Photo Tips #8 : A Lens Primer (Part 2)

Posted on Sunday 2 October 2011

Last time we talked about lenses we covered length, zoom vs prime and different mounts and manufacturers. Today I’ll be talking about the other big differentiating factor – lens speed (all those weird f numbers). We’ll also cover some other features you’ll see talked about such as autofocus, image stabilization and macro.

Lens Types & Cost

After reading the last post on lenses you may be wondering why lenses cost such radically different amounts, even while the length is the same. As an extreme example, Canon currently make 4 different 50mm lenses:

  • EF 50mm f/1.8 II Autofocus Lens ($100)
  • EF 50mm f/2.5 Compact Macro Autofocus Lens ($300)
  • EF 50mm f/1.4 USM Autofocus Lens ($400)
  • EF 50mm f/1.2L USM Autofocus Lens ($1500)

The prices are approximate, but you get the idea. A 50mm prime could cost anything between $100 and $1500! So let’s take a look at what’s going on here. There are a few differences in the names of these lenses, so lets break one down – starting with the cheapest.

  • EF – means it fits a Canon EF mount
  • 50mm – the length
  • f/1.8 – the maximum aperture (or speed) – more on this later!
  • II – this is the second revision of this model
  • Autofocus – capable of automatically focusing, almost all modern lenses have this

Now if we compare to the next one, the speed is different (f/2.5 vs f/1.8) and it’s a “compact macro” (we’ll discuss macro later but it’s basically a specialty type of lens for taking close up photos of very small things). Comparing to the third lens, the speed is different again, and now we have “USM”. This stands for “Ultrasonic Motor” and it’s a Canon term for a particular type of motor used in the autofocus mechanism (the Nikon equivalent is SWM). A lens with USM will focus faster and more quietly than one without. Finally, onto the crazy expensive one. Notice the speed has changed again and now we have an “L” after it. The L is significant – it marks the lens as being from Canon’s professional series – these have a red ring painted on the end so you can tell them apart easily. The L series lenses have better build quality, better internal design and generally give much better images – as well as being much more expensive, larger and heavier.

Canon EF 50mm f/1.2L
Canon EF 50mm f/1.2L

Aperture/Speed

Although there were a few differences between those four 50mm lenses, the most significant is probably the varying speed (or maximum aperture). The aperture is simply a hole in the lens that the light goes through before it hits the sensor. Most lenses can adjust the size of this hole, to allow more or less light to pass through, and the number quoted is a measure of how large the hole can be on this particular lens.

Before talking about why a larger aperture is both good and expensive, let’s touch on shutter speed. When you press the button to take a photo, the camera allows light to hit the sensor so the image can be recorded. In an SLR this is done with a physical curtain called the shutter, normally it’s closed, when you want to take a photo it’s briefly opened. In order for the image to be of the correct brightness we need to carefully control how long the shutter is open for – too long and the photo will be too light, too short and it’ll be too dark. Obviously in dark conditions we will need to open the shutter for longer than in very bright daylight to get the right amount of light onto the sensor. So varying the amount of time the shutter is open for (the “shutter speed”) is one way we can compensate for the lighting conditions. But there’s a problem with using shutter speed on it’s own – movement. If the shutter is open for too long then any movement (either the subject moving or the camera itself moving) will cause the picture to be blurred. This isn’t an issue in nice bright daylight, but indoors or in dark conditions then it can become hard to open the shutter for long enough to get a well-lit photo without it being blurred.

This is where the aperture comes in. If more light can get through the lens (i.e. a larger aperture), you can have the shutter open for less time and the same amount of light will hit the sensor in total. This means that a larger aperture allows for a faster shutter speed to be used with the same end result. It should now be obvious why a lens with a large maximum aperture is useful – it allows for faster shutter speeds which reduces the likelihood of blurring, particularly in tough lighting conditions. As for why it’s more expensive, making the aperture larger greatly complicates the design of the lens if you want to avoid distortion. You need more glass, more gears and a more complex design.

Finally let’s talk about the actual measurements used for aperture. What does “f/1.8” mean compared to “f/1.4”? Put very simply, the smaller the number the larger the aperture, so the $1500 f/1.2 is larger than the $100 f/1.8 (you’d hope so, given the price!).

How much larger? Well that gets (a lot) more complicated. The f-number is a scale made up of values (called “stops”) derived from a geometric series. Whe you move from one stop to the next you double (or halve) the amount of light hitting the lens (and thus, halve or double the required shutter speed). Most DSLR cameras allow for adjustment of the aperture to one-third stop values, so moving three steps (one stop) will double or halve the shutter speed.

Full Stops1.01.42.02.8
One Third Stops1.01.11.21.41.61.82.02.22.52.8

We can tell from the table that f/1.2 is a full stop faster than f/1.8, so the expensive lens will let in twice as much light as the cheaper one, allowing for shutter speed that’s twice as fast to be used. This is a very significant difference, and it’s why a lens with a larger aperture is called “faster” – it allows for a faster shutter to be used.

shutter-vs-aperture-montage
Relationship between aperture and shutter speed

In the image above, we see the results of taking the exact same shot 4 times with different settings. First, in the top left, we have the properly lit shot according to the camera’s exposure meter, which gave us an aperture of f/4 and a shutter speed of 1/250th of a second. In the bottom right we see an equivalent shot using f/10 and a shutter of 1/50th of a second. f/10 is almost 3 full stops smaller than f/4, and so the corresponding shutter speed is significantly slower. The top right shows what happens if we use too long a shutter speed – 1/50 is far too slow for an aperture of f/4 so too much light is allowed in and the image is too bright. The bottom left shows too short a shutter speed – the aperture of f/10 is too small when used with the shutter of 1/250 – not enough light means a dark photo.

Note: There are other effects of changing the aperture (most significantly depth of field), but more on those later.

Macro

As previously mentioned, a Macro lens is one specifically designed to allow you to take photos of very small things (flowers, insects, etc). This is usually achieved by allowing the lens to focus on subjects very close to the camera – as all telephoto lenses have some degree of magnification this close-up view will magnify the subject producing the desired effect. The design changes needed to achieve this will cause a Macro lens to cost more than an equivalent non-Macro, as can be seen with the comparison of 50mm lenses above. The Macro at f/2.5 is more expensive than the f/1.8 which is significantly faster.

Image Stabilization

One of the reasons for wanting a lens with a large aperture is to increase shutter speed and so decrease blur caused by movement of the camera. Another way to do this is to use image stabilization which is a system that detects and compensates for the movement of the camera. This can be a nice alternative to sheer lens speed, as it’s typically cheaper and works reasonably well. In the Nikon system IS is referred to as VR (Vibration Reduction). The really important caveat is that IS/VR is useless when the subject is moving, it only compensates for movement of the camera itself. So if you want to get a nice clear shot of that speeding F1 car you’ll still need a fast lens.

IS-montage
The Effects of Image Stabilisation

That’s it for the lens primer, check back for more photo tips soon!


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