All About the Glass

While the camera itself is a technological marvel, the lenses we use are equally miraculous. And for more complex than meets the eye.

Lenses come in many different shapes and sizes. Well, shapes are usually just cylindrical, but you know what I mean. There are small ones, very big white ones, heavy ones, plastic ones, zoomy ones, expensive ones, and everything in between. Lenses made by the camera manufacturer. Lenses made by third parties.

What are the attributes of a lens? Let's look at the numbers. Grab your lens (or lenses) and look for the numbers that describe your lens. The numbers might be around the front element.

The lens I grabbed for this example is a Sigma 18-50mm. My lens is made by Sigma; yours might be Canon or Nikon or Tamron or someone else.

18-50mm tells me two things. First, this is a zoom lens because it has a range of focal lengths. It goes from 18mm, which is quite wide angle, to 50mm, which is roughly equivalent to the way our eyes see the world. Photos shot at 50mm will look "normal". So this lens zooms from wide to normal.

The next number is 1:2.8. Remember Aperture is a ratio, so this is just another way of writing f/2.8. The biggest Aperture this lens is capable of is 2.8. Cool, that's quite wide, and it will help me get quite shallow depth of field, and I can shoot in lower light situations. Now, f/2.8 is just the widest this lens will do. I can still set it to smaller apertures using the settings on the camera.

Next is a bunch of letters that tell me about the quality of the glass and the type of coatings they used on the glass. Yay! Not particularly useful. You may have letters like "IS" or "OS" or "VR" - these all mean pretty much the same thing: your lens has some very clever technology built in to cancel out any shake in your hands while taking a shot. Is this gimmickry, or does it actually work? Usually, it works quite well. Does it do away with the need for a tripod? No, it doesn't work that well. It's technology, not magic.

And last is the filter thread diameter - 67mm. If you look carefully at the front of your lens, where the front element meets the barrel, you'll see there is a screw thread. This is where you can screw in a filter. When you need to know what size filter to buy, this is the number you need. But you probably won't need any filters except in some very rare scenarios. I'll talk about filters another time.

Here is another example: Canon 70-300mm 1:4-5.6 IS USM. Easy enough: It's a zoom lens made by Canon. It can zoom from 70mm, which is just a bit beyond "normal", to 300mm, which is what we might call "telephoto".

The next bit gets a bit trickier.  Notice the Apeture is stated as a range.  At 70mm, the maximum aperture will be f/4, but as we zoom in the aperture will get smaller, so that by the time we are at 300mm it will be f5/6. It's quite hard, and expensive, to design lenses that can zoom and retain the maximum aperture all the way through the zoom range.

We've covered IS already, and USM just describes the type of motor that drives the autofocus. Kind of like "GT" on a car. Just a bunch of marketing letters.

And this lens has a 58mm filter thread.

One more example: Canon EF 50mm 1:1.8 II Ø 52mm
Yes, that's right, it is the famous Nifty-Fifty! 50mm prime (not zoom), capable of f1.8, and this is the mark 2 version, with a 52mm filter thread.

Your Challenge

Take two photos of the same subject such that the subject is the same size in both photos. But use the extreme ends of your lens focal length range - very wide and telephoto.