Mar 112012
 

This series of articles attempts to explain some of the nuts and bolts of photography for people who want to get past the auto mode on their camera and gain more control over their photographs. I’ll be glossing over or sometimes completely ignoring some of the more advanced aspects in the interest of brevity.

Photography is all about light. It’s very important to get the right amount of light hitting the sensor (or film) so that the parts of interest are neither too light (blown out) or too dark. All photographic sensors can only capture a small range of light values compared the the human eye so adjustments need to be made to compensate for different light conditions. Once the light exceeds the maximum the sensor can handle, all detail turns white and is lost. Likewise, areas that receive insufficient light will disappear into blackness.

Most real life situations will contain a range of tones that exceed the capability of the sensor to record, and we, as the photographer must decide which parts are to be overexposed and/or underexposed. For instance, if you are photographing a person, their face is usually the most important part to get exposed correctly.

It’s important to understand that there are only 3 controls that effect the final exposure. These are: the aperture of the lens, the shutter speed of the camera, and the ISO sensitivity of the sensor. For film, this last item is pre-determined by the ISO (sometimes known as the speed) of the film and can’t be changed unless we load a different type of film into the camera. With digital cameras the ISO can be set by the photographer and is a valid tool in achieving the correct exposure.

Why not let the camera decide?

The auto exposure systems of modern cameras are quite good at getting the exposure correct most of the time, but there are some good reasons we may want to take over part or all of the job ourselves. Firstly, there are some situations where the camera will simply misinterpret the scene and expose the area of interest incorrectly. But the main reason is that we may want to use a particular aperture or shutter speed for creative reasons.

In addition to full manual mode, most cameras have at least 4 auto modes: full auto, where the camera decides everything including whether to fire the flash, program mode, where the camera sets the exposure, but allows the photographer to set an exposure compensation, and sometimes choose one of several different aperture/shutter speed combinations, shutter priority, where you tell the camera what shutter speed to use and the camera sets the aperture automatically, and aperture priority, where you set the aperture and the camera sets the shutter speed.

Many cameras also offer scene modes that allow you to tell the camera what sort of subject you’re shooting. This allows the full auto function to bias it’s results in favour of the type of subject. For example, in sports mode, the camera will aim for a fast shutter speed to help freeze fast action.

Advanced photographers will choose the mode most appropriate to the current shooting situation as they understand how each mode will creatively impact the photograph, although the full auto modes are rarely, if ever used.

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Now for some terminology. Confusion often exists for new photographers as some of these terms have different meanings depending on context.

f-Stop (or stop)

Literally, this refers to a series of fixed positions of the aperture on the lens, but the term is also used to describe a relative difference in light level. This is an important point that confuses some people. Increasing exposure by 1 stop doubles the amount of light being recorded and decreasing the exposure by 1 stop halves the light being recorded. Although the term stop actually refers to the setting of the iris on the lens, changing the exposure by a number of stops can be performed with any of the three exposure tools we have available to us.

Aperture

This is an area where several terms are used interchangeably that actually have different, but related meanings. The term aperture describes the diameter of the lens elements. However, describing this as a size in millimetres or inches is meaningless in photography because the relationship between the physical diameter of the lens and the amount of light it lets through is dependent on other characteristics of the lens. I’ll explain more about this in the article on lenses. Having a fixed amount of light pass through a lens is not very practical for most photography so a mechanism is fitted to lenses to reduce the size. This is reduction done with the iris.

Something that creates confusion for many people is that apertures are specified in what appears to be  a rather strange sequence of numbers. I’ll explain the reason for this in the article on lenses, but for now, just use this list as a guide. Each step in the example below is one stop.

< Smaller Aperture = Less Light < …………. > Larger Aperture = More Light >
f64 …. f32 …. f22 …. f16 …. f11 …. f8 …. f5.6 …. f4 …. f2.8 …. f2 …. f1.4 …. f1

Yes, that’s right. Smaller numbers mean larger apertures. When someone says they are setting the aperture to a smaller value, they are actually changing it to a greater number. For example, f11 is 2 stops smaller than f5.6.

Iris

The iris is an adjustable hole fitted inside the lens and is usually made from a series of interwoven metal or plastic leaves. It works much like the iris of the human eye in that it will reduce the amount of light passing through the lens as it closes down. When you are setting the aperture of a lens, you are actually adjusting the size of the hole through the iris. This is why the terms iris and aperture are often used interchangeably.

Shutter Speed

This sets the amount of time the sensor or film is exposed to the light. The longer the shutter is open, the more light will be collected by the sensor. Shutter speed in measured in fractions of a second such as 1/125th or 1/250th. Each step in the example below is one stop.

< Slower Shutter = More Light < …………. > Faster Shutter = Less Light >
1/4 …. 1/8 …. 1/16 …. 1/30 …. 1/60 …. 1/125 …. 1/250 …. 1/500 …. 1/1000 …. 1/2000

ISO

This is the sensitivity (or speed) of the film or sensor. Higher numbers mean greater sensitivity. Each step in the example below is one stop.

< Lower ISO = Less Light < …………. > Higher ISO = More Light >
100 …. 200 …. 400 …. 800 …. 1600 …. 3200

Why three controls?

If any of the three available controls can adjust the exposure, why do we need to worry about all three? Like most things in life, all three exposure controls come with consequences. How we balance these is dependent on how we want the photograph to look.

The aperture can only be adjusted over a comparatively small range due to cost, size and weight considerations, so we can’t cover all possible light levels with the aperture alone. There are also other, creative reasons we might want to use a particular aperture, such as depth of field. I’ll cover this in the article about lenses.

