Many felt disappointed by the luminosity of the 18-55mm lens kit, -don’t misunderstand, it really is an excellent lens, but in dim light situations, it shows its weaknesses-. A f/3.5 aperture falls short in these situations and if you increase the zoom the aperture will be decreased even more. At 18mm you’ll have an aperture of f/3.5 (which isn’t too bad) and at 55mm you’ll have a f/5.6 one. How can this be? Why can’t it remain constant? Try it yourself: shoot at f/3.5 and at f/5.6 and you’ll see the size doesn’t change. You can’t even see the blades of the diaphragm because a that aperture the diaphragm doesn’t do anything.
Some lenses, however, are capable of zooming and keeping a constant aperture. These are known as constant aperture lenses and they feature a small, neat trick to achieve this.
First, we must understand the f value isn’t defined by the diaphragm, but is the result of an equation. That’s why it’s written f/x, since the formula is f = focal distance of the aperture. We will explain it in more detail:
A focal distance of 50mm divided by an aperture diameter of 17,86mm gives us 2.8 (or f/2.8). Since this isn’t too easy to remember and it varies depending on the lens, using f values is much simpler because it’s a standard for pretty much every lens out there.
This gives us the first hint that explains why zooming alters the aperture. Since its length changes, the formula gives us a different result and therefore a higher f value.
Altering the zoom level can produce two different types of results depending on the lens we have: some lenses move the crystals in front of the aperture, while others move it behind them, or a combination of both.
Let’s imagine you’re seeing a ring through a magnifying glass. The ring is on the table and you can see what’s below it. Then you pick up the ring and start seeing through it, still with the magnifying glass. If you move the ring away from the magnifying glass, it’ll look smaller. If you get it near it, it’ll look bigger. In this example, the magnifying glass represents the crystals of the lens, and the ring is the diaphragm.
Let’s talk about the lens once again. If a lens moves only the frontal crystals –those in front of the diaphragm- and increases the focal length but also makes the diaphragm look bigger, it will keep a constant aperture. Since the focal length increases, increasing the size of the diaphragm as well makes the f value remain constant. In a diaphragm that behaves like this, the blades don’t block or let the light in, but the apparent magnification of the blades.
Why don’t al lenses do this, then? It’s very simple: Money. These kinds of lenses are much more expensive to make than a regular one. Are they worth it? Depends on what you’re going to use it for. For beginners it may not make any difference, but professionals who are exposed to dim light situations on a regular basis can find a valuable tool in these magnificent lenses.