Choosing a lens
Apart from the selection of a format and then brand, choosing a lens, or lenses, can be a big decision, especially for beginners.The focal length: From a camera operator's perspective, lenses are broadly grouped into four categories - wide-angle, standard, telephoto and macro - according to their focal lengths. It is often assumed that wide-angle lenses are for landscape photography and telephoto lenses for objects in the distance. As you will discover throughout this book, this is a myth. Any lens can be used for any subject and the choice of a lens's focal length, and hence the perspective that it offers, has to do with exactly what you want to communicate through your image.
When you are assessing a lens, there are a number of issues that need to be considered. Most important are the field (or angle) of view and perspective that it affords, which relates to the focal length; the range of f-stops (apertures) it offers (which directly affects depth of field, and which is referred to as the speed of the lens); and the resolution (sharpness), which will dictate how much the image can be magnified and how much detail there will be at any given enlargement. Lens resolution varies between brands and higher quality lenses are understandably more expensive than lower quality ones. Lens speed (the maximum available aperture) also affects the cost of a lens. Then there are issues of weight, or perhaps auto focus mode is a choice you would like to have available.
It might sound overwhelming but lens manufacturers do not set out to deceive. They clearly indicate what each lens is best suited for, and so the choice is yours. If two lenses seem identical in terms of their specifications and one is considerably more expensive than the other then it is most likely that the costlier lens will have a far higher optical quality. A rule of thumb is to buy a lens at a price you can afford, remembering that you can always trade up at a later date.
Lens perspective -200 mm telephoto lens - Conversely, if a telephoto lens is attached, the viewing distance is then altered, and, for a picture composed of the same elements, the background will appear closer and therefore larger than seen by the human eye. This is called a compressed perspective.
50 mm standard lens -When you use lenses of different focal length or change the focal length on a zoom lens, the field of view and perspective change. In short, focal length, distance, and the resulting perspective alter the apparent size and depth of subjects within a picture, i.e., the distance by which the background and foreground appear to be separated from each other.
20 mm wide angle lens - The most obvious effect seen is when a wide-angle lens is attached. The background objects will appear distant and quite small against objects in the foreground, and the wider the lens the more exaggerated this becomes.
Lens speeds - The two lenses shown here are of the same make and focal length. The first lens has a maximum aperture of f4, and the second lens has a maximum of f2.8. Immediately you can see the front element diameter on the faster lens (f2.8) is larger. The faster lens is also twice the weight, however it offers a higher resolution (sharpness), a brighter view for focusing and one extra aperture (f-stop) in speed.
The f2.8 also gives less depth of field when it is used on maximum aperture. Unfortunately it is twice the price! Nonetheless, choice of lens will depend on your type of work. I use the lighter more mobile f4 lens; a sports photographer using a monopod on the sidelines might prefer the f2.8.