Explaining the basics: focal length, aperture, fix or zoom lenses.

You just bought an SLR and are very confused by the technical terms? The sales guy went through it really quick and to be honest, you don't feel empowered at all to make the right choice? They read what's next !

I will try to explain it in the most practical, simplest way possible.

First of all, a lens name is usually like this : 18-55mm f4.5-5.6 FX VR. Yeah chinese !

• 18-55mm = focal length
• f4,5-5,6 = aperture
• FX VR= other features, less crucial though

Focal length 

...is "how much it zooms". The human eye focal length is about 33mm. Less than that, you camera will see wider than that, more it will be zooming compared to what you see. A 18-55mm lens goes from wide (18mm), for landscapes, to a little more than human (55mm) which is a good length for portraits. 

Fix or zoom?

Some time the lense is simply called "35mm" instead of "18-55mm". It means that it is a fix lens. Only one focal length. You can't zoom in or out with that, but it has a lot of key strengths: it is optimize for one thing so it usually performs best. It is also less complex hence cheaper, and finally it can shoot with less light due to aperture (keep on reading to cover that). Zoom lenses are more multipurpose, but image quality is a little under, it's pricier, and less performant in low light...but you can zoom in and out. Keep in mind: the large the zoom range, the worse the quality.

Aperture

Those funky "f" numbers. The number is a indication of how much the thing that blocks the light inside the lens can open. The more it can open, the more light can get it, the more you can shoot in low light. Seen stargate? The space door opens completely, that would be a perfect full aperture. 

Unfortunately, on a camera lens, it is hard to open that large. So the more you can open, the more expensive it gets. Keep in mind the f number is a ratio, so the lower it is, the bigger the aperture. For example, a large aperture lens would typically be f1.8 or f1.4. When F4.5 would be low aperture.

Fully close iris on the left, a perfect full aperture on the right, always can use a good sci-fi example !

• For fix lenses, f1.8 is about standard, very wide would be f1.4 or less. 
• For zoom lenses, you usually have a 2 apertures: one for the small focal length, one for the big one. For example, 18-55 f4.5-5.6 means that at 18mm, the lens can open up to f4.5, and at 55mm, is can open up to f5.6. Great zoom lenses usually have a constant aperture of F2.8, which is really good for a zoom, but those cost usually 1000$ and up.

Also large aperture (small f number) allows you to get those nice blur behind the subject.

Those other letters at the end (FX, VR, etc)

Those are not optical characteristics. They are features or version. For example, FX means the lens is for a full frame camera (if you need to read this article, I doubt you own a full frame camera). VR for Nikon means that it has stabilization against shaking, the name probably is different for every brand. 

G or D can mean that it's optimized for digital cameras, etc. These are not so important.

My personal must have, a 35mm fix f2.0 lens. Versatile, small, cheap.

Now what should you buy:

• portrait: a fix lens, 50mm or 85mm or 105mm. F1.8 or less. 
• landscape: fix or premium zoom, 14mm up to 28mm. f number is not so critical because you will shoot with a tripod, and use the low apertures anyway.
• travel: a human eye type of fix lens (35mm) or a versatile zoom that goes from 18mm up to 55 or 85mm. 

Feel free to read the other articles here to see how to make the most of it !