Sigma 120-400mm f/4.5-5.6 DG OS HSM APO

(From Sigma lens literature) The new APO 120-400mm F4.5-5.6 DG OS HSM telephoto zoom lens incorporates Sigma's original OS (Optical Stabilizer) function. It offers the use of shutter speeds approximately 4 stops slower. Three SLD (Special Low Dispersion) glass elements provide excellent correction for chromatic aberration.

SLRgear Review
June 30, 2008
by Andrew Alexander

Announced just before PMA 2008, the Sigma 120-400mm ƒ/4.5-5.6 is an optically-stabilized telephoto zoom lens. The lens is compatible with both subframe (APS-C-sized) digital camera bodies, as well as film and full-frame digital camera bodies. It is available now for Nikon, Canon and Sigma mounts. The lens is also available for Sony and Pentax mounts, however on these cameras optical image stabilization is not available (these bodies tend to have in-camera stabilization) and on the Pentax version, HSM focusing is not available.

The 120-400mm isn't a ''constant'' lens, in that as you increase the focal length, both the maximum and minimum aperture sizes decrease. The following table reflects the change in aperture with focal length:

The Sigma 120-400mm ƒ/4.5-5.6 takes 77mm rear-mounted filters, comes with a circular lens hood and tripod mount, and is available now for around $830.

Sharpness
The 120-400mm is a fairly sharp lens, but its performance is optimized for the wider range of its focal length spectrum. As you zoom in towards the 400mm range, image sharpness degrades significantly. Optimal image sharpness was achieved at either 120mm and ƒ/5.6, or 150mm and ƒ/8.

Wide open and wide-angle (120mm, ƒ/4.5) image sharpness is quite good, with test results showing 1.5 blur units across the frame. Stopping down to ƒ/5.6 improves central sharpness to 1 unit, but there is still some very slight corner softening. Diffraction limiting seems to set in at ƒ/11, but image quality never gets so bad, reaching 2.5 blur units at ƒ/22. Results at 150mm are almost the same, just a tad less sharp.

However, at 200mm and above, image sharpness starts to worsen. Our sample copy of the lens also showed some centering issues at this focal length. Sharpness showed results of between 1-2 blur units in a very small central region, while corner softness reached 4.5 blur units; as the lens is stopped down, quality improves dramatically at ƒ/8 and ƒ/11 (1 blur unit centrally, 2-3 units in the corners) and the image is generally soft at ƒ/25 (3 blur units).

Between 300 and 400mm, performance when used wide open (ƒ/5.6) is nothing to write home about - very uneven and soft (3-5 blur units at 300mm, and 5-8 units at 400mm). Stopping down to ƒ/8 or ƒ/11 is necessary to get any kind of sharpness out of the lens at this focal length. Interestingly, image sharpness is better at ƒ/22-29 than when used wide open at these focal lengths.

Full-frame performance is similar across all focal lengths and apertures, although corner softness is generally more of a factor throughout. We do note some interesting ''blips'' of extreme corner softness, particularly at 250mm. Optimum performance on a full-frame body is 120mm or 150mm at ƒ/8.

Chromatic Aberration
The 120-400mm ƒ/4.5-5.6 tolerates chromatic aberration very well, better at the wider end of its focal length range. At 120mm the lens shows less than 2/100ths of a percent of frame height CA generally, and less than 3/100ths in the corners. This is very good performance. As the lens is zoomed in and stopped down CA tolerance degrades slightly, but never objectionably. Even at 400mm and ƒ/5.6, performance is still good at less than 3/100ths generally and 4/100ths in the corners. CA performance is as good or better on a full-frame body.

Shading (''Vignetting'')
Corner shading isn't an issue with the 120-400mm mounted on a subframe camera body; the worst-case scenario is noted when using the lens wide open (ƒ/4.5-5.6), and even then we note the corners only a quarter-stop darker than the center. At any other apertures, corner darkening is insignificant.

Mounted on a full-frame body, however, light falloff is more of a problem. Used wide open, the corners of an image will be between 3/4 and a full stop darker than the center, with the worst cases being the wide angle at 120mm or full telephoto at 400mm. This shading dissipates as the lens is stopped down, showing less than 1/3 of a stop by ƒ/8-ƒ/11. At 400mm, corner shading never truly goes away unless you stop completely down to ƒ/29.

