August 2012

EF-S 18-135mm f/3.5-5.6 IS STM / EF 40mm f/2.8 STM

A powerful, easy-to-handle zoom lens for more enjoyable movie shooting, and a high-quality prime lens that is the thinnest and lightest in the EF series

Photo 1: EF-S 18-135mm f/3.5-5.6 IS STM

Photo 1: EF-S 18-135mm f/3.5-5.6 IS STM

Photo 2: EF 40mm f/2.8 STM

Photo 2: EF 40mm f/2.8 STM

In this month's technical report, we present two new EF lenses that went on sale in June: the EF-S 18-135mm f/3.5-5.6 IS STM (Photo1) and the EF 40mm f/2.8 STM (Photo 2).

(1) EF-S 18-135mm f/3.5-5.6 IS STM*1

Development aims

This lens is a powerful, high-performance standard EF-S zoom lens that was developed to be a perfect match for the EOS Kiss X6i, which was announced at the same time. As its name suggests, the lens features an image stabilizer and utilizes a stepping motor as the actuator that drives the focus lens.

In recent years, a multitude of users have taken advantage of the movie shooting function on our EOS cameras. However, we wanted to provide more casual users with the easy enjoyment of shooting in a diverse range of styles. Therefore, we made improving the functionality and performance of movie shooting, alongside still-image shooting, our primary development aim.

The stepping motor's inclusion was a large contributing factor in reaching this goal.

The next sections detail lens technology improvements in the optical, mechanical, and electronic areas.

*1. STM stands for "stepping motor".

Feature 1 - New six-group zoom lens design

Figure 1: Cross-section view of the EF-S 18-135mm f/3.5-5.6 IS STM optics

Figure 1: Cross-section view of
the EF-S 18-135mm f/3.5-5.6 IS STM optics

This lens's most striking feature when attached to the EOS Kiss X6i is how quietly and smoothly it maintains focus on moving subjects in Movie Servo AF mode. This performance is the result of the newly designed lens focus mechanism, which includes a stepping motor, and a newly developed six-group zoom system.

The newly developed six-group zoom lens, capable of delivering stunning image quality, consists of 16 elements in 12 groups (Figure 1). Each group has been positioned for optimal magnification, furnishing the lens with its 7.5x zoom magnification and 18-135mm focal length (equivalent to a 29-216mm 35mm-film focal length). The group positioning also enables both the lens image stabilization functionality and specialized AF functionality for movie shooting.

The inner focusing system of the lens is designed so that just the fourth group's single negative lens element moves to achieve focus, which helps boost Movie Servo AF performance.

The EF-S 18-135mm f/3.5-5.6 IS STM achieves better image quality at all zoom positions than previous models thanks to the inclusion of a high-performance aspherical element in the final lens group, which corrects for aberrations that occur while zooming, and the placement of an UD element in the front lens group, which corrects color aberrations (Figure 2).

Figure 2: MTF characteristics of the EF-S 18-135mm f/3.5-5.6 IS STM

Figure 2: MTF characteristics of the EF-S 18-135mm f/3.5-5.6 IS STM

How to read MTF charts
Bold lines: 10 lp/mm, fine lines: 30 lp/mm. Black lines are the MTF characteristics at full aperture; the blue lines at f/8. Solid lines represent sagittal MTF and the broken lines represent meridional MTF.

Compared to the current EF-S 18-135mm f/3.5-5.6 IS lens, the main optical design improvements are higher resolving power, a five millimeter reduction in overall length, shorter minimum focusing distance (from 45 centimeters to 39 centimeters), and a more powerful maximum magnification (from 0.21x to 0.28x). We also increased the number of aperture blades from six to seven to render more nearly circular bokeh out-of-focus effects that are key to many creative photographic expressions.

One of the lens' biggest features is hidden in the newly developed six-group zoom lens design. This is the electronic cam (also known as computer zoom (CZ)), which drives some of the lens groups, and is frequently used in Canon digital video camera lenses. A compensator lens maintains the in-focus plane on the imaging sensor plane while zooming to improve AF performance during movie shooting. For this lens, however, we switched the compensator lens drive method from the previous method, using a cam ring mechanically linked to the zoom ring, to a method that drives lens groups individually with actuators.

The new zoom system on EF-S 18-135mm f/3.5-5.6 IS STM is a hybrid of the characteristic EF-lens mechanical zoom lens drive and an electronic drive (CZ drive). This hybrid system allowed us to remake the lens into the ultimate zoom lens that retains the familiar zoom control feel while adjusting the AF based on an in-focus signal from the imaging sensor even when the camera's mirror is locked up during movie shooting.

Feature 2 - Stepping motor*2 and a new focus mechanism design

Figure 3: Sectional view of the focus mechanism

Figure 3: Sectional view of the focus mechanism

Users have been dissatisfied with the lack of smoothness, the operating noise, and the AF tracking performance of the focusing drive on past EF lenses when shooting movies. Therefore, in order to provide a better movie-shooting experience, we overhauled the mechanical drive mechanism in tandem with the new optical system's development.

