Canon's Little Beauty: The 40mm f/2.8 STM Lens

Earlier this month, Canon introduced its new STM lenses, including the EF 40mm f/2.8 STM lens. STM lenses include Canon's new Stepping Motor technology, which allows the lenses to smoothly and silently focus, and when used in combination with the new Movie Servo AF feature (found in Canon's Rebel T4i), achieve continuous AF while recording video. The new stepping motor technology in these lenses quietly focuses allowing the internal microphone to only record the sounds of the scene being shot.

It is exciting to have the opportunity to review a new piece of equipment, especially when it works well on so many levels.  Canon's new 40 mm f/2.8 STM lens is the new little beauty I am referring to.

This lens is the first pancake style lens produced by Canon in the EF lens mount.  Pancake style lenses are not new to the camera industry, but they have been used mostly by smaller-sized cameras.  This lens works beautifully on both the EF-S and EF mount cameras. I tested it on a full-size sensor cameras like the Canon 5D Mark III.  The smallish size of the lens is a typical 2.7 inches wide at the barrel and EF mount, but it is only 0.9 inches tall (or rather flat as a pancake) and 4.6 ounces in the weight department.  It sees a 57 degree 30 minutes wide, which is only 6 degrees narrower than my favorite 35 mm lens.  This made using the small lens for street photography at my local Arts Festival an inconspicuous event to most of the crowd and a lot of fun for me.

backlit with NO flare artifacts

About the "STM" lenses

The new "STM" designation on this lens indicates that it contains a new stepper type motor to handle the focus activity.  This is an entire new breed of lenses for Canon.  When shooting using 'Live View' or in 'Camera Video' the photographer lost the use of auto focus in the past.  This lens, along with future offerings from Canon in the STM style, can continue to provide auto focus in 'Live View' or during 'Video Recording' with certain new cameras.  The first compatible camera to utilize this technology is the new Canon Rebel T4i.  I would wager that this technology will echo into more offerings from Canon in the future.


The STM style motor is very smooth in focusing operations and is nearly silent, perhaps not as fast as the USM ultrasonic motors, but not as slow as the older arc form drives of yesteryear.  I did not experience any real slowness of focus or any other disappointment while shooting in a "hurried up" street condition.  As a test I shut the auto focus off and used the lens in trap focus (modified hyper-focal) condition.  I did this by setting the lens at 10' and the camera (Aperture Priority) at f/11 with 400 iso and let the camera choose the shutter speed.  This way any subject between 6' and 30' would be in focus because of the inherent depth of field without the need to wait for focusing to happen.  This worked fine, but was not appreciably faster than the superb internal auto focus system of the STM motor.


I was a bit worried because I did not have a lens hood available for testing.  However, I quickly found out this lens is not bothered by flare.  Even when I intentionally pointed it at a person backlit by the sun, there were no "flying saucer-like" flare artifacts shooting across the frame.  A slight decrease in contrast occurred, as expected, yet the tonality remained smooth throughout the image.  A remarkable performance for any lens.

One more great thing about this lens is that it is sharp, much sharper than expected from the appearance of its diminutive size.  Checking its specs at Imatest, it reveals a lens resolution of excellent from wide open at f/2.8 through f/11.  After f/11 there is an predictable drop in resolution due to diffraction, as in any lens. Chromatic aberrations are virtually non-existent at all apertures in this new lens.

Closing thoughts

One cannot go wrong with the new Canon 40 mm STM lens.  It is compact and lightweight, fast focusing, low distortion, and provides excellent sharpness.  Combined with the fact that in future generations it will continue to provide auto focus in 'Live View' and 'Video' modes, this is an all around winner.  With its affordable price what else could you want from a lens.

Sample shots

great angle of view

crisp fast focus

overall sharpness

fantastic color fidelity
40 mm stmCanonJuly 2012LensesReview