Canon EOS 70D: The Guide to Understanding and Using Your Camera (2014)
Chapter 18. Considering Lenses
When the EOS 70D was announced, Canon identified 103 lenses as compatible with the Dual Pixel CMOS AF system introduced as a major feature of the camera. There are dozens of other specialty lenses or older lenses that cannot operate in the automatic focusing and automatic exposure world of today’s lenses, but can be quite effective with manual adjustments on the EOS 70D.
Introduced in 1987, the EF lens system was Canon’s attempt to eliminate mechanical linkages between the camera body and the lens; the mechanical links were completely replaced with electrical communication between the camera and lens. Most of those older EF lenses can still be used, albeit manually, on the EOS 70D.
The EF lens series was designed for the 35mm film cameras of that date. Full-frame Canon bodies such as the EOS 6D, the EOS 5D Mk III, or the EOS 1D X require an EF lens because it provides an image large enough to fill their full-frame image sensor.
The popularity of the smaller APS-C cameras, such as the Rebel series and the EOS 40/50/60/70D cameras, created the need for high quality lenses that only had to fill the smaller APS-C image sensor. Thus, the EF-S line of Canon lenses was born.
The bayonet mounting system is the same for both the EF and the EF-S series of lenses. Though either an EF or an EF-S lens can be used with your EOS 70D, you should never attempt to attach one of your EF-S lenses to a friend’s full-frame body (or even your own...). The EF-S lenses are designed for smaller-dimension sensors, so the rear lens elements project further into the camera body than an EF lens, causing mechanical interference as the mirror attempts to sweep up out of the light path. This can result in significant damage.
EF vs. EF-S Lenses
Very simply put, EF lenses are designed for full-frame cameras, but can be used very effectively on APS-C cameras, such as the EOS 70D. However, EF-S lenses are designed for the APS-C DSLRs, and cannot be used on full-frame cameras.
The EF lenses cover a quality gamut from good to excellent. The L-series lenses are excellent, but expensive. These L-series EF lenses are visually identifiable by the bright red ring just behind the filter-mounting ring. You can expect these lenses to have the highest quality materials and workmanship, and to deliver the best in image detail.
But for most of us, that level of excellence may not be either necessary or affordable, and if we’re working with an APS-C camera (such as the EOS 70D), the EF-S class of lenses still offer high quality, but at a much lower price. The smaller image sensor introduces yet another interesting effect: the incoming image from any EF or EF-S lens mounted on an APS-C body will be effectively cropped, making the lens behave as though its focal length were 60 percent greater than marked. You can accomplish the same magnification of a 320mm lens by using a 200mm EF or EF-S lens on the EOS 70D. The downside is that on the EOS 70D (or any APS-C body), your $1700 EF 16-35mm f/2.8L II USM will behave as a 25-56mm lens, not the nice super-wide you paid for.
When Canon announced their mirrorless interchangeable-lens camera (MILC), the EOS M, they also introduced a new class of lenses appropriate for that camera, the EF-M series. No lenses in the EF-M series can be used on either full-frame or APS-C DSLRs, although Canon does provide an adapter that allows attaching EF and EF-S class lenses to the EOS M cameras.
UltraSonic Motor (USM) Lenses
Ultrasonic motor (USM) lenses appeared in 1987. Canon was the first camera maker to successfully commercialize the USM technology. EF lenses equipped with USM drives have fast, almost silent, and precise autofocus operations, and consume less power compared to other AF drive motors.
There are two types of USMs: the ring-type USM and the micromotor USM. Ring-type USMs allows for full-time manual focus (FT-M) operations without switching out of AF mode. Micromotor USMs are used to bring down the cost of the lens. It is possible to implement FT-M even with micromotor USMs; however, it requires additional mechanical components, and the vast majority of micro-USM lenses do not offer such capability.
Stepping Motor (STM) Lenses
Stepping motor (STM) lenses were first announced in June 2012. Three EF-S lenses and one EF lens featuring this technology are now available: the Canon EF-S 18-55mm f/3.5-5.6 IS STM (available as a kit lens with the EOS 70D), the EF-S 18-135mm f/3.5-5.6 IS STM superzoom (also available as a kit lens with the EOS 70D), the EF-S 55–250mm f/4-5.6 IS STM, and the Canon EF 40mm f/2.8 STM pancake lens.
Canon claims that this technology allows smooth and silent autofocus, and with compatible bodies (such as the EOS 70D), will provide continuous autofocus in Live View and video. Unlike USM, STM lenses use focus-by-wire to enable full-time manual mode, which means that rather than manually moving lens elements, rotating the focus ring commands the stepping motor to change focus.
Image stabilization (IS) technology detects camera motion and optically corrects for it. It only corrects for handheld motion; if the subject of the photograph is moving, IS will not stop it. It also can only stabilize so much motion, with expanded slow shutter speeds ranging from two to five stops, depending on the specific IS in the lens. Canon has released several versions of the IS system, including the following:
The first version, first used in the 75-300mm lens (1995), takes approximately one second to stabilize, provides approximately two stops of stability, and is not suitable for use on a tripod or for panning.
IS Mode 2, released in 1997, detects whether panning is taking place horizontally or vertically, and only compensates for motion in the plane perpendicular to the plane of panning.
In 1999, tripod detection was added, so that the lens could be used on a tripod with IS turned on.
In 2008, a new version of IS was released which allows up to five stops of stabilization.
In 2009, the Hybrid Image Stabilizer was introduced. In addition to correcting angular movement, Hybrid IS also corrects for shift movement, a significant benefit for macro photography.
In 2011, IS Mode 3 was added. This mode is similar to (and in addition to) Mode 2, except that stabilization is applied only when the shutter is released.
Some newer lenses include an Image Stabilizer that can automatically detect whether the user is panning and respond accordingly; these lenses do not have an IS mode switch.
All EF and EF-S lenses that support IS have the words “Image Stabilizer” written on the lens.