In the last posting, I discussed the various general types of lenses - normal, wide-angle, and telephoto - and the pros and cons of fixed (prime) lenses and zoom lenses. Now, I would like to get more specific about the selection of focal length and add some specialty lenses and lens accessories to the mix.
The focal length of a lens is the distance from the optical center of focus within the lens to the plane of the image sensor in the camera. Focal length is usually measured in millimeters (mm) and the longer the focal length the greater the size of the image on the sensor (sometimes referred to as magnification.) I mentioned that a lens with the field of view of our eyes in approximately 50mm when projected on a full-frame (24x36mm) sensor. However, most cameras use a sensor smaller than full-frame that is called a crop sensor. The camera is designed so that the crop sensor still 'sees' the full image so, effectively, the image is magnified by the crop factor of the camera. Typically, point and shoot cameras have a very large crop factor (from 2 to almost 6) so that the effective focal length of the lens is much greater than the measured focal length (compared to full-frame sensor.) This is why many point and shoot cameras are referred to as 'super-zoom' cameras and their zoom range is defined as the magnification beyond the shortest focal length of the lens (12x, 15x, 20x and even 30x.) Digital SLR cameras do not use this 'magnification' terminology but rather relate the effective focal length to that of a full-frame sensor.
So how does crop factor influence the effective focal length of the lens. Many dSLRs from Nikon, Sony, Samsung, and Pentax have a crop factor of 1.5 while most Canon dSLRs have a crop factor of 1.6. The crop factor is multiplied times the actual focal length to obtain the effective focal length seen on the sensor. In other words a normal 50mm lens on a Canon 1.6 crop sensor looks like an 80mm lens. So this 'normal' lens is working like a short telephoto (if you were shooting on a full-frame camera.) This effect has a tremendous advantage for a wildlife photographer who always wants a longer lens to create images closer to the subject. Effectively, a 200mm (medium telephoto) functions like a 320mm lens on a Canon crop sensor camera. A 70-200mm zoom functions like a 112-320mm zoom. This sounds like a real bonus but what if you are shooting landscapes with a wide-angle lens? Now your 24mm (medium wide-angle) lens acts like a 38.4mm (barely wider than normal) lens. This is part of the reason that landscape photographers like full-frame cameras - they can maximize their wide-angle ability.
So when purchasing a new lens you must carefully consider the type of photography you will be doing and the crop factor of your camera to select a lens that will give you the effect you desire.
There are also several types of specialty lenses that appeal to nature photographers. The first is a macro lens. A macro lens is specially designed for a very close focusing distance for its focal length. A true macro lens projects an image of the subject at its full size on the camera sensor - a 1:1 image. Macro lenses come in different focal lengths depending upon how close to the subject you can get. Typically, macros range from 60mm to about 200mm. These lenses are designed for ultra-sharp images captured at very close distances and are often used to shoot flowers, insects, and other small critters. Focusing a macro lens is a challenge because the the very narrow depth-of-field - often only a millimeter or two when wide open (f/2.8.) Depth of field is improved if the lens can be stopped down (f/11- f/32) but this usually means a very long shutter speed or supplemental light using a reflector or flash. Some general lenses have a close-focusing feature they refer to a a macro function. I have used this to advantage on my small Sony camera but it is not a true macro because you cannot focus down to a 1:1 size - mine is more like a 1:2 but it still focuses down to about 3-4 inches from the subject.
Another lens sometimes used by landscape photographers is a fish-eye lens. A fish-eye is an extreme wide-angle lens (6-10mm focal length) that is able to capture nearly a 180 degree view. These lenses cause extreme distortion of the image resulting in curving horizons than are usually interesting and can be very pleasing to view.
With all of the choices of lenses, what is the best general 'carry-around' lens for everyday photography? This is a very difficult question to answer and most photographers will tell you that a 24-70mm zoom is a good all around lens (best on a full-frame camera.) This lens gives you a mild wide-angle to short telephoto perspective and can be fast (f/2.8) and not unwieldly large. I like this lens a lot when I am in town shooting what-ever comes by, but it is not my favorite lens in the wilds. It is generally too short for birds and animals and not wide enough on my crop-sensor camera for great landscapes. It is fast for dawn and dusk shots but I almost always carry 25-30 pounds of other lenses and gear when I hike to take wildlife or scenic shots. Some people like super-zoom lenses (28-300mm) for general work. Often these lenses are slow (f/5.6 or worse) and some lack high quality optics so I have avoided the super-zooms.
Finally, how does one properly accessorize your lens? A lens case is a good thing to keep in the closet at home or to store your lens when you are not shooting, but I never use lens cases in the field because they add unnecessary volume to the gear and it takes too much time to get the lens out of the case. I keep my lenses in my pack with lens covers on front and back to protect from dust and weather.
There is controversy among photographers about using a UV/haze filter to protect the front of your active lens. Clearly, low quality glass in front of your expensive lens can degrade image quality and introduce lens flare on back-lit images so these should be avoided. I have chosen to use high-quality neutral (UV/haze) filters on the front of most of my lenses. It is a high cost but does keep dust off the much higher cost lens. I have never dropped or scratched a lens so I can't (yet) vouch for the protective effect of the neutral filter. Many (maybe most) professionals advise against a neutral filter over the lens. They doubt the protective effect if the lens is dropped and worry about the negative effect on image quality. Almost all professionals use a lens hood on all lenses. The hood protects the front of the lens from damage, reduces the likelihood of lens flare on back lit images, and has no negative effect on image quality. I always use a lens hood unless I need to rotate a polarizing filter or variable neutral density filter or am using a rectangular graduated neutral density filter. These filters will be discussed in the next posting.