Photography: The Basics of Exposure
Exposure - in the photographic sense - is defined as the amount of light that is allowed to pass through the lens and fall on the photographic film or image sensor (the photographic mediums in film and digital cameras respectively). A single exposure will allow a certain amount of light (measured in lux seconds) onto the photographic medium which will determine how bright or dark the photo is, the depth of field and amount of motion blur. Exposure itself is affected by three values: light, shutter speed and aperture, and the sensitivity of the photographic medium can be increased or decreased with film speed (in film cameras) or by adjusting the ISO value (in digital cameras).
The Factors Affecting Exposure:
Lighting is probably the most important part of the composure of a photo - without it photos would be rather dull... oh, and black. Ambient light - light produced from the scene you're taking the photo of - is crucial as it will determine the range of shutter speeds, aperture widths and ISO values that you are able to use. While ambient light is nigh-on uncontrollable, if shooting outside it might be an idea to return to the same scene at different times of day and, indeed, on other days as the light cast by the sun - or even the moon- will always be slightly different and may allow you to produce a completely different photo altogether.
As ambient light isn't always as required, one can introduce other forms of lighting to the scene - whether it be a simple desk lamp or more professional lights with umbrellas. One can also use reflectors to light a subject; often these take the form of large circles of material held up to reflect light on a subject, but they can be as simple as a white sheet of A4 held by someone you're taking a photo of to brighten up the bottom half of their face.
The most readily available form of artificial light - as the vast majority of cameras come with them - is the camera flash. Unfortunately camera flashes are constantly misused. Personally I never really use a direct flash as I find it creates harsh shadows, can often be seen in reflections off objects and tends to remove the atmosphere of the scene and replace it with a 'whitewashed' look. The easiest and cheapest way to remedy the problem is as simple as attaching some tissue paper or other translucent material (for instance, the plastic from ice cream tubs) over the flash unit to diffuse the light coming from it and soften its effect on the photo. If using an external flashgun it is often a good idea to either point it upwards or sideways to 'bounce' the light from it off the walls of the room or environment so as to not directly illuminate the subject - a good idea is to hold the flashgun by hand to increase manoeuvrability. Sometimes it's also possible to use a 'fill flash' which will give you a less intense flash for portrait photography - enough to remove unwanted shadow from the subject's face but not so much as to make them look lifeless. In doing these things the photograph will have a far more natural feel to it, as well as - in the case of portraits - being much kinder on the subject's eyes.
The speed at which the shutter in a camera moves, and hence the length of time that the photographic medium is exposed for, is termed the shutter speed. Shutters come in two forms, electronic and mechanical. Electronic shutters are only found in digital cameras and work as simply as just turning the image sensor on and then off. Mechanical shutters, on the other hand, are far more complicated.
Most modern-day SLR cameras use what's called a 'focal-plane shutter mechanism'; this mechanism works by using two plates (made from metal, plastic, rubberized material etc.) which slide over each other either vertically or horizontally - exposing a small part of the photographic medium as they travel allowing for fast shutter speeds of up to 1/8000 of a second (see diagram to the right).
However, when using a flash, the whole of the photographic medium needs to be exposed when the flash goes off; because of this the shutter moves as on the right. If it were to move as it does with fast shutter speeds, only a small sliver of the photo would be lit by the flash. The exposure method used with flash means that shutter speeds are limited to around 1/200 of a second - the same method is used for long exposures.
The slower the shutter speed the longer the exposure and hence the more light is allowed to pass through the lens and onto the photographic medium. However, with slow speeds, the photo is more susceptible to being out of focus; it is hence often a good idea to use a tripod or monopod when taking photos at a low speed as it will reduce camera shake - even more so if the camera being used has a remote shutter release or is set to a timer. Low shutter speeds also enable motion blur to be factored into the photo (see example pictures below). High shutter speeds are best used when taking photos without the use of any support or when photographing fast moving objects that you want to 'freeze' such as birds in flight or a person jumping.
An aperture is, by definition, a hole through which light travels. In photographic terms, the aperture is the hole in a diaphragm inside the lens of a camera which is widened and narrowed as the diaphragm moves. As the purpose of the diaphragm is to limit the amount of light reaching the photographic medium, it is also called a 'stop'. Below is a diagram representing 8 different stops and their relative f-numbers; note that the lower the f-number, the larger the aperture, and vice versa. The f-number itself is the ratio of the lens' focal length to the diameter of the aperture and each stop is an increasing power of √2 (roughly 1.4). However, knowing the physics behind the aperture values, while fairly interesting, has very little use when it comes to taking photos. The smaller the f-number, the wider the aperture and the more light it will let in. The larger the f-number, the narrower the aperture and the less light it will let in (Much like the pupil of an eye). Increasing the aperture by one stop will double the amount of light that passes through it.
It is here that the interdependence of aperture and shutter speed comes into play. A low f-number allows for faster shutter speeds to be used (the large aperture width allows enough light through the lens that the exposure time can be very brief) whereas high f-numbers require slow shutter speeds.
The effects that fast and slow shutter speeds can create have been previously explained and to achieve them a wide or narrow aperture would need to be used. However, aperture choice is not a slave of shutter speed; different f-numbers can drastically affect the photo too. The main effect of aperture is to increase and decrease the lens' depth of field. The depth of field of an image is the area either side of the focus point that's sharp and appears in focus. Large apertures create a shallow depth of field which is often very useful for portraits or macro photography as it will blur out the background and draw attention to the subject. However, with, say, landscapes, smaller apertures are preferable as they give a large depth of field - allowing the whole scene to be in focus. The distance at which the largest possible depth of field for any f-number is achieved is called the hyperfocal distance - the shortest distance at which a lens can be focused while keeping objects at 'infinity' (the furthest away objects) in focus. It is important to note that, at very narrow apertures, vignetting can occur - a phenomenon that causes the periphery of the photo to appear darker than the centre as only a small beam of light is passing through the aperture and hitting the photographic medium - causing the light intensity at the edges to be fainter.
The ISO value or film speed in cameras determines how sensitive the photographic medium is to light. In film cameras, film with high ISO values were deemed 'fast'; those with low values, 'slow'. ISO stands for International Standards Organisation and is a fairly arbitrary set of numbers (100, 200, 400, 800, 1600, 3200 etc.) The lower the ISO value, the less sensitive to light the medium will be, however, while at higher ISO values a faster shutter speed or narrower aperture can be used, increasing the ISO value increases the amount of noise in the photo; because of this, if a photo is being taken with the intent of using it on a poster or as a large print, it is best to use as low-an ISO as possible to minimise noise.
High ISO values do, however, have their advantages. If taking 'action' shots in a dark environment that you cannot control, for instance, a sporting event taking place indoors, in order to attain the necessary shutter speeds a high ISO setting is needed. High ISO values can also be used as a substitute for the camera flash in places where flashes are not permitted - such as museums and galleries - or where a flash would detract from the atmosphere - for instance, at parties.
Well, now you, the reader, should have a fair understanding of how lighting, shutter speed, aperture and ISO setting can affect the composition of a photo. So next time you get your camera out try putting it on manual rather than auto and experiment with settings - if a photo comes out too bright then it's 'over exposed', try increasing the shutter speed, using a higher f-number for the aperture or decreasing the ISO value. If the photo's too dark then it's 'under exposed', a slower shutter speed, lower f-number or higher ISO value is needed (or you could use a flash and / or additional lighting)
The best way to learn is to practice! You will eventually find that your photos look much better when individually tailored to the scene - not to mention getting a greater sense of achievement from good photos!
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