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Lens Notation Primer

Posted by Mario Stylianou Categories: Photography

As part of my series on buying entry-level dSLRs and lens buying, you may come across notations on a lens that are unfamiliar to you. If this all seems familiar, move on to my dSLR and lens guide. If not, read on to get a handle on what I mean.

Canon EF Lineup 2013

Canon EF Lineup 2013

Focal Length — wide-angle, telephoto, mm

The millimeter (“mm“) number refers to magnification in the focal length (how close or far a lens captures a photo) of a lens. Low numbers mean you can capture a wide field (“wide-angle“) of vision (sometimes more than regular human vision). High numbers mean you can zoom in on something far away (“telephoto“). If you like taking photos of landscapes or people in a small room, you’ll want a lens that goes down pretty low (24mm and less) into the wide-angle territory. If you like zooming in on stuff far away, you’ll want a lens that goes pretty high (100mm and more) in the telephoto category. Some lenses have a range that’s only low (10-22mm), some high (70-200mm), and some have a big range (18-135, 28-200). Lenses that go to a high focal length (super telephoto) are usually more expensive than lenses that go low focal length; also they’re bigger and heavier.

Zoom vs. Prime

The lenses I specified above are “zoom” lenses because you can zoom in and zoom out with them. Some lenses are specified with a single number (e.g. 50mm) and they’re called “prime lenses. With those, you zoom with your feet! To recompose a shot, you walk closer or farther away. While inconvenient, they are cheaper, lighter, and higher image quality. Of course, your eyeballs don’t zoom because they are prime lenses and you’ve probably been making do with those for a while.

  • Note: The zoom notation does not necessarily mean that they are good for zooming in on things far away. A 10-22mm is a zoom lens that only captures wide-angle photos (you can’t capture a bird far away). A 70-200mm is a lens that only captures telephoto images (you can’t capture a room full of people).

Aperture

The aperture notation on a lens dictates how wide the diaphragm in the lens opens up to let in light. It is written out with an F (e.g. “f/5.6”). The more light it lets in, the easier it is to take a low light shot. For every increment the f-stop number goes down, you double the light that goes in. For example, when taking a photo at an aperture of 2.8 and a shutter speed of 1/20th of a second, to get the same looking photo with a lens that only goes to 5.6 (it opens up only a quarter as wide — see link above for reference), you’d need a shutter speed of 1/5th of a second. And let me tell you, despite telling myself otherwise numerous times, the human hand rarely takes a steady shot at 1/5th of a second. It will be blurry because of unavoidable hand shake. Essentially, without boosting the ISO (see below) or having a lens with image stabilization (IS) you won’t get the photo to come out well with 5.6 but you will at 2.8. Lenses with low apertures are thus more desirable as they prevent blurry photos but also more expensive / heavy.

  • Zoom lenses occasionally have a range of minimum apertures (e.g. 18-55mm f/3.5-5.6). What that means is that it opens up to f/3.5 at 18mm but it can only open up to f/5.6 at 55mm. In between, it’ll be something between 3.5 and 5.6. Some zoom lenses have a single, fixed minimum aperture. All prime lenses have fixed minimum apertures because they don’t zoom.
  • Plan on shooting with a tripod all the time? Aperture isn’t as important since the camera will be completely stationary. On the other hand, if you are like me and think toting a tripod on your back absolutely everywhere you go for photos is crazy talk, aperture remains important.
  • Maximum apertures are rarely listed outside of the technical specifications as they’re rarely used in practice.

Image Stabilization (IS)

IS stands for image stabilization; very handy because it compensates for the unsteady hand effect I just mentioned. It’s not a silver bullet but the technology allows for significantly better shots. It is also cheaper to get a lens with IS than a lens with a larger aperture. Taking a shot at a shutter speed of 1/5th of a second with IS means it’s much more likely to come out passable than without. Of course, IS only compensates for your hand shake. If your subject is a moving pet or kid indoors, you still need a low aperture with a fast shutter speed to freeze the action or you’ll get motion blur as they travel across your photo. For non-action shots, having IS is as good or better than a low minimum aperture.

  • IS technology is sometimes described in reviews in terms of how many f-stops it can simulate. Typical technology compensates for 2 f-stops (e.g. 5.6 to 2.8) but some can do 3 f-stops (e.g. 4 to 1.4).

Ultrasonic Motor (USM)

With an ultrasonic motor (“USM“) autofocusing becomes significantly faster (especially in high action situations like people dancing or playing sports), significantly quieter (photographing people without drawing attention, not tipping off animals with focus whizzing noise), and you can manually focus the lens without having to flip the focus switch from Autofocus (AF) to Manual. If the switch isn’t flipped to manual, manually focusing a camera while on AF without USM technology causes damage. It’s handy but I don’t manually focus very often and autofocus works just fine for me most of the time. The split second improvement in focusing speed is noticeable but, personally, the biggest benefit from USM is when I hand my camera to someone else, I don’t have to worry about them accidentally rotating the focus ring instead of the zoom ring and damaging my lens.

Stepper Motor (STM)

STM stands for Stepper Motor and is a applied to a new range of Canon lenses which feature a new design of focus motors which, along with a new iris mechanism are designed to eliminate (auditory) noise during video recording. It’s a more precise version of a regular focusing motor but still has the same direct connection to the lens focus group, which means manual focus has to be implemented using a focus-by-wire arrangement whereby moving the focus ring by hand sends a signal to the motor to move the focus group. In comparison, a USM consists of a pair of concentric rings which vibrate at high frequency to rotate back and forth, an arrangement which permits the user to move the focus ring to directly move the lens element, achieving full time manual focus without damaging the motor. Stepper motors are better at producing smooth, precise incremental movements, such as those required by contrast detect AF and AF during video. Ultrasonic motors are better at jumping to the right focus point as part of a phase detection system. Unless you take a lot of video with your camera, STM will not make a big difference. In my videos, I’ve never noticed the focusing motor noise from my non-STM lenses and my subjects don’t move out of focus range but your mileage may vary.

Less important:

Macro

All lenses have a minimum focus distance. Anything closer and the lens will not be able to autofocus on it clearly and the camera won’t snap the photo (unless you switch it to Manual focus and force it, but then you’d still have a blurry photo). It’s rare that you run into an issue with minimum focusing distance unless you’re trying to take a big photo of something like a flower or insect (i.e. fill up the whole photo with it). However, if that’s something that you’re interested in, pay close attention to the minimum focusing distances. If they’re too long, you’ll need to get a “macro” lens which has a shorter focusing distance. Half of them are fairly affordable at entry level prices.

Diffractive Optics (DO)

A rare few telephoto lenses come with diffractive optics (“DO“) in their name. It denotes that the lens is using lighthouse technology (a Fresnel lens construction) and a special lens coating to dramatically reduce the amount of glass required for manufacturing compared to a conventional lens. The practical takeaway is that DO lenses are dramatically shorter in length and much lighter. Canon is working on making more lenses like this as there are only two now and neither are entry level.

Mario Stylianou

One Response so far.

  1. […] about lenses and words like focal length and aperture aren’t in your vocabulary, see my lens notation primer before reading […]


 

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