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I belong to a lot of photography groups and one thing I hear a lot of is “What lens is best for”. So I thought we would take a little look at what lens do.

First off lets look at how a lens works. ”A pinhole camera with a large aperture is blurry because each pixel is essentially the shadow of the aperture stop, so its size is no smaller than the size of the aperture (below left). Here a pixel is the area of the detector exposed to light from a point on the object. Making the pinhole smaller improves resolution (up to a limit), but reduces the amount of light captured. Diffraction limits the effectiveness of shrinking the hole, so at a point, making the hole smaller makes the image blurrier as well as darker (below center).”

According to an article from cambridgeincolour

The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position. Wide angle lenses have short focal lengths, while telephoto lenses have longer corresponding focal lengths. A zoom lens is one where the photographer can vary the focal length within a pre-defined range, whereas this cannot be changed with a “prime” or fixed focal length lens. 

The aperture (F-Stop) range of a lens refers to the amount that the lens can open up or close down to let in more or less light, respectively. Note that larger aperture openings are defined to have lower f-numbers (often very confusing). These two terms are often mistakenly interchanged. Lenses with larger apertures are also described as being “faster,” because for a given ISO speed, the shutter speed can be made faster for the same exposure. Additionally, a smaller aperture means that objects can be in focus over a wider range of distance, a concept also termed the depth of field. When one is considering purchasing a lens, specifications ordinarily list the maximum (and maybe minimum) available apertures. Lenses with a greater range of aperture settings provide greater artistic flexibility, in terms of both exposure options and depth of field. The maximum aperture is perhaps the most important lens aperture specification.

Portrait and indoor sports,theater, and wedding photography often requires lenses with very large maximum apertures, in order to be capable of a narrower depth of field or a faster shutter speed, respectively. The narrow depth of field in a portrait helps isolate the subject from their background. For cases where extreme depth of field is desired, then smaller minimum aperture (larger maximum f-number) lenses allow for a wider depth of field.

Determining what the lens will be used for will help in deciding which lens purchases are right for you. Someone who shoots strictly in-studio portraits may not have the same lens needs as someone who shoots wedding for instance. The important part of picking a lens is understanding how lens work, In learning the ins and outs of lens you will always know what lens you need for what situation. It isn’t good enough to learn solely about a particular lens, you need to be fully aware of how all lens work as well as knowledge of how your camera and it’s settings work. This is done by mastering the knowledge of how all lens work. There is a general principle to all lens. Understanding that principal as well as mastering knowledge of apertures and lens functions is important. One great way to do this is be experimenting with the lens you do have, as well as looking at the lens at work and understanding it’s function.

So some basics with some tips from DPpreview too

• F-Stop/Aperture: A smaller number means the lens has a larger maximum aperture and therefore gathers more light; an F2.8 lens collects twice as much light as an F4.
  
• Image stabilization systems reduce the blur caused by camera shake
• The first number used to describe a lens is its focal length; in combination with the camera’s sensor size, this defines the angle of view covered by the lens, with smaller numbers indicating a wider angle. Zoom lenses are named using two numbers which indicate the extremes of the range, for example 18-55mm for a typical kit zoom lens. Fixed focal length lenses which don’t zoom (also widely known as ‘primes’) just have a single number (e.g. 50mm).
• A standard zoom lens is a general-purpose lens that covers a range of focal lengths from wideangle to moderate telephoto. The most obvious example is the kit lens that comes with the camera (generally an 18-55mm F3.5-5.6 for APS-C), but this can be upgraded to something with slightly more range and better optical quality, or with a fast F2.8 maximum aperture. Often the second lens that photographers buy, the telephoto zoom effectively allows you to get closer to your subject, and is therefore useful for photographing such things as sports, wildlife, or children running around playing.
• Fast prime lens like the 50mm F1.8 are ideal for shooting indoor natural light. Primes also make excellent portrait and wedding lens! With its fixed focal length (50mm, 85mm, 100mm), it reproduces fairly accurately what the human eye sees – in terms of perspective and angle of view. At higher focal lengths (85mm or 100mm) you have an ideal lens for portraiture, because when coupled with a wide aperture they thoroughly soften any background detail, thus making it less likely to distract from the main subject.
• A wide-angle lens is a natural choice for capturing outdoor landscapes and group portraits.

Here is an exposure cheatsheet from Hetler photography that looks at understanding your camera settings and lens.