Lens basics
January 12th, 2012
Well this past Christmas must have been the year of DSLR because I’ve heard from a lot folks that got them as gifts. One of the main questions I’ve been asked is about what lenses to get. (I also have been asked how to use the darn thing. 🙂 ) The simple answer is “buy the best lens you can afford”. But that’s not really the best answer. It mostly depends on what you are going to be shooting and the environment in which you are going to be shooting
The 2 most basic qualities of a lens are the focal length and the maximum aperture. The focal length determines the field of view and is expressed in terms of millimeters. The more millimeters the less field of view. For instance a 300mm lens will have a narrow field of view (more magnification) compared to a 35mm lens.
The other quality is the maximum aperture. This is expressed as an F/stop and determines how much light the lens lets in. The lower the number the more light passes thru the lens. So an F/2.8 lens lets in more light than a F/4.5 lens.
The problem we run into is that these qualities are at odds. When you make a lens with lots of magnification, you have to use a longer lens tube and/or more elements. This reduces the amount of light hitting the sensor. In order to compensate, lens makers make the front element of the lens larger in diameter. This lets more light it but increases the price and weight.
Two common terms to describe these qualities are speed and length. For instance a 300mm lens is longer than a 50mm. Although the physical length may not hugely different between 2 lenses, because one has more magnification than the other, it is considered “longer”. The other term speed refers to the maximum aperture. In this case a faster lens has a wider maximum aperture than a slower lens.
A “longer” 70-200mm f/2.8 lens versus a “shorter, slower” 10-22mm f/3.5-4.5 lens.
So, in order to determine what lenses to buy you need to decide what you are shooting and your lighting situation. Next week we’ll discuss other lens attributes in more detail.
Thanks for reading. 🙂
-Ken