Taking Away the Mystery of Exposure

You are an artist.  You already know what you want to shoot.  Maybe it’s the creamy background and a sharp model.  Maybe landscape photography is your thing, and you’re looking to get the best from your lens.

Knowing how exposure works is the bridge to using your camera artistically.  By the end of this article, you’ll feel comfortable leaving Auto Mode behind and trying Aperture Priority, Shutter Priority and Full Manual modes.

You’ll know what Depth of Field is and how to ensure your subject is sharp. In addition, you’ll learn how to de-focus the background and avoid blowing highlights, among other common photographic techniques.

Understanding sensors helps us understand Exposure.

Inside your camera is a light sensor.  Whether you shoot with a compact camera or a DSLR, a sensor is responsible for converting light into a photo.

Light –> Lens –> Sensor –> Photo

Sony Sensor
Image of a Sony Sensor from https://www.sony.com/electronics/support/articles/S1F0528

Sensors are more or less rectangular and vary in size depending on the class of your camera.  But that’s not important right now.

What IS important to know is that sensors produce electrical signals depending on how strong the light is.  The stronger the light, the “stronger” the electrical signal.  This electrical signal is converted into a number to represent the strength of light hitting the sensor.

Sensors can measure light in millions of different locations along its surface.  We call these areas Pixels, and a photo is made up of all these readings.  That’s it.  Photos are nothing more than a collection of millions of sensor readings.

The designer of the sensor determines the sensitivity of your sensor.  Some sensors are better at detecting light than others, and there are many factors in determining how sensitive your sensor is.

For example, larger sensors tend to be better at taking photos in lower light because their size advantage allows them to have larger pixels – or areas over which to collect and measure light.

For your sensor to produce an image, it needs to be exposed to light for a certain amount of time. This is because it takes time to build up enough charge within the sensor’s electronics to produce useable readings. Therefore, the longer the light hits the sensor, the more charge builds up and the better the conversion from an electrical charge to a number.

However, If you expose your sensor to light for too long, the electrical charge will saturate and give the camera the same reading even if the light gets brighter. This is because the large electrical charge can’t be stored in its entirety.  This is called highlight clipping – the sensor has lost information in the bright areas, and your photo editor software will show this in the histogram as a pile-up against the right of the graph.

Skull Picture with Blown Highlights
Skull Picture with Blown Highlights

On the other hand, it’s also possible to clip black levels.  If your sensor isn’t exposed for long enough, there won’t be a sufficient build-up of electrical charge, and your sensor will interpret that as completely black, i.e., no information. So even if your eyes can see it, the sensor may need a longer time to expose a scene properly.

Bayer Array image from Wikipedia
Bayer Array on Wikipedia https://en.wikipedia.org/wiki/Bayer_filter

Your sensor measures light in black and white.  It’s true.  Your sensor has no idea what a colour is.  It simply reads brightness levels and never thinks about colour. Instead, manufacturers place a grid of coloured filters over the sensor’s pixels so that each pixel location receives one colour of light.  Either red, green or blue.  So, when the brightness level is read from the sensor at any pixel location, we can determine the colour because we know what colour the filter is above that pixel.

The most commonly used colour filter is the Bayer filter. It’s a checkerboard of red, green and blue with a heavy emphasis on green. 

The camera or your post-processing software can reconstruct the image by combining and interpolating pixels from the sensor’s readings, using the Bayer filter’s colour arrangement to estimate the best colour for each pixel in the final image.

Luckily we don’t need to know much about this process except how our exposure is affected.

Since the Bayer filter allows three different colours, it’s helpful to think about our image in reds, greens, and blues individually.  And it’s also useful to know that each of these colour channels can clip independently of each other.  In other words, as you approach over or underexposure, you may experience colour shift while one channel clips and the others don’t.

The Exposure Triangle

Copyright UnknownTo take a properly exposed photo requires your camera to balance three variables.

  • Shutter Time
  • ISO
  • Aperture Size

Shutter Time is the amount of time your shutter spends open while exposing your sensor to light.

ISO is your sensor’s sensitivity to light. Commonly used ISOs are ISO100, 200, 400, 800, 1600, 3200, 6400 etc.  Lower ISOs are less sensitive than higher ones.  ISO400 is twice as sensitive as ISO200.  ISO1600 is twice as sensitive as ISO800.  And so on.

