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 !

My DSLR lens guide

Most of us make poor choices when it comes to picking our first lenses. We are new to photography and think the camera body does it all. We believe the bigger the zoom the better the lens, we have no idea what those "f" numbers mean after the focal lens (ie 50mmf1.8)...

We end up buying a kit lens, or a big cheap zoom and we think "what a deal!" Well if you did so, you probably got ripped off.

So how to choose a lens? Well you have to look at the following:

• focal length
• aperture
• extra feature : price of filters, warrantee etc.

Let's start with nomenclature. What does 16-55mm F4-5.6 means for example?

• First the focal length. 16-55mm mean than the lens is a zoom going from wide angle focal length (16mm) to a street life / portrait focal length (55mm).
• F4-5.6 is the aperture, i.e. how much light can the lens let go thru onto the sensor.

1 /First choose the focal lens

...based on what you'll be shooting. Remember: a lens that does everything great does not exists. It simply doesn't, pick your fights :)

- You shoot landscapes and architecture: take a lens between 14 to 35mm: it's a wide angle. You will have way to much distortion for portraits, but it'll allow capturing most of your view field.

- You shoot life scene, candid, mixing street photography and portraits. Typical documentary style. Choose from 35mm to 50mm lenses: those are general focal lengths. Most journalists in the 60ies had only a fix 35mm or 50mm since it is small, light, and can do all. I'd recommend a fix 35mm on a DX camera as a first lens for a beginner into more arty photography.

- You shoot portrait, choose from 50mm to 135mm lenses. They allow you to detach the subject by having him very sharp, and background very blurry.

- Everything beyond 135mm is a telelens, for wildlife, paparazzi and anything that takes zooming. If cheap, it's probably crap. Those are usually not very light sensitive, and if they are they cost a fortune. I don't see the use for it appart for wildlife.

- Last but not least, the greater the zoom range, the lower the quality; so avoid 18-300mm lenses.

One question comes along quite often:

"if I have the budget, should I get 2 prime lenses, 24mm for landscape and 85mm for portrait, or one 24-85mm lens?"

- Well it very much depends on what you value most: convenience, or image quality. If you travel and care about not having to change lenses all the time, go for the zoom. But in order to compare to prime lens quality, you'll need a top one, usually costing more that $ 1000.

If you value image quality, don't care much about transport or have 2 camera bodies, go for the prime lenses. Less flexibility, but better images, and the sum of the two might still beat the zoom lens.

Wide angles are ideal for landscape and architecture.

2/ Now choose the aperture

The "f" thing is the maximum aperture of the lens. See the iris thing in Stargate? Well lenses have such a thing inside that opens when you shoot to let the light in, and onto the film or sensor.

The wider it opens (low f numbers), the more light can get it, the more sensitive (we also call it fast) the lens is. Fast lenses cost more of course, but they allow great stuff such has very little depth of field (distance on which things are on focus) or low light photography.

An f1.8 lens is very sensitive. Less is very very sensitive and usually expensive.

Prime lenses (no zoom) can easily be f1.8 or F1.4, when the priciest zooms are f2.8 at best. Zooms are less light sensitive due to mechanical constraints.

Also take into account the camera body you have: if you have a top of the range leica or nikon, allowing shooting at 1600 or 3200 without quality drops, you might be ok with a f3.5 lens. Otherwise, I strongly advice you spend extra bucks on an F2.8 lens at least. You won't regret it and won't have to upgrade one year later.

An example of short depth of field with a fast lens, notice that the image is on focus on 15 cm, creating a 3D feeling.

3/ Extra features

Stabilization: useful for low aperture lenses. Since you can't shoot at high shutter speeds, you are likely to get some blur from shaking. This can then be a nice feature. On a high ISO performance camera, you can shoot fast, you don't need it

Lens diameter : the larger the lens, the more expensive the filters. If you're tight on budget, avoid 72 mm large (diameter of the lens) since the filters will cost $50 each instead of $15 for a smaller lens.

You now have the tools to decide. No lens can be perfect for every purpose. I will insist once again on this : the lens makes the quality, not the camera. Unlike the camera bodies that will keep improving a lot, lenses won't. Spend the money on the lens !

photozone.de is good advice when it comes to chosing lenses.