The Digital Photo Guy

Deciphering Lens Specifications

by on Jan.23, 2011, under gear, Monday Morning Tips

What Do All Those Numbers and Letters Mean?

Each time I teach a class, I’m reminded that photography, like most disciplines, has its own strange jargon, acronyms and codes. This article hopes to be the “secret decoder ring” for Canon lenses.

Let’s take a typical Canon lens like the venerable EF28-135/3.5-5.6IS USM. EF stands for Electro-Focus, Canon’s trademarked term for an electronically controlled lens focusing system. The EF is usually left off unless it it the EF-S, a version designed specifically for Canon’s crop sensor line of entry level to mid-range digital cameras (all Digital Rebel variations and xxD series and 7D). EF-S lenses cannot be used on the 1D, 1Ds or 5D. Nikon uses DX to identify its lenses for crop-sensor digital bodies.

Next, the numbers 28-135 indicate the focal length range of the lens ON A FULL FRAME BODY. On a crop frame bodies such as all Digital Rebels and xxD/7D series, these numbers are multiplied by 1.6 to get the true field of view. Therefore, on a Canon Digital Rebel T1i or 7D, the actual field of view range is 28×1.6=44.8mm to 135×1.6=216mm. In 35mm photography, a 50mm lens is considered “normal” in that it closely matches the field of view of a human eye. So, the 28-135 ranges from just about normal to nearly 5x normal. For Nikon crop frame bodies, use 1.5 as the crop factor.

********************Commercial Break********************

Photoshop Elements in 6 Weeks, starts Feb 16
Flash Photography in a Flash, Feb 26
Palomar College – Spring 2011
Spring Desert Wildflowers, late March 2011
Free Photo Critique

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The numbers after the forward slash (/) indicate the aperture range of the lens. In this case, the maximum aperture at 28mm is f/3.5 and f/5.6 at the 135mm telephoto end. This is important for several reasons. Most consumer and prosumer cameras can’t autofocus with lenses slower than f/5.6 while pro bodies usually AF down to f/8.0. This means the Canon 28-135/3.5-5.6 can AF throughout its 28mm to 135mm focal length. Be wary of 3rd party lenses that have a range like 4.5-7.1. Such a lens can’t AF at the long end.

Moving on, in Canon lenses, IS stands for Image Stabilization, a technology pioneered by Canon and now emulated by virtually every lens manufacturer. Nikon’s version is VR for Vibration Reduction while Tamron uses VC (Vibration Compensation), Sigma is OIS (Optical Image Stabilization) and my favorite, Sony SSS (Super Steady Shot or Super Silly Superlative, take your pick.) Note that not all IS is equal. In Canon lenses, there are, at least, five generations of IS. The earliest version in the 28-135 was capable of about 2 stops of stability. This means that if you’re usually capable of handholding at 1/30 second, this version will let you handhold down to about 1/8 second. The latest versions introduced in 2008 and 2009 enable up to 5 stops so 1/30 now becomes 1 second, a bit of a stretch for all but the steadiest of hands.

Finally, USM in Canon lenses can mean whatever Canon marketers want it to mean. USM (UltraSonic Motor) comes in two flavors, micromotor and ring-style. Micromotor is exactly what it sounds like, a small electric motor built right into the lens. This is slower, less precise and less flexible than ring-style. Given these “features,” it’s not surprising that micromotor USM is used in lower cost, consumer lenses. Ring-style USM, on the other hand, is a marvel of engineering. It is fast, precise and has FTM (Full-Time Manual). FTM allows one to override the autofocus at anytime without pressing buttons, switching controls or jiggling switches. This is very nice when AF misfocuses and you need to nudge the focus. Nikon calls their version SWM for Silent Wave Motor. Again, Canon developed much of the original technology and holds many of the early patents on USM.

Workshops and Webinars

Photoshop Elements for Digital Photographers, Six Weeks starts Feb 16

In just 6 weeks (1 hour each week) learn to enhance and edit digital photos using Photoshop Elements (PSE). From the comfort of your own PC or Mac, attend via webinar every Wed evening, Feb 16-Mar 23 from 7PM-8PM. This intermediate level class is for photographers who want to jumpstart their photo editing skills. All techniques in this class also apply to Photoshop. Start with an overview of a basic workflow then dive into Selections, Cloning, Healing, ACR (Adobe Camera RAW), HDR (High Dynamic Range), plug-ins and output. Register HERE.

Flash in a Flash, Sat, Feb 26, 2011, 10AM-1PM

If your flash baffles and scares you, this workshop is for you. Learn to take control of your external flash (aka, Speedlite, Speedlight, strobe, etc) and make great flash photos that don’t scream, “This deer-in-the-headlights look created with FLASH!” Learn what all the knobs, dials, switches and menus do on the back of your expensive flash. Register HERE.

Palomar College – Spring 2011 Schedule Posted

Palomar College has posted my Spring 2011 classes. I’ll be teaching both Digital SLR for New dSLR Owners (Feb 1 & 3) and Beginning Adobe Photoshop Elements (Mar 1 & 3) via webinar so you can learn from the comfort of your own home computer. Back by popular demand is the my popular Hands-On Photoshoot (Apr 2) at Kit Carson Park in Escondido.

Spring Desert Wildflower Workshop, late March 2011

One of my perennial favorites, spring is a magical time in the desert when everything comes to life. Get down low for amazing macros of flowers. Get up high for gorgeous sunsets and landscapes. Make beautiful photos from almost every vantage point in the Anza-Borrego Desert State Park. More details as conditions in the desert firm up and I can better predict the spring bloom.

New Benefit for Students

Current and former students of The Digital Photo Guy can now receive free critiques of their work to judge their progress. Once a month, send me a recent photo for an in-depth analysis of what works, what doesn’t work and how you can improve the photo next time.

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2 Comments for this entry

  • Karen

    Great info…I learned some new things. I do have a question regarding the full-time manual focus on Canon lenses. What happens if you try to tweak the focus on one that isn’t full-time and it isn’t set on MF? Example: the EF-S 55-250IS zoom. ?Bad for lens? Thanks for your interesting info/tips.

    • Lee

      Hi Karen,
      Thanks for reading and, especially, commenting. Good question! As usual, the answer is, “It depends…” The focus ring on most micro-motor lenses won’t turn while you have it in AF mode. In fact, forcing the ring will strip gears and wreak havoc on the lens. However, some lenses such as the Canon 50/1.4 are equipped with a slip clutch to allow a “faux FTM” as it were. It’s cheaper little brother, the 50/1.8, has a true micro-motor that won’t allow the focus ring to be turned manually while in AF Mode. BTW, be aware that there’s a difference between USM micro-motor and an older micro-motor. Stay away from lenses with the older micro-motor.

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