The shutter speed has a much broader range of adjustment than the aperture, but there is also the problem of movement. For most slow moving subjects, the shutter speed will often need to be about 1/125th of a second or faster to prevent blurring, and for fast moving subjects this might need to be 1/500th of a second or even faster. Only when we have stationary subjects and a tripod can we start to use very slow shutter speeds.

In this photo I used a shutter speed of 1/5th of a second. Because the camera was on a tripod, the fish sculpture and background are sharply defined, but the person walking and the water from the fountain are blurred.

The ISO control adjusts the sensitivity of the sensor, but has its own set of limitations. A camera image sensor has what’s called a native sensitivity, which is the minimum value it can go down to. Most modern cameras have a native sensitivity of 100 ISO, so that’s as low as they can go. Conversely, if the ISO is raised too much, noise is introduced to the photo causing a grainy effect that looks unpleasant in most photos.  So why not leave the ISO set at 100? Because in all but the brightest conditions there is not enough light to use a reasonably small aperture and a reasonably fast shutter speed, so the ISO needs to be boosted. ISO works in a linear fashion. 200 ISO is twice as sensitive as 100 ISO and 400 ISO is twice as sensitive as 200 ISO. To put it another way: 200 ISO is 1 stop more sensitive than 100 ISO. Remember that each stop is double or half the adjacent value.

So how do these three controls work together?

Here’s a hypothetical situation. Suppose we are about to shoot a photo of a particular scene where these settings will result in a correctly exposed image: Aperture:f4, Shutter:1/125, ISO:100. Let’s first assume the subject is moving quite fast so we need to increase the shutter speed of 1/500 to prevent movement blur. If we just set the shutter to 1/500 without changing anything else, the resulting photo will be underexposed by 2 stops (because 1/500th is 4 times faster than 1/125th) and come out quite dark, so we need to compensate with some combination of the other 2 controls. We need to increase the exposure by 2 stops to get back to the correct exposure, and could do this by opening the aperture by 2 stops. However, we have a problem. The lens we are using can only open to f2.8 (its maximum aperture) which will only give us one more stop of light. So we have no choice but to increase the ISO to 200.

Another thing we could have done in the hypothetical situation above was to leave the aperture at f4 to allow a deeper depth of field and raise the ISO to 400. Perhaps we needed even greater depth of field in addition to the fast shutter speed. We could set the aperture to f8 and would need to compensate by raising the ISO to 1600. At such a high ISO the picture will start to become quite grainy (noisy) and that’s a trade-off the photographer has to decide on.

 Do we need to always need to adjust all three settings?

Everything I’ve described so far has been using the camera in full manual mode. I strongly recommend that everyone becomes familiar with manual mode, but most advanced photographers will only use full manual mode in controlled lighting situations such as studio shooting, or static shots such as landscapes. In other situations, where the light or subject is constantly changing, it’s more practical to use aperture priority or shutter priority, often with exposure compensation to correct for mistakes in the camera’s exposure metering system.

There are often creative reasons why we may want to use a particular aperture or shutter speed, so we pre-set that and allow the camera to adjust one or both of the other settings to obtain the correct exposure. For example, when shooting wildlife, I usually want to keep the aperture between f8 and f11 to obtain the greatest sharpness from the lens while allowing a reasonable depth of field. I will monitor the shutter speed the camera is choosing and if it is getting too slow I’ll manually raise the ISO.

A significant advantage we have now days with digital cameras is that we can instantly preview shots and adjust our technique if we aren’t getting the expected results, but unless we understand how the three controls effect the result, it may take considerable trial and error to achieve the photo we are aiming for.

Exposure aids.

It is difficult to judge exposure just by viewing the photograph on the little screen on the back of a camera, especially in bright light conditions, so we need to rely on a couple of tools provided by most cameras.

The first is the highlights display. This display mode will show any areas that are overexposed by flashing the overexposed areas. The highlights display mode is a very quick way of determining if any important areas of the photograph are blown out, but it’s useless for determining under exposure.

We can gain much more information about how the photograph is exposed by using the histogram display. The histogram will give us information about how much of our photograph is exposed at various brightness levels, but it can fool us in some situations.

All a histogram tells us is the distribution of various shades in the picture, it can’t tell us if a particular part of the picture is overexposed or underexposed. That said, it does help to determine what is happening, exposure-wise within the photograph when used in conjunction with the image itself. The histogram is a graph of the brightness levels, from the darkest on the left through to the brightest on the right.

This picture contains large areas of very bright tones so the histogram shows a high level of data at the high end of the graph. We can tell that most of the photo is not over exposed because the large peak representing the bright areas is not pushed all the way to the right of the display. What it doesn’t really tell us is that there are some small areas on the flowers that are overexposed. We can see some small amount of the photo has reached the end of the display, but we can’t tell from the histogram if those small areas are something we do not want blown out.

In the two pictures below, the girl is exposed identically and yet the histograms are quite different. This is due to one picture having a light background and the other having a dark background.

You will notice that the dark picture has a lot of information on the left (dark) area of the histogram. At first glance, this might seem to indicate underexposure, but notice that there is data pretty much right up to the highlight end of the histogram indicating a full range of tones. Likewise, the picture with the light background has a lot of data squashed up at the right end which would indicate overexposure. In this case the background is overexposed, but that’s okay as that was the effect I was after. In both pictures her face was correctly exposed.

The Wikipedia entry on image histograms has a bit more information that may help: http://en.wikipedia.org/wiki/Image_histogram

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I hope this article goes some way toward helping you gain full creative control of the exposure of your photographs. If you have any questions, feel free to post them in the comments below.

  One Response to “Photography Basics – Part 1 – Exposure”

  1. Peculiar article, just what I wanted to find.

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