Distortion
The 120-400mm is very well-tuned to produce a distortion-free image. Our testing notes a very slight pincushion distortion in the corners, more evident at full telephoto than any other focal length (-0.1%). This effect is statistically more noticeable on a full-frame camera body (-0.2%) but in practice, these distortion effects will be difficult to note.

Autofocus Operation
Equipped with Sigma's HSM autofocus motor, the 120-400mm focused very quickly and quietly. Autofocus results can be overridden at any time by simply turning the focus ring.

Macro
The Sigma 120-400mm ƒ/4.5-5.6 isn't designed as a macro lens, but its maximum magnification of 0.24x and minimum close-focus distance of 150cm (just under five feet), provides reasonably good macro performance.

Build Quality and Handling
The lens is a fairly complex design with 21 elements in 15 groups, including 3 super low-dispersion and APO glass elements. It's solid - almost four pounds of lens, coated with a textured finish with makes it very easy to handle. The diaphragm consists of nine aperture blades.

The lens has two switches to speak of, including one for disabling autofocus and another for selecting the optical stabilization mode. Stabilization can be set to vertical and horizontal, or vertical only, to allow for panning shots. A windowed distance scale is found close to the metal lens mount. The filter mount is plastic.

Zoom and focusing rings are large and nicely textured, with raised rubber ribs. The focus ring is closer to the lens mount, less wide than the zoom ring. The focus ring travels about 120 degrees during manual focus, and is nicely damped to allow for precise adjustments.

The lens extends slightly during zoom operations, adding 2.5 inches to the length of the lens at 400mm. The zoom ring travels about 90 degrees through its focal length range. Zoom creep isn't much of an issue for the lens, though you can get it to extend if you jostle it with any force; walking with the lens held down, it would probably extend over time. For this reason Sigma has included a zoom lock switch to lock it at 120mm for travel.

The lens hood for the 120-400mm is bayonet-style, and heavily ribbed inside. The hood adds 3.5 inches of length to the lens when attached and reverses onto the lens for compact storage. Also included is a removable tripod mount, which makes a good carrying handle as it has finger grips molded into it. Sigma also thoughtfully includes a sling to carry the lens if desired.

Filters are attached to the rear of the lens, taking 77mm filters. There is no rotation of filters during focus or zoom operations. We are still putting together our specifications for optical image stabilization testing, but our initial impressions of the 120-400mm are good, and that it works as advertised.

Alternatives

Canon EF 100-400mm ƒ/4.5-5.6L IS USM ~$1,400
Much sharper lens, especially at 400mm wide open. CA performance is worse; distortion and corner shading are about the same.

Nikon 80-400mm ƒ/4.5-5.6D ED AF VR ~$1,400
Slightly sharper, but more susceptible to chromatic aberration. Corner shading is about the same, but there is slightly more distortion in the corners.

Pentax 55-300mm ƒ/4-5.8 ED SMC DA ~$400
Pentax doesn't really produce a lens in this range; the closest is this consumer-level model, which we haven't tested yet. No in-lens stabilization, but then, most Pentax bodies now have in-body stabilization.

Sony 75-300mm ƒ/4.5-5.6 SAL-75300 ~$230
Sony doesn't produce a lens in this range; this is the closest point of comparison. Sharpness is very poor at 300mm, CA is slightly worse, but corner shading is about the same. Distortion is slightly worse. No in-lens stabilization, but all Sony bodies support in-body stabilization.

Conclusion
Sigma has produced a nice telephoto zoom here, and the inclusion of optical image stabilization will be a welcome upgrade from its previous offerings in the similar range. This stabilization is necessary, as good image sharpness in the telephoto range is only obtained when stopped down to ƒ/8 or ƒ/11. Using a smaller aperture in this range usually demands a slower shutter speed, which is usually a recipe for blurry images when using long-zoom lenses. With image stabilization, you get a steadier platform.

In the final analysis, it becomes fairly obvious whether this lens is for you; Pentax and Sony users really don't have any comparable options in this range, and while the Nikon and Canon offerings are slightly better choices optically, they're more expensive. If absolute image quality at full telephoto is your objective, you may have to invest slightly more money into a telephoto prime, but shooting stopped down to ƒ/8 or ƒ/11, you can get fairly good images out of this lens, and with image stabilization, you don't even need a sunny day or a tripod anymore.

Sample Photos

Please note, sample images for the sub-frame sensor body (in this case, the Canon 20D), are temporarily unavailable and will be uploaded shortly.

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.