Sunday, 11 October 2009

Photography Basics

All art forms have their techniques, which should be learnt, but photography is probably unique in the amount of technical expertise that is required to achieve good results. Some of this technique involves the use of additional equipment, such as flash, strobes and filters, but before any of this equipment is used, the basics must first be mastered. Despite the obvious differences, film and digital photography have much in common and while the method of capture has changed, the terminology is equally applicable, even the ISO or ASA.

Film and Sensor Sensitivity

In the days of film, film was categorised by its American Standards Authority (ASA) sensitivity, which later was reclassified by the International Standards Organisation (ISO); it is this latter designation that has been passed on to the digital age. It was often referred to as the speed of the film, because the higher ISO ratings allowed higher shutter speeds to capture faster motion. The slowest film in general use was ISO 25, but the most commonly used film was ISO 100, which was ideal for everyday use in a variety of conditions. Digital cameras typically start at ISO 100 or ISO 200 and some can go as high as ISO 12,800, something that was unheard of in the days of film, although some slide film could be “pushed” quite high, as much as two stops above its rated ISO in some films. The amount of noise (assuming equal noise reduction algorithms) is dependent on sensor pixel density. The more pixels in a given area, the greater the resolution, but the greater the noise characteristics. The smaller sensor size of compact cameras, is therefore more likely to produce high levels of noise, due to higher pixel density, when compared to DSLR cameras with the same megapixel resolution.
Each time the ISO doubles, it lets in twice as much light and is the equivalent to a full stop by that definition. However, higher speed films tended to have larger grains, giving a grainier appearance in the resultant prints. Likewise, increasing the ISO of a digital sensor increases the noise that is present. The combination of increased noise and higher levels of noise reduction, either in camera or in post production can soften the detail in an image. The amount to which this matters, depends on the subject matter, the more detail there is in an image, the more important it is to preserve the detail. An example would be the fine textures in bird feathers, where detail is vital.

Aperture

The aperture is measured as a function of the focal length of the lens; it is inversely proportional, so is usually defined by f/x, where f is the focal length and x is a number which represents the aperture size. Because it is an inverse, increasing the f number actually decreases the size of the aperture, so f/2.8 is wider than f/5.6. Aperture has a very important effect on the image and when used correctly, can be the make or break of an image. It can also be used to good creative effect. Each full stop allows twice the amount of light in as the previous aperture. Many modern cameras can be adjusted by a third of a stop. The full stops most commonly encountered are f/2.8, f/5.6, f/8, f/11, f/16 and f/22. There are narrower and wider apertures, but they are only available on certain lenses.
The size of the aperture has a big effect on the depth of field, this is the amount of the image that is in sharp focus, from the foreground to the background. A narrow depth of field results in a very small amount of the image being in sharp focus, while a deep depth of field results in most of the image being in sharp focus.
Wide apertures (such as f/2.8) are very good for portrait and wildlife photography. This is because they result in a narrow depth of field, which throws the background out of focus, helping to isolate the subject from an otherwise distracting background. Conversely, narrow apertures, such as f/16 result in a deep depth of field, which is an effect that is needed in landscape and architectural photography, so that as much of the image as possible is in focus.
Caution should be exercised though. Lenses are generally less sharp, especially at the corners, at their widest aperture, so often it is better to “stop down”. This is the practice of closing the aperture slightly. At the other end of the scale, with narrow apertures, you start to see the physical limitation called diffraction, this is where the light is bent as it passes through a narrow aperture and is a function of physics and not the lens. The resultant side effect of diffraction is again image softness. Some lenses start to exhibit the effects of diffraction earlier than others and natively sharp lenses are still able to produce sharp images at apertures affected on other lenses. Generally, diffraction sets in from around f/11, but it isn’t usually until beyond f/16, where the effects start to visibly affect sharpness. Sometimes, a scene is so expansive, that a very deep depth of field from a narrow aperture counteracts the diffraction to a great enough degree to make it worthwhile, but it is best not to go any narrower than f/16 unless absolutely necessary.
Just to complicate matters further, it isn’t just aperture size that affects the depth of field. Sensor size also has a role to play, the larger the sensor, the narrower the depth of field. Compact cameras have a much deeper depth of field, which makes it much harder to produce background blur, but does make keeping the whole landscape in focus easier. At the other end of the scale, full frame cameras (cameras with a sensor size equivalent to traditional 35mm film) have a much narrower depth of field, so much more care needs to be taken when preparing landscape shots. So-called cropped sensor DSLR cameras are somewhere in the middle, as they have a smaller sensor than full frame, but a much larger one than compacts. In addition, lens focal length is important, the longer the focal length of a lens, the narrower the apparent depth of field, as the increased focal length compresses the scene, making it easier to throw the background out of focus.

Shutterspeed

Shutterspeed controls the speed at which the shutter flips up and down, letting light onto the film or sensor. The longer the shutterspeed, the greater the amount of light that enters the sensor or film. Each full stop shutterspeed allows twice as much light as the next fastest. Like aperture, modern cameras are usually able to be adjusted by a third of a stop. Examples of full stops include 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec and so on. As can be seen, it is much easier to predict the effect on light, just by looking, than with the aperture. For example, 1/250 sec is twice the shutterspeed as 1/125 sec, so therefore will let in half the light. Faster shutter speeds allow the capture of fast motion, so are ideal for sports and wildlife. Slow shutterspeeds allow for motion blur, so are ideal for creating the silky effects of water in some landscapes.

Putting it all Together

The key to a good photograph though, is to get all the different aspects right and to do that, you need to understand how they interact. To get the right exposure, you need to get adequate light into the camera and it is how you control the different aspects, that determines the exposure. If you increase the shutterspeed by a full stop, you are halving the amount of light, therefore to compensate, you must either open the aperture by a full stop or increase the ISO. So for example, if you change the shutterspeed from 1/125 sec to 1/250 sec, you must open the aperture from f/5.6 to f/2.8 or increase the ISO from 100 to 200. Likewise, if you close the aperture from f/8 to f/11 to get more depth of field, you must decrease the shutterspeed, say from 1/30 sec to 1/15 sec or increase the ISO. It all sounds pretty complicated and it is to a degree, but the key is to practice; eventually it all comes together. Also, there are ways you can reduce the amount you have to think about each time, by using either aperture (Av) or shutter priority (Tv). By using these modes, you can set the camera, so that only one of either the aperture or shutterspeed respectively is varied by you. In aperture priority mode, you control the aperture and the camera adjusts the shutterspeed accordingly, if you need a faster shutterspeed, you can increase the ISO. The same principle applies with shutter priority. The more you practice, the easier it is to remember and eventually deliberately change things to get different effects, adding creativity. You’ll make mistakes to start with, probably alot of them, but it will come and remember, even the most seasoned of photographers make mistakes, they just don’t show them to people.

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