At very high magnification, depth of field is extremely shallow even at small apertures, a fact of life with any lens. The defocused areas should not be interpreted as a sign of low resolution. (Sigma SA-5; at 1:1; f/16; Bogen/Manfrotto tripod; PhotoFlex reflector panel; Ektachrome E100 VS at EI 200.)

· Instead of internal focusing (common on longer lenses) this one incorporates helical focusing; the front barrel does not rotate so using a polarizer is a pleasure. However, the effective aperture does reduce in size in extreme close focusing because of light loss due to extension (the lens barrel increases in length). It's f/4.2 at a 1:2 reproduction ratio and f/5.5 at 1:1. With in-camera TTL metering, exposures will be accurate. When accessory light meters are used (not common in extreme close-up work) exposure should be based on the effective aperture as per the chart in the Owners Manual.

· The focus ring does not rotate during F operation if it is pushed forward to the "AF" mark. This nicety ensures that your hand does not impede it while autofocusing. Remember to set the ring to the correct position when autofocusing; this speeds up the operation and avoids the risk of damage to the motor during.

· The test sample was for the Sigma SA-5 camera so it included an Autofocus/Manual Focus switch also found on the Canon mount lens. Lenses for other cameras do not include this control as all functions are set on the body. Naturally, a mechanical aperture ring is included on lenses intended with Pentax AF and Nikon AF mount. The smallest maximum aperture is f/45 (f/32 with models intended for Nikon and Pentax), which is useful for maximizing depth of field. (To determine depth of field at any aperture, use the camera's preview control (if so equipped) as no scale or chart is included.)

Although this Sigma Macro lens can be employed for extremely high magnification photography, it is also useful for more moderate close-focus work. (Sigma S-5; at 1:3; f/22; Bogen/Manfrotto tripod; PhotoFlex reflector panel; Ektachrome E100 VS at EI 200.)

· A focus limiter switch is provided. Use it to restrict autofocus operation to only very short--or great--focusing distances. This helps maximize focus acquisition speed especially in autofocus as the AF system does not search the entire range of possibilities. No focus adjustment is required when using infrared film.

· A non-slip, non-reflective, and scratch-resistant matte EX finish is used. Sigma includes a screw-in lens hood, although it is not really necessary except in extreme sidelighting. The front element is recessed so stray light rarely strikes it in any event.

· A distance scale (in feet and meters) is provided and the reproduction ratio from 1:4 to 1:1 is marked on the internal barrel that extends in focusing. This is useful when you want a specific level of magnification. Simply set the focusing ring to the desired reproduction ratio and move the camera back and forth until the subject appears sharp. I used the Adorama Macro Focusing Rail for maximum convenience during my tests, but you can move the entire tripod if you don't have such an accessory.

Mechanical Performance. Overall, I would rate mechanical quality as very high. This lens mounted onto a Sigma SA-5 with an authoritative click and fit perfectly. There was no looseness or "play" whatsoever. Manual focus operation with the 1.5" wide knurled/rubberized ring is smooth and well damped for a familiar feel. For focusing from infinity to 12.3", a 300° turn is required. I found this useful because the slightest change in focus makes a significant difference in extreme close-up work. Setting the precise point of focus proved to be easy and convenient with the AF 105mm f/2.8 EX Macro.

In close-focus work, stopping down to f/22 is often necessary for adequate depth of field, and the Sigma 105mm EX Macro lens produces excellent results. Even smaller apertures can be selected but the effects of diffraction then begin to degrade sharpness, as with any lens. (Sigma SA-5; at 1:2; f/22; Bogen/Manfrotto tripod; PhotoFlex reflector panel; Ektachrome E100 VS at EI 200.)

Autofocus operation is fairly fast with the Sigma SA-5 but not silent, producing a low pitched hum. With any AF system, response is quickest at wide apertures such as f/2.8 because more light is transmitted to the sensors. This is an f/2.8 lens at least at common focusing distances. Effective aperture does diminish in extreme close focusing, but autofocus is rarely practical in such photography. Because depth of field is minuscule, it's best to focus manually--on the most important subject area, such as the stamen of a flower.

