Digital SLR Handbook. John Freeman

Чтение книги онлайн.

      Shift and tilt lenses are one of the many specialist lenses available to the DSLR camera user. They are especially suited to architectural photography, as they eliminate the problem known as converging verticals, where buildings appear to taper towards the top.

      > Canon EOS 1DS, 24mm f/3.5 TS-E lens, 1/400 sec, f/11.

Ghosting and flare

      Another problem with DSLR camera lenses, particularly long lenses, is ‘mirror’ reflection. The sensors in digital cameras are different from their film equivalents and have a reflectivity that creates flare and ghosting inside the lens. For example, if you were to use a 300mm telephoto lens with a protective glass flat in front of the first lens element, any light entering the lens from a bright light source would be reflected off the sensor and back onto the protective glass, causing ghosting.

      To eliminate ‘mirror’ reflection, some DSLR manufacturers now make their lenses with a meniscus lens (one convex and one concave side with equal curvatures) used in place of the flat protective glass. The meniscus lens means that light reflected from the sensor forms an image in front of it and then disperses. As most light that’s dispersed does not hit the reflective elements, ghosting and flare are prevented.

      Fringing

      What we perceive as white light is, in fact, a combination of different colours uniformly mixed so that we don’t see any one colour in particular. If we shine this light through a prism, it will disperse, creating a rainbow-type spectrum caused by refraction because the individual colour wavelengths are focused at different points. To some extent, the same thing happens with photographic lenses. Called ‘chromatic aberration’, it appears as ‘fringing’ along the edges of the subject matter in photographs. In good-quality lenses, this is corrected by a combination of different types of optical glass with different dispersal and refraction qualities. In lens construction, these are known as ‘elements’ and are placed in the lens barrel in a series of ‘groups’. It is this combination, together with the quality of the glass, its shape and coating, that makes one lens superior to another, even if it’s the same focal length. This also explains why some lenses cost considerably more than others.

      Viewed from top to bottom and left to right, this sequence of views over the River Thames shows how different lenses capture the same scene from the same viewpoint.

Accessories

      Having selected your lenses, there are several accessories you should consider buying, not only to help you make the most of your camera and lenses but also to protect them.

      UV and skylight filters

      The first is a UV or skylight filter. Both make virtually no difference to colour temperature and exposure, although the UV filter does absorb UV light without cutting visible light, reducing haze on sunny days, while a skylight filter reduces the blue cast caused by sky and water reflections. Despite such minimal effects, I have one of these filters permanently attached to my lenses to protect them from dust and scratches – after all, a filter costs only a few pounds, whereas a lens might cost thousands.

      Lens hoods and shields

      I always fit a lens hood whenever I am shooting, not only to cut out flare and prevent stray light from entering the lens, but also to protect it from knocks, which could damage the front element. Although most lenses come with a lens hood, they are not always adequate in certain lighting conditions. For this reason I would recommend a separate lens shield that is attached to the lens with a flexible arm. This can be varied to cope with extremely bright conditions and is more effective than a hood. Consider your own eyes when the sun is bright: it is a normal reaction to raise your hand to protect them or to wear a cap that has a large peak.

      Without a lens hood, flare can enter the lens, as it has in this shot, ruining the photograph.

      Lens extenders

      A lens extender fits between the camera body and the lens. Depending on its strength, it increases the focal range of the lens. For example, if you’re using a 200mm lens with a 2× extender, the effective focal length goes up to 400mm. Although this entails a loss of speed, giving an f/2.8 lens, say, a maximum aperture of only f/5.6, the portability of the extender and its low cost compared to that of a 400mm lens far outweigh this reduction.

      UV filter

      Lens hood

      Extension tubes and bellows

      As well as an extender, I always carry a set of extension tubes. Again, these fit between the camera body and the lens, and enable you to get extremely close to your subject. They normally come in sets of two and can be used individually or doubled up for greater magnification. However, once fitted, your lens will not be able to focus on infinity. Extension bellows perform a similar function but are bulkier and, therefore, more suited to indoor work.

      2x СКАЧАТЬ