Small ApertureWhy some lenses cost more than you can imagine.
Lenses with wide maximum aperture are big, heavy and expensive. Besides the coolness factor of showing off large lenses, what drives people to buy these monsters and why should we even care?

What is Aperture?

Aperture is the circular opening of your lens.
You can control the size of the opening with the aperture setting on your camera. The camera then controls the lens. Depending on the lens you are using, you can dial in a different maximum aperture setting (smallest f-number). Lenses with large maximum aperture settings thus, need to be physically large as well, to deal with the wide opening.

What this does for you:

A narrow aperture results in highly collimated (bundled) rays, which will produce sharp images.

Creativity:
This just means that a narrow aperture will give you a large depth of field.
The opposite is also true. A wide aperture will give you a shallow depth of field.
A shallow depth of field is the technical term for a very selective focus. It is a critical element in our arsenal of tools to create better images. Some pictures work well with large depth of field (landscapes, architecture) and some pictures command a shallow depth of field (portraits, product photos).
The lens size limits how wide our aperture can get and thus, may hinder our creative options.

Capability:
A wide aperture admits more light to the image sensor (or film). As more light reaches the sensor, we can dial in shorter exposure times. In low light situations (dusk, dawn, forest) this may be the difference between being able to shoot handheld or having to set up a tripod and possibly missing a shot. Freezing high-speed action with very fast shutter speeds is another creative option that we get with wide aperture lenses. (Read my tutorial on Exposure for more details!)

What to buy

Always get the fastest lens you can afford, or that you want to afford. People refer to lenses with wide maximum aperture as being “fast”, because of the impact of wide aperture on shutter speed.

Image Stabilization, ISO and Flash

Image Stabilization (Canon), Optical Stabilization (Sigma), Vibration Reduction (Nikon) and Super Steady Shot (Sony) are all synonyms for shake reduction technology. By measuring movement and automatically compensating for it by moving the elements of your lens, you can get significant improvements in sharpness of handheld pictures shot at slow shutter speeds. This sounds complicated, but it just means that you can go up to 3 stops (8times) slower on your shutter speed and still get sharp pictures despite what common wisdom dictates.

Fast Lens or Image Stabilization (IS)
A lens that is one-half stop faster than an equivalent lens from the same maker can cost twice as much or more. If all you are concerned about is being able to shoot handheld, spend your money on a lens with Image Stabilization. You get much more for your money in terms of low light sensitivity.
However, remember that IS does not prevent motion blur. If you have a moving subject (wildlife, sports), a fast lens is your only way to freeze the speed besides increasing ISO sensitivity.

ISO sensitivity
Although you can increase your ISO sensitivity to cope with low light situations, you will get more digital noise (grain) in your pictures. Modern Digital SLRs can easily go up to ISO800 sensitivity and still produce good results. You can find more information about ISO in my Exposure Article.

Flash
You may decide to use flash instead of buying expensive lenses, but the range of flash is very limited. Maximum flash sync speeds are limited and images taken with flash look unnatural and flat. Many people will not notice noise as a problem and subconsciously ignore it. You should not use flash unless you absolutely have to.

How do you read maximum lens aperture?

When you shop for lenses, you will often find numbers like these:
70mm-200mm f/4
18mm-200mm f/3.5-6.3
These indicate the focal length and the maximum aperture (at min focal length and at max focal length).

The first example comes from a Canon Lens of mine. It has a maximum aperture of f/4 throughout the entire zoom range of 70-200mm.
The other lens has a changing aperture range of f/3.5 to f/6.3.

The f-number in the lens description indicates the maximum aperture. The f-number is the ratio of effective focal length and effective lens diameter. The lens diaphragm blades in your lens control the effective lens diameter.

All you need to remember is this:
When you compare lenses of equal focal length, the ones with a smaller f-number (larger maximum aperture) are preferable.

What you should know about Aberrations.

Aperture and Image Stabilization are not the only features driving the price up. Optical built quality and lens glass coating determine Lens Aberrations. chromatic aberration can be extremely detrimental for image quality. It is not easy to fix and requires extra effort in the image post processing.
Chromatic Aberrations show as purple or blue fringes around high contrast edges.
Encyclopedia Britannica gives very detailed explanations about all sorts of optical aberrations.

But wait, there is more:

For the thorough readers I recommend these subjects:

Bokeh: It refers to the looks of the out-of-focus area in pictures. Good Bokeh render the out of focus areas blurry while lenses with bad Bokeh tend to have sharp edges in out of focus elements that can distract from the main subject.

Vignetting is the darkening of the image corners, by uneven illumination of the image circle.
Sunrays hitting the lens directly cause Lens Flares.

Don’t obsess, digress

Buy a lens, put it on your camera and go out have fun with it. At some point, you will have obsessed enough about your lenses. You will always find flaws if you keep looking.
While some people are still reading technical specifications, others are already out there taking beautiful pictures of the world around them. You should try to be the latter.