Two guide bars direct the movement along the optical axis of the fourth lens group's focus lens - which functions both as a focus lens and as a compensator lens while zooming (Figure 3).

The focus lens is driven by a stepping motor connected directly to a lead screw. The rotating lead screw meshes with a rack on the focus lens barrel that creates a drive force along the optical axis.

This mechanism has two advantages.

  • The guide-bar support is well suited to precise, minute focus lens movements because there is very little stress put on the lens barrel's movement.
  • Due to its simple construction, the stepping motor-rack mechanism immediately converts stepping motor rotations into a precisely controlled, direct drive force.

The mechanism described above enables the focus lens to make quick movements when acting as a compensator lens during zooming and to make very precise movements on the order of microns while autofocusing. The mechanism both quiets and accelerates autofocusing, compared to previous lenses, even when shooting still images with a current APS-C size camera.

In addition to these mechanical changes, an electronic ring system was introduced as a replacement for the previous mechanical focus ring.

On this lens, the focus lens is independently controlled and driven with the mechanism described above. But this makes it difficult to design a mechanical linkage from the focus ring to move the focus lens. Therefore, the focus ring's rotation is detected electronically, and a controller, based on sensor signals, directs the stepping motor, moving the focus lens. Although this electronic ring system does not have any mechanical coupling, fine control and smooth focusing adjustments are still possible because of the focus ring's smooth mechanical movement and the optimized control system. Also, full-time manual focusing remains for making fine adjustments while the shutter button is held halfway down after autofocusing has finished when in the auto focus mode (One Shot AF).

*2. A stepping motor is a motor that is synchronized to a pulsed drive voltage. For each pulse signal it rotates one step. For this reason, it is also called a pulse motor. Stepping motors are noted for their responsive starting and stopping characteristics and level of fine control.

Feature 3 - New control engine manages all functions and performance

We dramatically boosted the control circuitry's processing power to support the electronic cam system's zoom lens and the electronic ring system's focus ring as well as to facilitate other new functions and reach new performance levels, including greatly improving the AF performance while shooting movies.

By adding a new co-processor to the CPU, the lens's control engine can process multiple functions in parallel by sharing work with the CPU.

This parallel processing, working behind the scenes, is what permits the very quiet, fluid Movie Servo AF performance of the lens, and maximizes its functions and performance, including the previously mentioned new functions.

Feature 4 - Excellent image stabilization suited for movie shooting

Photo 3: Dedicated EW-73B hood for the EF-S 18-135mm f/3.5-5.6 IS STM

Photo 3: Dedicated EW-73B hood for
the EF-S 18-135mm f/3.5-5.6 IS STM

The lens uses a shift-type image stabilizer that shifts part of the fifth lens group (elements 13 and 14) perpendicular to the optical axis to counter camera shake. The image stabilizer provides a stabilization gain equivalent to about 4 f-stops in shutter speed. A function detects from a vibration gyro's output signal whether the camera is panning or held still and automatically switches to the best image stabilization mode for each situation.

The lens is also equipped with the new Dynamic IS function,*3 which expands the angular range of the IS correction when shooting movies.

Dynamic IS is useful when recording while walking because it corrects camera shake over a wider angular range than when shooting still images at the wide-angle end.

Other features include a zoom ring lock lever that prevents the zoom from extending while attached to a strap-held camera. The lens also accepts the petal-shaped EW-73B hood with a bayonet mount (sold separately) (Photo 3).

*3. Included for the first time on an EF lens. This function operates only when shooting movies.

This report has focused on the lens's movie shooting functions and performance, but our designers did not overlook upgrades to its still image functions and performance. Attach the EF-S 18-135mm f/3.5-5.6 IS STM lens to your APS-C size camera, and you will discover a new, advanced zoom lens for outstanding utilitarian use, with enhanced image quality, and a faster, quieter AF than previous lenses.

When combined with the EOS Kiss X6i, you benefit from 100 percent of the lens's functionality. Pick up this new camera-lens combination and enter the new, engaging world of EOS movies.

(2) EF 40mm f/2.8 STM

Development aims

The EF 40mm f/2.8 STM has such a low profile that you will barely notice it when carrying your camera. It was developed for taking casual snapshots without intimidating subjects with a large lens. Easy for everyone to use and delivering excellent image quality, this unprecedented compact pancake lens is our thinnest and lightest EF lens.

The following sections detail its features.