An Aperture is an adjustable hole within the lens through which light passes into the camera, striking your sensor or film.  The aperture can be made larger or smaller to allow larger or smaller amounts of light to strike the sensor.

Exposure Explained

If you would like to know how your camera calculates a proper exposure, or if you’re willing to take your camera off of Auto Mode, then you need a solid grasp on the three variables, Shutter Speed, Aperture and ISO and their relationship to each other.  When you change one variable, it requires you to adjust another to keep exposure balanced. But, don’t worry; it’s not difficult.  Here is how your camera achieves proper exposure.

Shutter

House with a shutter .Stock Photo. Copyright Unknown.

Each time you take a photo, your camera has to decide how long to expose your sensor to the light entering your camera.  The length of time your camera exposes its sensor to light is called the shutter time or exposure time.  Holding your shutter open for the correct amount of time leads to a well-exposed photo. Conversely, leaving your shutter open too long leads to a bright photo, and too short leads to a dark photo.

In some cameras, such as DSLRs, the shutter is a physical gate that moves out of the sensor’s light path to begin exposure.  Once the exposure is done, the shutter returns to its resting position, blocking light from reaching the sensor.  Other cameras don’t have a physical shutter. Instead, they initialize the sensor at exposure time and read the data from the sensor at the end of the shutter time.

Canadian Forces Chinook Helicopter
A fast shutter time freezes this helicopter’s blades – Taken at Canadian Forces Base Borden, Canada.

When shutter times are below 1 second, they are written as fractions.  So, 1/60th of a second is about twice as long as 1/125th of a second.  A shutter time of 1/500 is five times faster than 1/100.  1/1000 is very fast, but 1/10 is very slow.

I need to talk a little about terminology to keep confusion to a minimum.

Shutter time is the amount of time you leave the shutter open to take a photo.  When I use the term “increase shutter time,” it means to increase the amount of time the shutter is open.  For example, I can increase my shutter time by changing from 1/500 to 1/60th of a second.

Shutter Speed is also used to specify the amount of time you leave the shutter open to take a photo.  However, when I say “I’m increasing my shutter speed,” it means to shorten the amount of time my shutter is open for a photo. Increasing shutter speed means decreasing the amount of time it’s open for.  I can increase my shutter speed by changing from 1/60 to 1/125th of a second, for example.

Effect of Shutter Speed Choice

Squirrel motion blur
A long shutter speed shows this squirrel’s movement – Taken on a ladder in my driveway.

Choosing a long shutter time allows your subject to move during the exposure.  You can use a long shutter time in creative ways to lengthen star trails, smoothen waterfalls and show movement. Experiment with shutter speeds of 1 second to 20 seconds to really show movement.  Of course, you’ll need a tripod.

Shorter shutter times are important for reducing camera shake or capturing action. 

If you are hand-holding your camera, your body’s shakiness will translate into a blurred image unless you choose the proper shutter speed.

The general rule of thumb for choosing a shutter speed:

Shutter Speed is faster than   1 / focal length

For example, hand-holding a camera while shooting with a 200mm lens requires a shutter speed of 1/200th of a second or faster.

Of course, this doesn’t take into consideration optical stabilization, known as OS.  Pentax bodies have OS built right into their sensor array, and many companies such as Sigma, Canon, and Nikon have OS built into their “Pro” lenses.  When using stabilization, you can reduce the shutter speed by 2 or 3 times without worrying about blur induced by your arm’s movements. Of course, OS doesn’t prevent action from blurring,  but you will not see your body’s jerky movements in your photos.

ISO

ISO is an inaccurate label we use for describing the sensor’s sensitivity.  Since our sensors are electronic, we can tell the sensor to amplify its light readings, simulating a more sensitive sensor.  Therefore, we can change ISO at will.

All camera sensors have the ability to change their ISO.  Becoming more sensitive to light means a brighter photo, and less sensitive means a darker photo.

Film shooters will remember choosing film based on the shoot location.  ISO100 for daylight and ISO1600 for indoors.  ISO1600 is “faster” than ISO100, making it more suitable for indoor shooting.

Digital camera shooters have the luxury of turning a dial to adjust their sensor’s ISO.  Your camera likely allows you to adjust ISO whether you have a DSLR, Mirrorless or Compact.

