Closer look: Canon RF 100mm F2.8L Macro IS USM


A closer look at Canon’s RF 100mm F2.8L Macro IS USM

Canon has just announced a native RF contemporary to its popular EF 100mm F2.8L Macro lens. The RF 100mm F2.8L Macro IS USM is an all-new design, and becomes Canon’s first medium telephoto macro lens to offer a maximum magnification ratio of 1.4x, which means you can project images onto the camera’s sensor 1.4x larger than life-size.

Close focus

The optical design comprises 17 elements in 13 groups. At its maximum magnification, you can fill your frame with a subject just 25 x 17mm in size (1 x 2/3″). What does that mean in terms of working distances from your subject?

At 1x magnification, the focus distance is 28cm, while your working distance is 11.2cm (accounting for the length of the lens barrel). At the maximum 1.4x magnification, the minimum focus distance is 26cm, while your working distance is a mere 8.6cm (3.4″). At such close working ranges, you’ll want to watch for any shadow cast on your object by your camera and lens (and you’ll definitely want to remove that lens hood). Ring lights and flashes are quite useful when using these types of lenses.

See that ‘Lock’ button on the end of the lens barrel? It locks and unlocks a very special control ring on the lens…

SA Control

The Canon RF 100mm F2.8L Macro offers an unusual feature: a dedicated spherical aberration control ring that allows the photographer to change the style of the bokeh for the desired effect (we’ve seen something similar in Nikon’s ‘Defocus Control’ lenses in the past). This ring essentially moves an optical element (or elements) within the lens to change the spherical aberration properties of the lens.

A positive shift leads to ‘soap bubble’ bokeh by concentrating light rays peripherally within the circle-of-confusion (CoC), also softening foreground bokeh. A negative shift leads to very soft defocused backgrounds and foregrounds – potentially at the cost of some sharpness at the focus plane – by concentrating light rays more centrally within the CoC (see this example from Nikon’s 105mm F2 DC lens). Keep in mind that for the sharpest results at the focus plane, you’ll want to keep the SA Control ring at its ‘0’ position.

Nine aperture blades ensure circular out-of-focus highlights, though it’s worth mentioning we are now seeing lenses offering even more circular bokeh with 11-blade apertures.

SA Control for smoother backgrounds

Graphic assembled from Wikipedia illustrations and images from ‘Depth of Field and Bokeh’ by H. H. Nasse of the Camera Lens Division at Zeiss.

While the negative SA shift in less hard-edged bokeh, much like the effect of an apodization filter (such as the one in the Canon RF 85mm F1.2L USM DS), it’s important to note that the effect here is entirely different.

Let’s take a look at what the SA Control ring is doing. At its ‘0’ setting, the optics have minimal spherical aberration (SA). It’s never quite zero SA for real lens, but for the purposes of discussion, we’ll pretend that at ‘0’, the lens behaves like a ‘perfect’ lens with no SA (uppermost graphic). Such a lens will have a uniform circle-of-confusion (CoC); that is, bokeh should be smooth and out-of-focus highlights will appear as Gaussian, even, uniformly-lit circles (see inset grey circle: uppermost graphic, right). You can see why: light rays from an object that falls into focus ahead of the image sensor (grey vertical line) diverge evenly before hitting the image plane, whether they’re rays that entered the peripheries of the lens or ones that entered more centrally.

A lens with under-corrected spherical aberration (lower graphic) gives a very different CoC, one that for objects behind the focus plane is brighter on the inside with a continuous fall-off as you move radially outward (see inset grey circle: lower graphic, right). This is what turning the SA Control ring in a negative direction will give you. Background bokeh will appear even smoother than a perfect lens, and out-of-focus highlights will have feathered edges. The light ray diagram shows us why: marginal light rays from our object of interest that entered the peripheries of the lens focused farther ahead of the image sensor (at the marginal focus point indicated by the black arrow) and diverged more by the time they hit the image sensor. So they’re less concentrated, more diffuse, leading to those feathered edges. Light rays from that object that entered the lens more centrally focused closer to the image sensor (at the paraxial focus point indicated by the orange arrow) and so diverged less before hitting the image sensor and, hence, are more concentrated or tightly bundled together, leading to the CoC’s brighter center.

