Photography: Learning the basics of exposure control

In the learning process of photography, one of the first things you need to know are the exposure controls in a camera because it controls how a photograph you take turns out.

The Exposure triangle

Exposure is how much light per unit area is reaching the plane surface of an image sensor i.e. the amount of light the image sensor is being exposed to.

Exposure of an image depends on four elements viz. ISO, aperture, shutter speed and luminescence of the scene. And out of these four elements, most of the time only 3 can be controlled (unless you're in a studio 🤷) viz. ISO, Aperture and Shutter Speed. These three elements constitute what is known in photography as the Exposure triangle

In my previous blog post, I mentioned ISO, aperture and a little bit about shutter speed. This blog post will further elaborate on how they specifically affect a photo.

1. ISO

As mentioned before, ISO is the sensitivity or receptivity of the image sensor of the camera to light hitting on its surface.

ISO set at 800

What does it affect?

Increasing the ISO will brighten the image, but also comes with a drawback of increasing image noise thus, resulting in a less clean looking image. Modern cameras can handle high levels of ISO without too much noticeable noise. But it's always good to keep the ISO as low as possible, and adjust the exposure by controlling the aperture or shutter speed.

2. Aperture

Aperture is the opening in a camera lens that lets light in and hit the sensor. The opening is mechanically controlled by aperture blades that widen or narrow the hole according to the setting.

Aperture set at F4

It is expressed in "F" values, such as F1.8, F2.8, F4, F8 and so on. New camera users often get confused on the F-values of aperture. If the value is lower, then the aperture is wider, and if it is of a higher value, then the aperture is narrower.

F1.2 vs F8

How does it affect?

A wide aperture setting will allow more exposure while simultaneously decreasing the depth of field, and vice versa for a narrow aperture setting. Depth of field is the distance between the nearest and the farthest objects in sharp focus in an image.

So, if the aperture is wide open, it will isolate the subject in focus from the background or foreground by blurring them out, thus creating what is known as "Bokeh" effect. This setting is commonly seen in portrait photography.

Now, a narrower setting will decrease exposure, while at the same time, increasing the depth of field i.e. increasing the number of objects in sharp focus. This setting is common in landscape photography when everything within the frame should be in focus.

Focal length affecting the depth of field

The focal length of a lens will affect the depth of field in a photograph. For example, when a photo is taken with a 200 mm lens at f2.8, it will produce a much shallower depth of field (more background or foreground blur) than a 16 mm lens at the same aperture focused at the same distance.

3. Shutter speed

A camera shutter is a part of a camera that allows light to pass for a determined period of time by opening and closing a mechanical shutter (or electronic in modern mirrorless cameras). Shutter speed is the time period between the opening and closing of the shutter mechanism.

Shutter speed set at 1/250 sec.

What does it affect?

A fast shutter speed allows less light to pass through while making the camera able to capture fast moving objects without motion blur, and a slower speed allows more light in while motion blur also sets in for fast moving subjects or camera shake when a tripod is not used.

Shutter speed affecting how a moving object is photographed

But it doesn't mean you always have to take every photo at high shutter speeds. Slower shutter speeds, such as 10 sec., 15 sec., 20 sec. etc. can be used to take artistic shots such as light trail photos and astrophotography (I'll post a blog about long exposure photography another day).

Photo of  light trails by Javier García on Unsplash

Astrophotography by Thomas Ciszewski on Unsplash

Exposure meter

Now, the above mentioned elements must be used to balance what is known as an exposure meter in the camera. An exposure meter shows how well the image is exposed on to the sensor.

In the picture of the exposure meter above, a small black rectangle can be seen beneath the "0" in the middle of the scale, that represents the current exposure of the image. If that indicator is seen on the right side, the image will be over exposed. If it's seen on the left side, the image will be under exposed. So, in most situations, it is necessary to keep it in the middle or near it for a correct exposure by either increasing or decreasing the ISO, Aperture or Shutter speed.

The distance between the numbers seen in the exposure meter is called a stop of light. So, if an image is under or over exposed by a stop of light (or more), the same unit should be compensated by increasing or decreasing either of the ISO, Aperture or Shutter speed settings.

What happens when an image is not properly exposed?