Many camera sensors use ISO 100, 200, 400, 800, 1600, 3200, 6400 and higher.   Using a higher ISO lets you shoot in darker locations, such as indoors.

Increasing ISO 100 to 200 doubles the sensor’s light sensitivity. This lets you increase your shutter speed by double.

Using a higher ISO and a more sensitive sensor can reduce the amount of time your shutter is open.  You can do this because the sensor simulates gathering light more quickly and is essentially done the exposure sooner.

Pro Tip: use the lowest ISO setting that you can get away with.  Raising ISO may be a tempting way to freeze the action and eliminate camera shake, but it will introduce noise into your photos.  Lightroom and others have great noise-removal options that should help clean up your photos where high ISO is unavoidable.  Each camera is unique and will have a usable ISO range.  My Pentax K5 is usable from ISO80 to ISO6400, and for an impossible shot, I would push it to its max of ISO128,000.

Effect of ISO Choice

Noisy Image of a Skull
Noisy Image of a Skull

Raising your ISO adds noise to your photo.  Most modern DSLRs can produce “clean” photos from ISO80 to ISO6400 or higher.  But, if you keep raising ISO, the noise will inevitably creep into your photos.

Lightroom and others have the ability to remove a fair bit of noise from your photo, so shooting at ISO6400 and higher isn’t the death sentence of a photo anymore.

If you need to stop action and are comfortable with noise in your photo, raise your ISO until your shutter speed is fast enough to capture the action.

Aperture

2 Cardinals on a fence
Two "models" in my backyard demonstrate how narrow depth of field is with 200mm@f/4.5

So far, I spoke entirely about time.  Specifically, the amount of time to hold open the shutter and the effect of ISO on that time.

I left the Aperture Topic until last because, in my mind, it’s one of the most creative settings on your camera, and it’s the most technical. So read what you need and ignore the rest – it’s not all required to take great pictures, but learning the theory helps you take better pictures in challenging situations and gives you a solid base on which to be creative.

Left, 50mm lens set to f/8 Right, 50mm lens set to f/2.8

An aperture is nothing more than an adjustable hole somewhere in your lens, through which light passes before entering your camera, and striking your sensor.

The aperture is made of several individual blades overlapping in a circular pattern.  When the blades are adjusted, the aperture becomes larger or smaller, depending on your needs.  If you want more light, we open up the aperture.  If we want less light, we decrease its size.

The aperture diameter is measured in f-stops, a century-old optics system for describing the size of a hole.  Here are the most common f-stop numbers:

Common f-Stops

1         1.4         2         2.8         4         5.6         8        11        16        22        32

Diameter = Focal Length / f-stop

Aperture is usually expressed as a fraction of its focal length, i.e., f/2.8 or f/11.  There is a mathematical reason for this, which I’ll cover below.

Your camera and lens combination may have f-stops listed between these, but these are the standard f-stops recognized by photographers across the world.

Each f-stop represents the diameter of your camera’s aperture.  This particular f-stop chart starts at 1, where the aperture is the largest and progresses to 32, where the aperture is the smallest.  

Each f-stop on this chart represents an aperture with an area that is half as large as the f-stop to the left.  Each f-stop is also twice as large as the f-stop to the right.

Looking at the f-stop list, we can see that you can change your lens aperture from f/5.6 to f/4 to double the light entering your camera.  How does this affect shutter speed?  Well, as you’ve already guessed, your shutter speed can be faster now since you’re letting in more light.  How much faster?  Twice as fast.  If your shutter speed at f/5.6 is 1/500th of a second, your new shutter speed would be 1/1000th of a second at f/4. 

Effect of Aperture Choice

Depth of Field

Algonquin Park - 2010 - Pierce
Narrow Depth of Field with a Tamron 300mm f/2.8. (C) Andrew Shirley

If you could measure your photo’s sharpness, front to back, you would find it blurry at the front, sharp at the focal point and blurry toward the back.  This is a natural physical phenomenon known as Depth of Field, DOF.  It’s what lets you take gorgeous tack-sharp photos of models with beautiful creamy blurred backgrounds.

The size of your aperture has a huge effect on DOF. For example, a large aperture like f/1.4 will have a very narrow depth of field compared to a small aperture like f/11.  Use a large aperture to blur the background. Do you want maximum detail front to back?  Use a smaller aperture such as f/11 or f/16 to maximize depth of field.

