DSLR lens uses
If you just bought a shiny new digital single reflex camera, you’re probably thinking about the types of lenses you’ll need. Most likely you’ll get one or more that cover a range from around 18-200mm.
The numbers refer to the focal length of the lens. Simply put, the bigger the number, the greater the magnification at a distance.
50mm to 70mm focal lengths are considered normal. Smaller numbers are wide angle lenses. And bigger numbers are telephoto. *
Many photo enthusiasts and pro photographers gravitate to lenses that are fast. Fast lenses have very wide apertures that let more light pass through during a given time span. Photographers may also buy many different single focal length, wide aperture lenses known as primes.
Lens focal lengths and subject matter
Some lenses are better suited for certain subject matter than others. Here is a general guide:
Walk around: 24-70 f-2.8; a good general purpose focal length.
Birds/Wildlife/Sports: 70-200mm f-2.8; 300mm f-2.8; 600mm f-4. A wide aperture allows for faster shutter speeds and can blur out a chain link fence.
Portraits: 135mm f2; little to no distortion; shrinks the nose and blurs the background.
Landscapes/Interiors/Cars: 10-20mm. Tilt the lens up a little to correct for curvilinear distortion or shoot verticals indoors; also good for intentional distortion.
Bugs/Macro: A true macro is always a non-zooming prime lens. Some zooms say macro but they won’t produce a one to one image.
Architecture: Ultra wide lens less than 21mm; a tilt shift lens allows you to correct distortion.
Poparattzi: 18-200mm or 28-300mm. Buy both for when an irate celebrity breaks one.
Weddings: Two of each of the above (except the 300mm, 600mm and the poparattzi ones). Also, at least two camera bodies. Any professional wedding photographer will tell you that something always goes wrong and you need to carry backups for a once in a lifetime event.
One all purpose lens?
Can you photograph all you want with one all purpose lens? The answer is sort of.
Even with just a 50mm 1.8, you would have a lens with a roughly normal field of view and a very wide aperture with little or no distortion. Get really close to the action and it is like a telephoto; step back and it is like a wide angle lens.
Why not use a zoom such as the 18-200mm or the 28-300mm? You could, but to get such a great zoom range they sacrifice some quality and usually have a small widest aperture such as 5.6. Ok for vacation photos but frustrating if it is your only choice.
Sometimes you can’t move so you buy a zoom lens that goes from wide angle to telephoto. If I had to recommend just one it would be either a 17-55mm 2.8 for small sensor cameras, or the 24-70mm 2.8 for full size sensors. They offer a versatile zoom range from wide angle to short telephoto and are still fast.
As you get closer to and further away from your subject the relationship of the sizes of the stuff in the foreground and the background change. The change is controlled by how close you are to the subject and would be the same if using a zoom lens or a prime.
For example, if you are very close to a person with a wide-angle lens then the background is expansive; buildings at a moderate distance are small by comparison. But if you get far away using a prime lens, or zoom in with a telephoto lens, the subject and buildings (trees, fountain, etc.) are about the same size.
So even if you find one zoom you like for everything, you will still have to move yourself sometimes to get the perspectives and spacial relationships you want.
*Important note: some DSLRs have a crop factor, also knows as focal length multiplier, that increases the effective focal length of a lens. For example, a 50mm used on a camera with a FLM of 1.6 has the effect of an 80mm lens (50mm X 1.6).