieces. In this lesson, we will see what exactly exposure is, and how we can use three camera settings to modify it. We will talk about a very important tool for reviewing exposure: the histogram. In subsequent lessons, we will talk about each of the three controls (shutter speed, aperture and ISO) in more detail.

Sean Makin © 2017. Nikon D610 | 86.0 mm | ƒ/11 | 1/80s | ISO 100 - Histogram showing bright highlights.

A photograph, as the name suggests, is a record of light. Exposure is simply the amount of light to which the sensor is exposed. I’m sure you’ve seen photos taken indoors without a flash, and found them to appear too dark. Those photos are underexposed - not enough light was allowed on the sensor. You probably have also seen images that are too bright, with pure white in large areas. Those are overexposed.

There is not one correct exposure of a given scene. Depending on what you are trying to say with your image, you might actually over or underexpose on purpose. For instance, a scene where the main source of light is behind the subject could be underexposed to create silhouettes against the sky. Or a portrait might be carefully overexposed to create a bright and playful feeling. What we will generally consider a good exposure is one with an even (but not necessarily linear) distribution of lights and darks - from pure black to pure white - with no details lost to either shadows or highlights.

Chelsea London © 2018. Fujifilm X-T10 | 56 mm | ƒ/2.0 | 1/800s | ISO 320 - Histogram showing an even exposure.

Modern camera bodies include one or several light meters whose role is to measure the quantity of light and give an idea of what the correct exposure should be. What you will do with this information will depend on the shooting mode you are using. In auto, the camera will choose all the settings for you. Semi-automatic modes (aperture or speed priority modes) will allow you to choose one setting, and then set the remaining settings for you. You can also choose all the settings for yourself using manual mode. Regardless, the light meter on your camera will tell you what the “good” exposure is.

Three parameters control the quantity of light to reach the sensor: aperture, shutter speed and ISO. Let’s see briefly how they work with an analogy.

Imagine that your sensor is a bucket. Light is water coming from a pipe (your lens) into the bucket. What you want to achieve is a good exposure – just the right quantity of water - to the rim but without spilling any on the floor. You can achieve that by doing three things:

You can change the diameter of the pipe. The wider it is, the more water will come into the bucket.
You can modify the time during which the pipe is open, the longer you leave it open, the more water will come through.
The water is not very pure. There is a filter above the bucket to remove impurities. You can decide how fine the filter is: the coarser it is, the more water will go through, but at the price of more impurities making their way into the bucket.

You can decide to modify any of these parameters as you wish to achieve your perfect bucket, with some limitations. You can’t have a pipe of infinite diameter, there is a maximum size. Likewise, your filter can’t be too coarse or you might get trash in the bucket and it would be unusable.

Important: all three parameters are bound together. If you modify one and want to keep the same exposure, you need to modify anothern. If you want to use a pipe with twice as much area (doubling the flow), you need to either cut the flow duration by half or use a twice as fine filter. Modifying a single parameter will result in a modification of the bucket’s content.

As you probably guessed already, the diameter of the pipe corresponds to the aperture. The duration to the shutter speed. The filter to ISO. Things get even more interesting because each of these parameters has another consequence besides modifying exposure. Aperture changes depth of field, shutter speed can introduce motion blur and ISO (can) influence the noise levels.

For the analytical:

Let’s look at some numbers that your camera might display when it is metering a hypothetical scene. When you put your camera in manual mode, you should see three numbers in the display; for instance f/8, 1/50, ISO 400. What this is telling you is that the aperture is f/8, the shutter speed 1/50th of a second, and the ISO is 400. What other properties these numbers affect will be covered in the following lessons. But for now, let’s take a look at how modifying them changes exposure.