When an image is under or over exposed, it will be either too dark or too bright respectively. The dark areas or highlights in an image will be clipped i.e. will lose detail in those areas and will not be able to get recovered in the editing process.

But again, it's not necessary to always go for the correct exposure according to the meter. In mirrorless cameras, the live exposure preview can be seen in the viewfinder. So, by looking at the image and judging the lighting situation and the scene, the image can be under or over exposed to achieve great creative photos. DSLRs can also use the live view mode to see how well the image is exposed, but just not in their viewfinders.

          

         Over exposed sky to in a photo by
         Joshua Fuller on Unsplash

Underexposed building for making a silhouette in a
photo by ZEKERIYA SEN on Unsplash

   

How can these settings be controlled?

In auto mode, the camera automatically adjusts the exposure settings depending on the lighting situation of the scene but taking photos in auto mode almost always result in photos looking like ordinary snapshots. So, its a good thing to know how to control the exposure settings manually to take photos according to what the photographer wants. Even modern smartphones also allow manual exposure control.

These settings can be accessed by changing the mode dial on a camera to either Aperture priority, Shutter priority, Program or Manual mode.

Manual mode

In manual mode, all the three elements of exposure i.e. ISO, aperture and shutter speed can be controlled manually and individually to correctly expose the image. Taking photos in manual mode requires a lot of practice and experience to be perfected, but gives access to full control on how the image turns out. The manual mode in the mode dial is represented by an "M".

Aperture priority mode

Aperture priority mode only allows you to control the aperture of the lens. The shutter speed is automatically controlled by the camera, and the ISO can be set to a preferred level or can also be set to auto. The aperture priority in the mode dial is represented differently in different brands of camera manufacturer. For example, in Canon it is represented by "Av" (Aperture Value), in Nikon, Sony, Fujifilm and Lumix, it is represented by an "A".

Shutter priority

Shutter priority is the mode that allows the user to control the shutter speed, while the aperture is automatically adjusted by the camera,  and the ISO can either be manually set or automatically controlled by the camera. The shutter priority mode is represented by "Tv" (Time Value) in Canon cameras, while it is represented by an "S" in other brands like Sony, Nikon, Fujifilm and Lumix.

Program mode

Program mode or program auto mode is the mode where the shutter speed and aperture is automatically set by the camera but also allows the user to change the settings while keeping the exposure level same. The program mode is represented on the mode dial by a "P".

After choosing one of these modes, the values of these settings can be increased or decreased by using the dials usually located around the shutter button or at the back of the camera.

The exposure histogram

Along with the exposure meter, there is also a histogram that indicates how well the image is exposed. It is the graphical representation that shows the amount of pixels in a image at each exposure level. A live histogram can be seen when taking photographs in a camera that shows real-time exposure levels of the photo, and can also be seen in photo editing softwares.

Now, it is necessary to learn how to read the histogram of a photo to check how much of the image is exposed correctly and how much is under or over-exposed.

The histogram below shows the areas where the pixels in a photograph are located in the different exposure levels:

Histogram of the bird photo I took in my first blog post

To read a histogram, all you need to know are the two axes that represents the number or amount of pixels in different exposure levels; the vertical axis or x-axis represents the number of pixels, and the horizontal axis or y-axis represents the exposure levels.

In the horizontal axis, it can be seen that lesser value or nearer to the left means darker the image, and more value or nearer to the right means a brighter image.

So, if there are more bright areas in a photo, there will be a higher number of pixels in the right side (the light side), and for darker photos, more pixels will be located at the left side (the dark side).

While setting the exposure, it is important to keep the histogram from getting over or under exposed by not letting the pixels represented there touch the left or right most bars which represent black and white areas respectively.

If a photo is taken with that kind of histogram, it will be impossible to recover the details from the white or black areas as all the details will be lost. This phenomenon of losing details in black and white areas in a photo is called "clipping" or "exposure clipping".

A few common examples of different shapes of exposure histogram are shown below:

This YouTube video by Sean Tucker explains how to master the exposure triangle with demonstrations:

If you read this far, it may seem overwhelming at first, but remember, there are only 3 elements controlling the exposure in the camera; ISO, Aperture and Shutter speed.

Master these settings with practice and you'll be taking correctly exposed photos soon.

This concludes the blog post. Thank you for reading.