Sharpness

Aperture choice affects the sharpness of your photos.  Very few lenses are sharpest at their maximum aperture.  For example, a 50mm f/1.4 lens will take a fine picture at f/1.4 with a narrow DOF, but stepping the lens down to f/2.8 or f/4 will sharpen up the lens considerably.

The same thing happens on the smaller side of the aperture.  As your aperture becomes smaller and smaller, light becomes susceptible to diffraction, where you begin to lose sharpness. Therefore, it’s always best to avoid your largest AND smallest apertures for maximum sharpness.   If sharpness is your primary factor, then you should be shooting between f/4 and f/11 for most lenses.  Of course, your mileage may vary.

Chromatic Aberration

Shooting wide-open (at your widest aperture) can also result in colourful errors in your photos called chromatic aberration.  Near the edges of your photos, you may see purple or green lines where your lens design breaks down and colours don’t line up correctly at your sensor.  It seems modern designs and lens coatings have corrected much of this, and your post-editing software likely has an option to reduce or eliminate this annoyance.  Chromatic Aberration isn’t as serious as it once was in Film since we can digitally edit our photos now. Still, ironically enough, we seem to have better lenses now than when we really needed them.

Vignetting

Another problem with shooting wide-open is vignetting near the corners of the photo.  Vignetting is a darkening of the photo, and it becomes more noticeable when shooting wide-open.  Personally, I like a little vignetting in many of my photos – but I choose to put them there in post-processing. If you need to clean up vignetting, it’s not difficult in Lightroom or other software that allows you to use a radial graduation filter to brighten the corners of your photo.

For the Math Geeks

We could do the calculation to see the actual opening size of the aperture if you knew the focal length of your lens.  For example:

A 200mm lens at f/2.8 has an aperture with a diameter of

200mm/2.8 = 71.42mm

A 200mm lens at f/5.6 has an aperture size of

200mm/5.6 = 35.71mm

And since we know Area = Pi * (1/2D) ^ 2

A1 = Pi * (1/2 * 71.24) = 3992.34

A2 = Pi * (1/2 * 35.71) = 1001.28

Comparing A1 and A2, we see the area of the aperture at f/2.8 is 4 times larger than the aperture at f/5.6.

This makes sense – remember the f-stops go like this:

1         1.4         2         2.8         4         5.6         8        11        16        22        32

Moving our f-stop from 5.6 to 4 doubles our aperture’s area.  Moving from 4 to 2.8 doubles it again, giving our aperture a total of four times the area.

This means we can increase our shutter speed by four times to achieve the same exposure when we move from f/5.6 to f/2.8.  Remember, increasing shutter speed means making it quicker.  In our example, you can change your shutter speed from 1/100 to 1/400.

PASM – Taking Your Camera Off Auto

It’s effortless to take a photo – you point, click and let the camera choose the appropriate aperture, shutter and ISO.  Most cameras have semi-auto modes that let you control one aspect of the exposure triangle while the camera chooses the other two.  Those modes are known as Aperture and Shutter priority modes.  In addition, cameras usually have a Program and Manual mode.  Some cameras use slightly different labels but mean the same thing.  For example, this Pentax body has Shutter Priority labelled  Tv mode – short for Time Value.  It also labels Aperture Priority mode as Av, meaning, Aperture Value.

My Pentax K10D

Program mode is a little different on each camera.  But, it lets you set up your camera the way you like it and recall those settings simply by invoking Program mode.  In that regard, Program Mode is like a preset. Again, each camera is different, and the Program mode on a Nikon is different from Pentax. Your camera’s manual will have a full explanation.  You can easily set up Program mode for certain types of shooting and recall those settings when it’s time. For example, shooting sports would favour higher shutter speeds, whereas portrait mode would favour wider apertures. Landscape settings may need a tiny aperture to maximize the depth of field.

Placing your camera in Aperture mode (Av) will let you choose an aperture. Your camera will choose the appropriate shutter speed and ISO for a properly exposed photo.

Using Shutter mode (Tv) will let you choose a shutter speed while the camera chooses the aperture and ISO.

The last mode I will mention is Manual mode, which, as the name suggests, allows you to choose aperture, shutter and ISO settings. In addition, the camera may display a graph showing you the resulting exposure, making it easy to “tune” your settings, but you are in charge of making any changes.

