A beginners explanation of the key elements of a camera; how the relationship between shutter speed and aperture size determine exposure, and the relationship between stops and exposure value (EV).

For pretty much any photographic image, there are two features it must have. Firstly it must be in focus, at least somewhere in the image, and secondly, it must be correctly exposed.  Okay, okay, I know there are lots of ‘arty’ images out there that don’t conform to either, but for most of us in the real world, those are the rules.  I’m going to leave focus aside for the moment and concentrate on exposure: what we mean by this, how we achieve it, how we control it and modify it.  Now of course, if you leave your camera on auto, it will do a pretty decent job for you.  But that’s not very satisfying, and it’s not very creative.  It’s a bit like buying a painting; it may look great on you wall but it doesn’t make you feel like a painter.  So if we want to be photographers, not just a ‘guy (or girl) with a camera’ pressing the shutter button, then we need to have some understanding of what is actually going on inside the camera in order to take the ‘autopilot’ off, and take control ourselves.  The second consideration is, if you want to produce a standard image with textbook ‘correct’ exposure then leave it on auto and your camera will do that for you, at least most of the time. However, if you want to create something more interesting, more individual, then you need more control over how the camera captures the image, and that requires you to take the wheel yourself.

One of the biggest problems nowadays, for people new to photography, is that cameras are so damn sophisticated.  There are so many controls, settings, menus and sub-menus that it can feel like learning to drive in a fighter jet.  Just where do you start to take control?  in order to help understanding, let’s pare back our camera to its simplest form.  Every camera, whether it is the latest top-of-the-range, full-frame, mirrorless, or an 1839 Daguerreotype, is fundamentally a light-tight box with a hole (the aperture) on one side to allow some light in, and some light sensitive material on the opposite side, and a means of blocking the from light reaching the light sensitive material (the shutter) .

The light-path between the aperture and the light sensitive material is periodically unblocked (the shutter opened) allowing light to reach reach the light sensitive material. This material is then altered in some way by the light, and this begins the process of creating an image. What exactly that material is, has changed many times.  The 19th century Daguerreotype used copper plate coated with silver; early 20th century cameras mostly used glass plates coated with silver salts; then of course film took over; and now we have solid-state sensors that convert light in to electrical signals, but the the basic design is exactly the same.

So we talk about ‘exposing’ the plate, or film, or sensor, to light in order to create an image.  I’ll stick with sensors from this point, as that is what we use now. The amount of light hitting the sensor determines the lightness of the image.  The more light hitting the sensor, the lighter the images, the less light, the darker the image.  We talk about images being correctly, under- or overexposed.  By underexposed we mean that the mid-tones are too dark and we start to lose detail in the shadows.

Underexposed image of a tokay gecko on a wall

By overexposed the mid-tones are too bright and we start to lose detail in the highlights.

Overexposed image of a tokay gecko on wall

So we control the degree of exposure primarily by controlling the amount of light hitting the sensor, and we do this in two ways.  We can vary the length of time that the sensor is exposed to light (i.e. the duration the shutter is open) and we can vary the size of the aperture allowing light to reach the sensor.  So let’s look at those mechanisms in turn.

Shutter speed

The shutter is a pair of metal curtains, located in front of the sensor.  They act to block light passing through the aperture from reaching the sensor, except for the period the shutter is opened.  This is the same mechanism used in film cameras. However, today most mirrorless cameras, and some DSLRs, have an electronic shutter, where the sensor is switched on and off to produce a similar effect.

The duration the shutter is open is known as the shutter speed.  There are a range standard shutter speeds we see on most modern cameras, e.g. 1 second, 1/2 second, 1/4 second, 1/8 second, 1/15 second, 1/30 second, 1/60 second, 125 second, 1/250 second, and so on.  You will notice that each is (with minor exceptions) half the duration of the previous one.  Half a second allows in half as much light as a one second shutter speed; 1/125 allows in twice as much light as 1/250 of second. So changing from one ‘standard’ shutter speed to the next nearest either halves or doubles the amount of light hitting the sensor, depending on the direction of change.  Having or doubling the amount of light reaching the sensor is known as changing it by one stop.  Understanding the concept of stops (and exposure equivalent values, which I will explain further on) is fundamental to photography.

