Studio Tests (APS-C Format)

The 100mm F2.8 L IS USM Macro performs very well even on the resolution-hungry APS-C format. Sharpness is high and fairly even across the frame, chromatic aberration is low, and distortion and falloff minimal. Despite not being on its 'native' format, the results are close to the excellent Olympus Zuiko Digital 50mm F2 Macro, and clearly better than the older Canon EF 100mm F2.8 USM Macro.

Sharpness The lens is very sharp wide open, with just a slight drop off towards the corners. Best results are seen in the F4 - F5.6 range, at which point the measured sharpness is likely to be limited mainly by the 50D's relatively aggressive optical low-pass filter. Diffraction progressively reduces sharpness on stopping down further, with apertures smaller than F16 giving extremely soft results.
Chromatic Aberration There's a tiny amount of lateral CA, but at just 0.05% in the corners it's rarely likely to be visible in normal use.
Falloff We consider falloff to start becoming noticeable when the corner illumination falls to more than 1 stop below the center. As usual for a full-frame lens on APS-C, falloff is negligible.
Distortion Distortion is to all intents and purposes zero.

Macro Focus

The 100mm gives true 1:1 macro, which reveals a lot of fine detail on a high resolution APS-C sensor. The measured closest focus is 29cm, giving a 13cm working distance from the front of the lens to the subject.

Central sharpness is very high even at F2.8, with the corners just a little behind. Optimal results are seen around F8; apertures of F16 and smaller inevitably start to suffer badly from diffraction, and should probably be avoided unless extreme depth of field is required (F32 is very soft indeed).

There's a slight hint of red/cyan fringing from lateral chromatic aberration towards the corners, but you're unlikely to see it outside of doing copy work.
Macro - 21 x 14 mm coverage
Distortion: None
Corner softness: Low
Focal length: 100mm (160mm equivalent)

Specific image quality issues

Chromatic aberration

The EF 100mm F2.8 L IS USM Macro shows a slight degree of lateral chromatic aberration (color fringing at high contrast edges towards the edge of the frame), and while it's nothing like what you'll see from many zooms, it can very occasionally be visible in 'real-world' shooting. A quirk in the lens's CA profile also means it is more obvious on APS-C cameras - but it's also easily corrected in software if required. For example, we found that using Adobe Camera Raw 5.5, a 'Fix Red/Cyan Fringe' of -13 did the job nicely.

Longitudinal chromatic aberration is also present at larger apertures (which is only to be expected), and takes the form of magenta fringing around high contrast edges in front of the plane of focus, and green fringing behind. Because of the extra magnification imposed on the image by the smaller sensor, this is also distinctly more visible on APS-C; it's also much less straightforward to deal with in software (as it's dependent on the image content). It's not hugely intrusive though, and decreases gradually on stopping down, becoming insignificant by F5.6.

Lateral CA
Longitudinal CA
Canon EOS 50D, F7.1, camera JPEG Canon EOS 50D, F2.8, camera JPEG
100% crop, lower right 100% crop, in front of focal plane
100% crop, CA corrected (ACR) 100% crop, behind focal plane

Diffraction softening at small apertures

One aspect of lens performance that's literally impossible to get away from is the gradual softening of the image due to diffraction at small apertures. The 100mm F2.8 macro stops all the way down to F32, which will give extremely soft results at the pixel level on APS-C. Of course this comes with the very real benefit of increased depth of field, so the trade-off can sometimes be worthwhile (especially when shooting closeups). But it's important to appreciate just how much the image degrades in the plane of sharpest focus.

The example below illustrates this much-misunderstood effect, looking at the depth of field of the picture as a whole alongside the pixel-level sharpness of 100% crops from selected regions. Stopping down brings progressive benefits in terms of depth of field, but detail in the region of sharpest focus (by the eye) is starting to soften at F8, and very heavily blurred at F32 (all the sharpening in the world won't bring it back). Against that, though, detail on the ear is most defined at F32, at which point it's coming close to that in the plane of focus.


The choice of aperture for any specific shot is dependent upon both the desired aesthetic, and the final output size. The F32 shot here would look OK in a 6" x 4" print, for example, but very visibly lack detail at 12" x 8". For most purposes we found apertures around F11 to give about the best compromise between fine detail and overall depth of field on APS-C. It's also important to understand that diffraction isn't a specific flaw with this lens (or indeed any other), but simply a direct consequence of the immutable laws of physics.