These modes are very convenient and implemented quite well in modern cameras.  They are there to help you be creative by letting you concentrate on a single variable of the Exposure Triangle while the camera takes care of the other two.

One of my favourite modes is Aperture Mode.  This lets me choose an aperture while the camera worries about things like shutter speed and ISO.  Choosing my aperture lets me keep things sharp and control for the DOF.  It’s a very creative mode, but it’s also great at controlling other aspects of your exposure too.

For example, by adjusting my aperture, I can also control shutter speed since they’re tied together in the triangle.  In Pentax’s Av mode, I can set ISO to a single value (or a range), then have the camera choose the shutter speed as I adjust the aperture.  If I find my shutter speed is too slow, I can open up the aperture or increase ISO.  But controlling aperture ALWAYS leaves me in charge of the creative aspects of my photography.

Exposure Compensation 

My Pentax K10D

Each time you press your shutter button, your camera calculates an exposure (whether in Auto, P, A or S).  The calculated exposure is often correct, but not always.  For example, strong back-lighting can cause severe under-exposure of your subject. However, the camera doesn’t know backlighting is unimportant – it just wants a balanced photo.  And so, with the camera seeing the image looking too bright, it will calculate an exposure to preserve all of those bright details, underexposing your subject in the process.

To compensate for exposure errors or to have more creative control, many cameras have “exposure compensation.”  Pentax bodies have an Exposure Compensation button right next to the shutter button.  You hold down the button while adjusting a dial.

Adding 1-stop of exposure compensation tells your camera to take its automatically calculated exposure and double it.  So, if you are shooting in A mode, the camera will adjust Shutter Speed or ISO to accomplish a 1-stop brighter photo by doubling the shutter time or doubling the ISO, or a combination of both.

F-stop and Aperture Area Table

Below is a table of aperture areas given a focal length and its f-stop number.  You really don’t need to know these numbers, but they show a general consistency where each aperture is about twice as large as the aperture to the right or one-stop up.

Focal Length mm11.422.845.68111622
1000785398.16400713.35196349.54100178.3449087.3925044.5812271.856490.893067.961622.72
500196349.54100178.3449087.3925044.5812271.856261.153067.961622.72766.99405.68
400125663.7164114.1431415.9316028.537853.984007.131963.501038.54490.87259.64
30070685.8336064.2017671.469016.054417.862254.011104.47584.18276.12146.05
27057255.5329212.0014313.887303.003578.471825.75894.62473.19223.65118.30
21034636.0617671.468659.014417.862164.751104.47541.19286.25135.3071.56
20031415.9316028.537853.984007.131963.501001.78490.87259.64122.7264.91
13514313.887303.003578.471825.75894.62456.44223.65118.3055.9129.57
703848.451963.50962.11490.87240.53122.7260.1331.8115.037.95
501963.501001.78490.87250.45122.7262.6130.6816.237.674.06
401256.64641.14314.16160.2978.5440.0719.6310.394.912.60
35962.11490.87240.53122.7260.1330.6815.037.953.761.99
24452.39230.81113.1057.7028.2714.437.073.741.770.93
20314.16160.2978.5440.0719.6310.024.912.601.230.65
16201.06102.5850.2725.6512.576.413.141.660.790.42
12113.1057.7028.2714.437.073.611.770.930.440.23
1078.5440.0719.6310.024.912.501.230.650.310.16

Conclusion

The exposure triangle describes the relationship between Aperture size, Shutter Time and ISO.  Adjusting one of these variables requires adjusting another to keep the exposure balanced.  If you decrease the size of the aperture, you need to increase shutter time or ISO to compensate.  If you increase the size of the aperture, a corresponding decrease in shutter time or ISO is required.

But, it’s an easy balancing act requiring nothing more than basic math. If you can multiply by 2, you can balance the exposure.  Even if you choose to ignore the math, you generally know how to balance exposure, getting you closer with each successive test shot.

The wonderful thing about shooting with a digital camera is that you get an intuitive sense of exposure.  Digital cameras don’t cost anything to take a test photo, so experiment away and try longer or shorter shutter times along with larger or smaller apertures.  You’ll have exposure mastered in no time. 

And most importantly, break all the rules!

If you find anything here useful, I’d like to hear about it in the comments below!