|Introduction | Part 1: STM series | Part 2: Best performing Primes and Zooms | Part 3: Best performing wide angle and telephoto lenses|
Ahead of our comprehensive lens recommendations for the Canon EOS 70D, we thought we would take a look at the optical quality of Canon’s new STM lenses, which with their smooth practically silent focus movements seem well suited to the video oriented features of the camera. Read onto find out how well each of the three lenses in the range perform.
With the introduction of the EOS 650D / Rebel T4i in June 2012, Canon announced two lenses bearing the new ‘STM’ moniker, the full-frame EF 40mm f/2.8 STM and EF-S 18-135mm f/3.5-5.6 IS STM. The STM designation refers to a stepper-motor mechanism, which is sometimes referred to as a pulse motor. These models are designed to complement the existing range but eschew the firm’s ultrasonic AF motors, which were designed to refocus quickly with large movements then make smaller focus corrections. This tends to result in irregular, hesitant focus movements that’s of little consequence with stills but is a somewhat unsettling experience during video-capture.
In conjunction with guide bars, the stepping motor moves the focus lens using a lead screw and rack which results in very smooth, precisely controlled movements (in the order of microns) and is especially beneficial during AF tracking during video capture. With no gears (found on the firm’s Micro USM motors, for example), low operating noise is another advantage.
Canon aren’t the first to adopt this type of AF technology; it’s used widely in mirrorless camera systems but with the adoption of Hybrid AF systems on the EOS 650D and latterly the EOS 700D, EOS M and EOS 100D, and now the dual pixel AF CMOS found on the EOS 70D, the STM lenses should be well-matched.
Although a full-frame model this so-called ‘pancake’ lens is the first and only prime bearing the STM designation to date. With its 6 element, four group design including one aspherical element for reducing aberrations it’s an unusually complex for an ultra-compact model measuring just 22mm in length and weighing 130g. As a 60mm equivalent on the EOS-70D it is somewhere between a ‘normal’ and short-telephoto lens, however its portability and imaging characteristics, not to mention the accessible price (around $249) make it an attractive option.
With a DxOMark lens score of 19 points, the 40mm f/2.8 is the highest performing of the three STM lenses announced thus far. Sharpness is good though on the EOS 70D it isn’t as consistent across the frame as it is been noted on full-frame cameras though centrally sharpness remains high. Distortion is very low and vignetting mainly at full aperture is also low, but chromatic aberration isn’t particularly well controlled.
As a ‘kit’ lens the 18-55mm is less compelling than the 18-135mm f/3.5-5.6, which was launched alongside the 40mm f/2.8 but as around $149 when bought with the camera (saving $100 if bought separately), it may be a useful addition. This model has image stabilisation and takes 58mm filters like its DC motor driven sibling,but it has more complex optical construction of 13 elements in 11 groups and an inner focus mechanism that avoids unwelcome extension or the rotation of the front element, perfect for polarisers or square filters.
In terms of optical quality this lens is very close to the overall performance (and ranking) of the DC motor version but there are some improvements thanks to the more complex optical formula. Sharpness,however, isn’t one of the highlights. Resolution is at its highest at the wider end of the range of focal lengths but it has disappointing uniformity through the range. Not only that, but distortion (particularly at 18mm) and chromatic aberration are higher than we would like to see.
This lens was announced with the 40mm f/2.8 and, by virtue of the wide range of focal lengths, is the most versatile of the three. As with the 18-55mm, it’s purpose designed for the APS-C format while quite a large and heavy lens by comparison (it weighs 480g and measures 76.6x96mm un-extended) it is the equivalent to a 29-216mm.
As well as featuring image stabilisation this lens unusually has one UD glass element (for reducing chromatic aberration, but also sports one aspherical lens in its 16 element / 12 group construction. This lens is also available with the Canon EOS 7D as part of a kit, saving about $200 on the regular price of $549.
With an overall DxOMark lens score of 13 points this lens is a similar performer to the 18-55mm, which is good considering the wider range and extra flexibility that brings. As with the 18-55mm sharpness is on the low side, just 9P-Mpix out of potential 20P-Mpix on the EOS 70D, and suffers from the same poor uniformity across the imagingfield. At 18mm, distortion is quite strong and chromatic aberration is noticeable if you look for it (though it is lower than the other STM kit option). Overall, this is the lens to go for between the two.
If you have a Canon EOS 70D and a favorite lens, we would very much like to hear from you. Please leave a comment below, stating what lens it is and why you like it.