Sigma 50mm f/2.8 Macro
By Jim Tanner

Center Sharpness
The DxO Blur Index viewer on the main SLRgear page for the Sigma 50mm f/2.8 lens and Figure 1 below display quite a bit of information about the Sharpness of the lens itself and how it performs relative to the six lenses we include in this 50mm shootout. As you move the aperture slider on the Blur Index viewer from its lowest position (f/2.8) to its highest (f/45), you see that the Sigma f/2.8 images start (f/2.8) pretty sharp and flat and remain so until f/16, a very good performance. Diffraction limiting first starts to take effect around f/11 (but would still be invisible to even the sharpest eyes at that point), gradually worsening such that at f/22 the image begins to soften noticeably. Diffraction effects are very evident in the f/32 and f/45 range, where the plots are very soft and wavy. But, no other lens in this 50mm shootout has apertures greater than f/22, so we cannot compare these high-aperture Sigma values to others. (The softness at f/32 and f/45 is so pronounced one wonders why Sigma bothered to include those settings.) But the sharp-flat plots from f/2.8 to f/16 indicate a very wide range for good, sharp photos.

Figure 1. Center Sharpness: Sigma 50mm f/2.8 versus Six-Lens Average

Now, more about Figure 1, which depicts (in purple) the center-frame sharpness for the Sigma f/2.8 over its full aperture range. Also shown (in black) is a plot of the group average (of the six lenses employed in this review) blur value for the aperture range (f/1.2-f/45) spanned by the six lenses included in this shootout. The group average is shown at apertures where data is available from two or more lens (f1.4-/22). For the f/2.8-f/22 range the "six-lens composite" and the Sigma f/2.8 lens show essentially identical performances Remember, DxO says that blur differences of less than 1 blur unit are usually not discernible by the average observer. Over this f/2.8-f/22 aperture range the f/2.8 lens has an average blur factor of 1.28 blur units against a group average of 1.22. This is a very good center sharpness performance for the Sigma 50mm f/2.8 lens. That said, the diffraction-forced high blur values for f/32 and f/45 apertures render these apertures of questionable value to the most photographers. (You'll certainly get loads of depth of field, but then nothing in the frame will be particularly sharp, either.) Note that, for some reason, the minimum aperture on Nikon and Pentax mounts is f/32, the f/45 minimum only being available on Canon, Konica-Minolta (now Sony) and Sigma mounts.

Corner Sharpness
As we remarked earlier, the DxO Blur Plots indicate good (sharp, flat) corner performance for the Sigma f/2.8.

Figure 2. Corner Sharpness: Sigma 50mm f/2.8 versus Six-Lens Average

This is verified in Figure 2, which shows curves of the Corner Sharpness measured for both the Sigma f/2.8 (purple curve) and the group (black curve). Just as in the center-sharpness graphs of Figure 1, these two curves are essentially identical for the range f/2.8-f/22.The group average for this aperture range is 1.5 blur units while the Sigma f/2.8 average is 1.6 blur units; again, a visually imperceptible difference. Once again, diffraction-induced softness renders the f/32 and f/45 corners very questionable. However, the Sigma 50mm f/2.8 lens displays excellent corner sharpness over the f/2.8-f/16 range.

Chromatic Aberration
The DxO Chromatic Aberration graph depicts the Maximum and (entire image) Average Chromatic Aberration for the Sigma 50mm f/2.8 lens over the full aperture range of the lens. There is nothing particularly significant about the data; that is, there are no unusual swings or noticeably high values over the full spectrum.

Figure 3. Chromatic Aberration: Sigma 50mm f/2.8 versus Six-Lens Average

The two purple curves of Figure 3 depict the same data shown in the graph we just discussed, a graph that is included with each SLRgear lens review. The black curves of Figure 3 show the group maximum CA and average CA at nine apertures in the f/1.4-f/22 range. Clearly, the Sigma has a poorer performance than does the group over the f/2.8-22 aperture range. Over this range the Sigma average values of (max = 4.9, ave = 2.7) are higher than the group values (max = 3.8, ave = 2.0). Only one lens (Canon f/1.2L) had a poorer Chromatic Aberration performance than that of the Sigma 50mm f/2.8, so chromatic aberration appears to be the Sigma's biggest weakness. That said, the values here aren't all that high, many users would likely be entirely satisfied with the results shown here.

Shading (Vignetting)
The DxO Shading (Light Falloff, Vignetting) graph (the usual slrgear presentation) indicates an essentially constant Shading value of about 0.1 EV over the full aperture range of the Sigma f/2.8. No surprises here, it's a generally good performance by a good lens.

Figure 4. Shading: Sigma 50mm f/2.8 versus Six-Lens Average

The purple curve of Figure 4 repeats the DxO graph and adds the group performance (black curve) over the f/1.4-22 aperture range. Over the (f/2.8-22) range the group average is 0.10 EV while the Sigma f/2.8 has an average value of 0.13 EV. In the shading competition, the Sigma 50mm f/2.8 lens performs very similarly to four other lenses, with only the Nikon f/1.8 having a slightly higher, but still very acceptable, shading performance.

Distortion
DxO Analyzer performs two measurements for distortion by a lens, the Maximum Distortion and the Average Distortion over the entire frame. The Sigma f/2.8 has the lowest values (max = 0.03%, ave = 0.01%) of the six lenses tested, a solid improvement relative to the group values (max = 0.14%, ave = 0.06%) and handily winning the Distortion duel. - This would be an excellent lens for architectural photography, where the straightness of lines near the edges of the frame is of paramount importance.

AF Operation
Because the Sigma f/2.8 has Macro capabilities, the user is given the choice of full-range (4 cm lens-to-object to infinity) or limited-range (about 10 cm lens-to-object to infinity) focusing. Using the limited-range selection, the infinity to closest focus (about 10 cm) is accomplished regularly in less than a second. Starting from a 4-cm focus, it takes a little longer, perhaps on the order of a couple of seconds. In lower light situations, the lens does search a bit, but appears to focus quite accurately. At closest focus, the width covered with the lens on the 1.6x Canon 20D is slightly over 2 cm, excellent macro performance indeed.

Build Quality and Handling
The Sigma f/2.8 has a very solid feel to it; perhaps not tank-like, but certainly more than acceptable. The switches (AF/M, Limit/Full) are a bit small and stiff, but are reasonably easy to use. The fact that it uses a conventional rather than ultrasonic motor for focusing means that you have to explicitly switch to manual focus operation before you can adjust the focus ring by hand. Once you do though, the focus ring operates smoothly, and with a good range of travel, allowing precise focus adjustment. In autofocus mode, the non-ultrasonic motor and gears make a little noise, but not nearly as much as many lenses we've tested. Overall, this seems like a well-constructed and nicely performing lens.

The Competition
The Table below summarizes the performance of the six lenses over the (f/2.8-f/16) aperture range, the largest range that is common to all six lenses in the shootout. In a later report, we will expand this table graphically by showing the full-aperture-range performance for each lens against each other lens for Center Sharpness, Corner Sharpness, Chromatic Aberration, and Shading. This will be a total of 60 graphs which reveal the dirty details of this intense competition.

Conclusion
As its name reveals, the Sigma50mm f/2.8 Macro stands out in this six-lens 50mm shootout as the only lens offering Macro capability. Additionally, it delivers sharp (both center and corner) photos over quite a large aperture range f/2.8-22, with little shading and almost zero distortion. It does exhibit mild chromatic aberration. If you don't need anything more than an f/2.8 maximum aperture, this is clearly an excellent buy.