Feature 1 - Excellent image quality from the thinnest, lightest EF lens

Figure 4: Cross-section view of the EF 40mm f/2.8 STM optics

Figure 4: Cross-section view of
the EF 40mm f/2.8 STM optics

Figure 5: MTF characteristics of the EF 40mm f/2.8 STM

Figure 5: MTF characteristics of
the EF 40mm f/2.8 STM

The lens's most obvious feature is its shape and size: a diameter of 68.2 millimeters, a length of 22.8 millimeters,*4 and a weight of approximately 130 grams, making it the thinnest and lightest in the EF lens series.

The optical layout*5 has been revamped and the latest optical design techniques, including an aspherical lens, have been used to form an extremely compact, high-quality 40mm f/2.8 optical system.

The lens has six elements in four groups (Figure 4), with a diameter nearly the same as the 23.5-millimeter-size of a 10-yen coin and an overall glass thickness of just 24 millimeters. The positioning of a GMo aspherical lens element in the rear lens group corrects aberrations and renders sharp images from the frame's center to the edges while keeping the overall size compact (Figure 5).

All lens groups are extended to achieve focus. The minimum focusing distance is 0.3 meters and the maximum magnification is 0.18x. Lens placement and optimized coatings both control flaring and ghosting and achieve excellent color balance. The seven-blade circular aperture produces beautiful bokeh out-of-focus effects.

*4. The dimension measured from the mount surface to the leading lens surface.
*5. Replacing the gauss lens with a front concave-convex group was key to making the optics more compact because the group fits closely to the second concave group.

Feature 2 - Stepping motor

Photo 4: Gear drive assembly

Photo 4: Gear drive assembly

Like the EF-S 18-135mm f/3.5-5.6 IS STM, the EF 40mm f/2.8 STM has a stepping motor to drive the focus lens. Because compactness was the main priority, however, this lens uses a gear drive (Photo 4) instead of the lead screw and rack mechanism described above, which offers better quietness and smoothness. A stepping motor was chosen because stepping motors and their peripheral units, including the speed-reduction mechanism, can be made smaller than DC motors. The gear linkage's final output gear turns a cam ring, which drives the focus lens.

In terms of operating noise while recording movies and the continuous AF's smoothness, this lens equals or betters the performance of the EF 24mm and 28mm f/2.8 IS USM lenses presented in the May technical report.

Feature 3 - Electronic ring system's focus ring and advanced design

Photo 5: ES-52 hood for the EF 40mm f/2.8 STM

Photo 5: ES-52 hood for
the EF 40mm f/2.8 STM

The electronic ring system of the lens is similar to that on the EF-S 18-135mm f/3.5-5.6 IS STM, helping to make both lenses more compact. This lens also comes with full-time manual focusing, which allows fine adjustments to be made while the shutter button is held halfway down after autofocusing has finished when the camera is in One Shot AF mode.

Like the EF-S 18-135mm f/3.5-5.6 IS STM, the control circuit of this lens has switched to a new control engine that makes it possible to control the stepping motor, the electronic ring, and other new functions.

The exterior design gives the sense of an advanced lens that fuses a feeling of luxury with functional beauty. The molding processing and silhouette make the lens easier to operate, the focus ring's design providing both operability and a delicate texture, and the high-grade platinum ring makes a smart-looking impression.

The optional ES-52 lens hood (Photo 5) can be used attached to the reverse side of a lens-front filter holder (52mm diameter) or the reverse side of a filter.

Note that when this lens is used with the EOS-1D X or the EOS 5D Mark III, the AF sensor's usable AF points and metering patterns follow Group D. (See the camera user manual for details on the group classifications.)

The EF 40mm f/2.8 STM has the ideal profile for everyone to take casual street snapshots or anything else that crosses their path. It is especially good for capturing people's natural expressions because they are not conscious of the lens pointing at them. Why not try it out for yourself?

Model EF-S 18-135mm f/3.5-5.6 IS STM EF-S 18-135mm f/3.5-5.6 IS
Lens construction 16 elements in
12 groups
16 elements in
12 groups
Aspherical elements 1 1
UD elements 1 1
Aperture blades 7 (circular) 6 (circular)
AF drive Stepping motor DC motor
Min. focusing distance 0.39 m
(at all focal lengths)
0.45 m
(at 135 mm)
Max. magnification 0.28x
(at 135 mm)
(at 135 mm)
Filter size 67mm 67mm
Max. diameter 76.6mm 75.4mm
Overall length 96mm 101mm
Weight ~480 grams ~455 grams

Table 1: Specification comparison between the EF-S 18-135mm f/3.5-5.6 IS STM and the EF-S 18-135mm f/3.5-5.6 IS

Model EF 40mm f/2.8 STM
Lens construction Six elements in four groups
Aspherical elements 1
Aperture blades 7 (circular)
AF drive Stepping motor
Min. focusing distance 0.30m
Max. magnification 0.18x
Filter size 52mm
Max. diameter 68.2mm
Overall length 22.8mm
Weight ~130 grams

Table 2: Specifications of the EF 40mm f/2.8 STM