Propiedad Intelectual e Industrial

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This photo of autumnal trees, taken early one misty morning, is a good example of a low-contrast image. In this instance, the narrow contrast is caused by the weather conditions and while – due to the lack of contrast – the image may look quite fl at and one- dimensional, the result is atmospheric and faithful to the original scene.

Nikon D200, 100–300mm (at 270mm), ISO 100, 1/20sec at f/11, tripod.

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These two illustrative histograms demonstrate how the tonal extent of both a high- and low-contrast image differs. High Low

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'High key' and 'low key' are terms used to describe exposures that are predominantly light or dark. High key refers to an image that is light in tone overall. An image that is dark, with the majority of the tones occurring in the shadows, is deemed low key. While you might imagine that due to their nature, high- and low-key images would lack impact, in reality this approach can create striking results.

High key is a photographic style where the image is predominantly white or brightly lit; in other words, there is little mid-range tonality. Low key is where the subject is surrounded by dark tones and in which there are few highlights. Both styles intensively use contrast and can be used to convey differing moods. High-key images are light, bright and often considered positive, while low-key images are often dramatic and atmospheric.

High-key images have little or no shadow and lack contrast, with the subject rendered in a light tone similar to that of the background. There are few middle tones and, in addition to the tone being bright, it will often be quite even across the image. One of the best high-key subjects is people,

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High-key images are mostly light in tone, often with both the subject and background brightly lit. Photographing a swan against a light, misty background created a simple, high-key result. Nikon D300, 70–200mm (at 200mm), ISO 200, 1/200sec at f/7.1, handheld. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DEH_Ch1.indd 38 DEH_Ch1.indd 38 27/11/2012 16:0927/11/2012 16:09

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Low-key images are dark in tone and rely on either highlights or colour to highlight the subject’s shape and form.

Nikon D300, 150mm, ISO 200, 1/15sec at f/16, tripod.

with the sitter being photographed against a white background, often dressed in white or light clothing. Exposure levels generally need to be high, but images shouldn’t be overexposed. With low-key photographs the tone is dark, and the controlling colour is usually black. Special attention will usually be given to the subject’s shape, form and curves – often emphasizing them with highlights – to provide the picture’s interest and impact. Low-key images tend to have a reasonably high level of contrast.

Before you take a picture, it is useful to identify whether or not your subject qualifi es as high or low key. Cameras measure refl ected light, opposed to incident light (see page 16), so they are unable

to evaluate the absolute brightness of the subject. Cameras employ sophisticated algorithms to try to circumvent this limitation, which estimates the image’s brightness. This estimate will often place brightness in the mid-tones and, while this is acceptable for most subjects, it will often result in high- and low-key images being incorrectly exposed. Therefore, high- and low-key images often require a degree of manual exposure adjustment relative to what the camera would do automatically. For example, high-key images often require longer exposure than recommended, with low-key images needing less exposure time.

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Histograms for high-key images will often show peaks to the far right, while low-key images will show peaks grouped left of the mid-tone point.

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While all camera settings with the same EV number will create the same level of exposure, the resulting pictures can differ greatly. These images were both taken using the same EV. However, due to the different aperture and shutter speed combinations employed, motion is recorded very differently. Remember that the shutter speed dictates the amount of motion blur and the relative aperture determines the level of depth of fi eld.

Nikon D300 12–24mm (at 12mm), EV 10, ISO 100, polarizing filter, tripod. (1) 1/15sec at f/8 and (2) 1sec at f/32.

The EV concept was developed in Germany during the 1950s in an attempt to simplify choosing among combinations of equivalent camera settings. Every combination of lens aperture, shutter speed and sensitivity refers to an exposure value for a given ISO. The EV is a number that – when used in

The ‘law of reciprocity’ (see page 56) states that the relationship between aperture and shutter speed is proportional. As a result, a ‘technically’ correct exposure can be made by using a variety of lens aperture and shutter speed combinations. For example, if an exposure of 1/125sec at f/5.6 is correct, then it is also possible to employ settings of 1/250sec at f/4 or 1/60sec at f/8 and maintain the same amount of light reaching the sensor. The exposure value (EV) number represents all combinations of aperture and relative shutter speed that can be selected to produce the same level of exposure.

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41 f/1.0 1.4 2.0 2.8 4.0 5.6 8.0 11 16 22 32 1 sec 0 1 2 3 4 5 6 7 8 9 10 1/2 1 2 3 4 5 6 7 8 9 10 11 1/4 2 3 4 5 6 7 8 9 10 11 12 1/8 3 4 5 6 7 8 9 10 11 12 13 1/15 4 5 6 7 8 9 10 11 12 13 14 1/30 5 6 7 8 9 10 11 12 13 14 15 1/60 6 7 8 9 10 11 12 13 14 15 16 1/125 7 8 9 10 11 12 13 14 15 16 17 1/250 8 9 10 11 12 13 14 15 16 17 18 1/500 9 10 11 12 13 14 15 16 17 18 19 1/1000 10 11 12 13 14 15 16 17 18 19 20 1/2000 11 12 13 14 15 16 17 18 19 20 21 1/4000 12 13 14 15 16 17 18 19 20 21 22

are using 200 ISO, then you need to adjust the above settings by -1 stop.

a specifi c ISO rating, typically 100. If the ISO sensitivity is different, then you will need to

conjunction with an exposure value chart – gives the appropriate combinations of exposure settings that maintain the same amount of light reaching the sensor. For example, 0 EV is equivalent to an exposure setting of f/1 at 1sec at an ISO sensitivity of 100. Each time you halve the amount of light collected by the image sensor – for example, by doubling the shutter speed or by halving the aperture – the EV will increase by one. Basically, each one-unit change in EV is equal to a 1-stop adjustment in exposure. High EV numbers will be used in bright conditions requiring a low amount of light to be collected by the camera’s sensor to avoid overexposure, while low EVs will be employed when there is less available light and a greater degree of exposure is needed to avoid underexposure.

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EV charts

The relationship between shutter speed and lens aperture is proportional – make an increase in one value, and you must make a proportional reduction in the other to maintain the same level of exposure and vice versa. Therefore, simple tables of exposure values can be calculated relatively easily for any given aperture.

Exposure value charts like this may look quite daunting at fi rst, but they are actually quite straightforward to interpret. The value on the left relates to the shutter speed in seconds and the value along the top refers to lens aperture. Typically, an EV chart will include an aperture range from f/1 to f/32 – it is unusual for a camera lens to have a range exceeding this. While the concept of EV may not prove quite so useful or relevant to photographers today, they do allow you to take photographs fairly reliably under certain lighting conditions without a light meter.

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ISO (International Standards Organization) is a numeric indication of a photographic material’s sensitivity to light. This standard measurement was originally used to show the speed of fi lm. However, since digital cameras use an image sensor instead, the term is now used to refer to the ISO equivalent. Therefore, ISO signifi es how sensitive a digital camera’s sensor is to the amount of light present. When using fi lm, photographers had to change fi lm to alter ISO speed. In contrast, digital photographers can quickly and conveniently alter the ISO rating for individual images.

A digital camera’s ISO range varies from camera to camera, but many today have a large and useful sensitivity range, typically ranging from either ISO 50 or 100, up to a staggeringly high 204,800 on some DSLRs. The ISO setting you employ has a huge bearing on exposure, being directly related to the combination of shutter speed and aperture needed to obtain a correct result. For example, at low sensitivities, more light is required to enter the camera in order to expose the image. Therefore, either a longer shutter speed or larger aperture