Aperture size

We also have a second means of regulating the amount of light reaching the sensor; we can change this by changing the size of the aperture. If we make the aperture twice as large, then twice as much light will hit the sensor (if the shutter is open for the same duration) if we make it half as large it will allow half as much light to reach the sensor.  This is preety self evident, I’m sure.  And this modifying the aperture by halving or doubling is known as changing it by …. one stop. Whilst the shutter speed values are pretty intuitive, aperture values are not.  They are known as f stops, and commonly values range between f2.8 and f22 (sometimes f1.4 – f32).  These f stops also change in standard increments, but at first glance they are quite meaningless: f2.8, f4, f5.6, f8, f11, f16, f22.  Even more puzzling, the largest number denotes the smallest aperture.  The simplified answer for this is that the f value is not a physical measure of the diameter of the aperture, it is a ratio between the focal length of the lens and the aperture: f value = focal length/aperture diameter.  From a practical stance, none of this theory matters to the photographer.  What does matter is knowing that each f stop allows in half or twice as much light the next full stop to the left or right, and that the smaller the number, the greater the aperture diameter, the greater the amount of light reaching the sensor.  Most modern cameras work on a click-stop principle.

A modern lens showing the aperture wide open

A lens showing the aperture stopped down to smallest size

So that the size of the aperture, rather than smoothly variable from smallest to largest, changes in a series of steps.  So when we change aperture on our camera, either by rotating a ring on the lens or, more commonly nowadays, a dial on the camera body, we feel a series or positive clicks.  These steps correspond to the f stop values listed above, plus (on modern cameras) 1/3 of a stop intervals.  So, for example, on our camera we might find f values of F2.8, f3.2, f3.5, F4, f4.5, f5, f5.6. The figures in bold are full stops, the figures in lighter text are 1/3 and 2/3s of a stop up or down.  The beautiful symmetry of the stop system is this.  Say for example, our image is correctly exposed at a combination of shutter speed 1/250th and aperture size f8.  If nothing else changes, then we know that if we change the aperture to f11 (one stop smaller) then the image will be underexposed by one stop. Equally, if we change the shutter speed to 1/500th, the image will be underexposed by one stop.  If we change the shutter speed to 1/500, and simultaneously change the aperture to f5.6, then the exposure stays exactly the same, because the former acts to half the amount of light reaching the sensor, and the latter acts to double the amount of light, thus the combined effect is that the total amount of light reaching the sensor is exactly the same.  The obvious questions are: why have two separate mechanisms to achieve the same control over the amount of light, and why would one alter two controls in order to produce exactly the same exposure.  There are several considerations here, and to detail them all would distract from the main purpose of this blog, so I’ll most of those for a future blog.  However, a key consideration is motion blur. If we have an aperture of fixed size, we can still achieve correct exposure by changing the shutter duration; this is in fact, exactly how a pinhole camera works.  The problem comes with photographing moving objects. The faster a subject is moving, the faster the shutter speed needs to be in order to freeze the motion and stop the subject appearing blurred.  This explains how, in some pinhole camera photographs, streets can appear empty of people or vehicles. because the aperture is so tiny, the shutter speeds often need to be very long (many seconds).  Fast moving objects move across the aperture too fast to register on the image. But even modern DSLRs, if we set the shutter speed too slow, then a fast moving object (e.g. a bird in flight or a sports player) becomes blurred in the image produced.

Exposure Values

The final concept I want to talk about here is Exposure Value (EV or sometimes Ev). As explained above, different combinations of shutter speeds and aperture sizes will still produce the same image exposure. Exposure values assign one value to all the combinations that produce the same exposure.  In practical terms, exposure values are mostly interchangeable with stops.  Underexposing an image by one EV is the same as underexposing by one stop.  We tend to refer to stops when taking about shutter speed of aperture.  When we want to vary exposure in automatic or semi-automatic modes (not something I’m covering here) then we start to refer to EV and vary it using the Exposure Compensation button; but that’s all for another blog.

What I’ve missed out

In order to keep things fairly simple, I haven’t talked at all about the third factor in what is often called the exposure triangle, namely ISO.  That will be the subject of another blog soon.