SA Control for a dreamy or ‘soap-bubble bokeh’ effect: you choose!

You’ll get quite the opposite effect for foreground bokeh. Note we’ve moved the image sensor (or lens, it’s all a matter of perspective!) such that the object now falls into focus behind the sensor. Now the peripheral (marginal) light rays are more concentrated in the CoC, while the central (paraxial) ones are less concentrated. Hence the outer edges of the CoC are brighter while the inside is darker, leading to a ‘soap bubble’-like effect. For a ‘perfect’ lens (that again, doesn’t exist), nothing changes: background and foreground bokeh are rendered equally.

Things are reversed if you turn the SA Control ring in a positive direction: now background bokeh takes on this ‘soap bubble’-like effect, while foreground bokeh appears smoother.

Also contributing to the dreamy look of images taken with the SA Control ring turned in either direction is a slightly softer plane of focus. You’ll note that all light rays don’t neatly converge to one single, tight point at the ‘best focus’ position for the lens with SA above compared to the ‘perfect’ lens. That’s because the ‘best focus’ point is actually a point in between the best focus positions for the marginal and paraxial rays. So even in-focus objects will never appear quite as sharp as with the SA Control ring set to ‘0’ (or a lens with no SA).

SA Control varies based on subject distance

The amount of SA control varies based on the focus distance, and peaks somewhere in the middle of the focus range. At the minimum focus distance (maximum magnification of 1.4x) , SA control is essentially unavailable, and the SA control ring will have no effect. At 1x magnification, or 0.28m focus distance, you’ll get a small amount of SA control as you turn the ring.

By 0.5x magnification, and from a focus distance of 0.38m to 1m, you’ll have the maximum amount of SA control: that is, turning the SA control ring will dramatically vary the spherical aberration properties of the lens. Between 5m and infinity, you’ll have a diminished but still modest amount of SA control. Canon tells us you can shift the SA control as much in a positive direction as you can in a negative direction throughout the focus range.

Optical Image Stabilization (I.S.)

The Canon RF 100mm F2.8L Macro IS USM offers a CIPA standard rating of 5 stops of optical image stabilization. When combined with newer EOS R bodies featuring 5-axis in-body image stabilization, that number goes up to a whopping 8 stops of shake correction, thanks to coordinated stabilization between the lens and image sensor. Image stabilization is particularly important with macro lenses and such high magnification ratios, as any hand movement will be greatly magnified. However, at close working distances, as with any macro lens, the effectiveness of image stabilization drops, and you won’t get that 8 stop figure. The nice thing, though, is that at close focus distances, Canon’s Hybrid IS – introduced in the original EF 100mm Macro – kicks in. This attempts to correct for horizontal and vertical translation in addition to pan and tilt.

Canon has included two Nano USM focus motors in this lens, which should lead to fast, and quiet, autofocus. We’ve been very impressed with the speed performance of Canon’s Nano USM technology, and with such small working distances, the large relative changes in distance should, theoretically at least, be easily handled by the ‘Dual Nano USM’ setup in this lens.

Canon claims that focus breathing has been reduced, which should be welcome news for videographers.

Controls, switches and availability

A customizable control ring is included for direct access to settings like aperture, ISO, and exposure compensation. A focus limiter allows you to limit focus from infinity to 0.5m so as to speed up autofocus by ensuring the lens never hunts all the way to close focus distances. You can also limit the focus range to only 0.26m – 0.5m. Also included on the lens barrel are an AF/MF switch and a switch to turn image stabilization on or off. Unlike some Canon lenses, a dedicated I.S. option for panning is not included because the lens can automatically detect and optimize the IS system for panning shots.

A ‘Super Spectra Coating’ helps to eliminate ghosting and flare when shooting into or around bright light sources. A fluorine coating on the front element helps prevent buildup of dust, moisture and fingerprints. The lens is quoted as being dust- and weather-resistant, and weighs approximately 730g (1.6 lbs).

The Canon RF 100mm F2.8L Macro IS USM will be available from July 2021 with a recommend price of $1,399. A tripod mount ring with adapter will be sold separately.

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