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I have decided to write this primer for two friends who have recently acquired decent small cameras, in both cases the amazingly inexpensive Nikon V1.

It is obvious that camera user’s manuals assume a knowledge of photography that no beginning photographer can ever possess. Accordingly I want to write a series of blogs about what it is that we’re doing when we take a photograph from a camera’s internal technical perspective. At the outset I want to add that photography is a compelling blend of art and technology in the present era. And last year, for the first time, more images were made with phones than cameras—but if you are reading this, I assume you want to make better, or different, images than can be made with a phone, wonderful though these can be. My objective in this series is to help you make the images you want to make, by showing how certain physical constraints (inherent in any camera’s design) can be used consciously.

It will be helpful to think about photography from the perspective of what it used to be like because, despite the plethora of art filters and buttons and dials on modern cameras, the act of making photographs fundamentally still relies on manipulating four variables.

On all film cameras since photography’s beginnings, including the huge, tripod-mounted glass negative slide cameras, there were only two controls, in addition to the lens focussing ring (this latter control tells the camera what you want in focus): the aperture ring on the lens and the shutter speed dial on the camera body (or on the lens in really old cameras). Aperture describes the device that controls the amount of light passing through the lens at any setting, and shutter speed describes the length of the exposure. The combination of the two yields the actual amount of light that gets to the film or sensor. The third variable was the speed of the film one chose to put in the camera body. When I use the term “speed” here, I am talking about the film’s emulsions sensitivity to light. So, for example, Kodak’s old Tri-X black and white film was a high-speed emulsion for the era. Tri-X had a light sensitivity described with an ASA rating of 400 out of the box. And this speed could be altered by the way the film was processed as well. The fourth variable is your framing (composition).

In the modern era, the light sensitivity denoted by the ASA scale (the ASA rating on the side of the film box) has been replaced by the ISO scale. Choosing a film with a speed of 400 ASA is the same as setting your camera’s ISO to 400. In the digital era film has been replaced by a sensor. And the higher ISOs are achieved by simply driving the sensor harder electronically. And the better the camera, the less negative impact making the sensor work harder has. Modern digital technology means having a huge range of film stocks at your disposal all the time.

At its heart a camera is simply a light gathering device. You point the lens at you want to photograph and you press the shutter when the composition or framing is pleasing to you. Let us consider each of the three variables’ effects on the images you make in turn.

In photography, for historical reasons (and for simplicity of balancing either variable’s effects on the final image), the relationship between aperture and shutter speed is this: each number on the aperture ring lets in half the amount of light as the number preceding it (so 2 is half the amount of light as 1.4), and each shutter speed describes the same relationship—so 1/60″ lets in twice the light as 1/125″. Your camera might not have these rings or dials, but the relationship is the same whether you change aperture or shutter speed by dials or other controls.

This relationship is why an exposure of 2 on the lens and a speed of 1/125″ gives the same exposure as 2.8 and 1/60″. “Exposure” means the brightness of the image. This equivalency can be described another way which will become useful below: the doubling and halving of light, whether via shutter speed or aperture control, is described by the term ƒ stop: each full division or number on a lens is a ‘stop’ more or less than the adjacent number; and so on the shutter speed dial: each of the divisions (like 1/60″ and 1/125″) is a stop difference in exposure, addressed immediately below. Modern cameras allow you to select ⅓, or ½, stops as well, for finer control over the final exposure. Changing exposure via aperture or shutter speed is exactly the same as far as the brightness of the final image is concerned, but there is more to an image than ‘correct exposure’.

Correct exposure

If we consider a camera as a light gathering device, then we need to know how much light and the length of time we want that light to shine on the sensor. At the core of this section is the notion of correct exposureAssuming we want the object we photographed to be seen much the same way we saw it, then the amount of light will need to be assessed and controlled. This is what the metering system in the camera is for. These days with a decent camera, we can judge the exposure on the sensor by what the image on the back of the screen looks like. If it looks too dark, then we can add exposure compensation. This means that we can lighten or darken what the camera thought was the ‘correct exposure’ by adding additional light (a ‘positive’ compensation) or by reducing it (‘negative’ compensation).

We do this when the meter is fooled—think about the last phone camera image you took of someone who was standing against a light or bright background—chances are her face was dark, or silhouetted, and this was because the camera chose an exposure in reference to the bright background. In this case, if we can (and with any decent camera you can) we would add about one stop (positive exposure compensation of +1 on the dial). A second image taken under the same circumstances will then look more like what our eyes saw. Exposure compensation simply lets more or less light in than what the camera thought was ‘right’. Another example is when you shoot something that is dark overall; the camera again will be fooled and make the image too light—in this case, you apply an amount of negative compensation, and the dark object will be captured at the darkness you want. The lovely thing (among many lovely things) about digital photography is that experimenting and learning is instantaneous, and costs nothing more than what you have already paid.

Aperture

Camera lenses have something called an aperture control. This term refers to the physical size of the hole the light comes through: the bigger the hole, the more light. But because of the optical physics of camera lenses there is more to the story then just how much light you want to come in. It turns out that as you make the size of the hole smaller, the depth of field (DOF, or the area in the resulting image that is in acceptably sharp focus) increases.

In the case of old-fashioned pinhole cameras, the depth of field is infinitely deep so that everything in the image from the closest to the furthest object in frame is in sharp focus. With modern lenses we can say that as you decrease the size of the hole (or in photography parlance “stop down”) the depth of field gets deeper. Conversely, the wider open (or the larger) the hole in the lens, the more shallow the depth of field becomes. Think here of portraits you have seen where only the eyes are really sharp and the rest of the face is less sharp and the whole background is soft and blurry. This is ‘shallow DOF’ in action. For many photographers controlling depth of field is the major artistic control that they use. Conversely, for landscape photography, a deep DOF is usually chosen. These are guidelines only, of course, and may be broken for artistic effect.

As a side, and related note, this is why lenses designed for portraiture typically are faster: that is, when the aperture is fully open, such lenses have a lower ƒ stop number than the zoom lenses that come with the camera. Another way this is described is that portrait lenses have ‘faster’ maximum apertures: they can gather more light than ‘slower’ lenses under the same conditions.

Tomorrow, I will try to pull this all together when we add the second major variable to the story, shutter speed. Until then.