Quite short at infinity focus, the barrel does extend by 2.5" when set for the minimum focusing distance. Since the internal barrel is so light in weight, overall balance on a tripod does not shift noticeably. At times, I do want a longer focal length in a macro lens: for greater working room between the lens and subject or for a narrower angle of view to avoid including a large expanse of a cluttered background. Like most macro lenses however, the Sigma AF 105mm f/2.8 EX will not accept a tele-converter directly. A short extension tube would solve that problem but one was not supplied for my tests.

Although the Sigma 105mm EX Macro lens includes autofocus, it's best to disengage this feature in extreme close-up photography. Set focus manually for the most important subject element instead of allowing the system to make its own "decision." (Sigma SA-5; at 1:2; f/22; Bogen/Manfrotto tripod; PhotoFlex reflector panel; Ektachrome 100EC.)

The loss of light transmission with lens extension did make focusing more difficult in low-light conditions, but then I remembered an important fact. My own macro lens--like some other affordable models--has only an f/5.6 maximum aperture and only provides magnification to 0.5x (half life-size). In comparison, the Sigma AF 105mm f/5.6 EX Macro is still ahead on both counts.

Performance Evaluation. I used this macro lens often in greenhouse gardens for close-ups of tiny blossoms or details inside larger flowers. The primary advantage of shooting indoors is that there is less wind-induced motion. The fans do move the subject slightly, but not nearly as much as the wind outdoors would. After discarding the slides blurred by subject motion, I made the following notes while reviewing the others under an 8x loupe.

· At a 1:2 (and lower) reproduction ration, macro performance as excellent overall with the optimum results obtained at f/16. This produced very high sharpness, definition of intricate detail, and contrast at the f/stop we all use most frequently in such photography. (I did not hesitate to shoot at f/11 or f/22 either when those f/stops were required to achieve a specific effect.) Center sharpness was high while edge sharpness nearly matched that level. I considered the slides suitable for a fine 11x4 print, impressive indeed for a lens in this price range.

· At the 1:1 reproduction ratio (full 1x magnification) optimum results were obtained at f/16 and f/22, small apertures essential for adequate depth of field. Image quality is suitable for a fine 8x12 print or a full page reproduction in a book or magazine.

No matter how high the optical potential of this Sigma Macro lens, the slightest camera or subject movement will lead to unsharp images. By shooting indoors, using professional techniques, I was often able to avoid motion blur. (Sigma SA-5; at 1:1; f/16; Bogen/Manfrotto tripod; PhotoFlex reflector panel; Velvia 50 at EI 100.)

· At very wide apertures such as f/2.8 or f/4, sharpness and definition were lower, especially at the edges of the frame. Frankly, this is merely academic. In close-focus photography, depth of field is far too shallow at wide apertures, so most such work is done at small apertures.

· When desired, I was able to stop down even further, to f/45 for maximum depth of field. This is tempting for anyone who finds the range of sharp focus so frustratingly shallow in extreme close-up photography. However, as with any lens, stopping down beyond f/22 does extract a penalty: due to the effects of diffraction, image sharpness is degraded. This is an optical fact with any lens, caused by the bending of light rays as they pass through an extremely small aperture. Select f/32 to f/45 only when depth of field is more important than a razor sharp image as in some interpretive (non-documentary) photography.

· At greater focusing distances from 8' to infinity, optical performance was very good indeed. A 105mm focal length is great for portrait photography, and this lens would do well in that application, too. For the optimum results, shoot in the f/5.6 to f/11 range to take advantage of the optical "sweet spot." When the background is defocused, you'll notice a very pleasing effect in the out of focus highlights. This is caused by the eight (vs. the usual six) blades in the diaphragm which make the opening nearly circular even at wide apertures.

Conclusion. In order to avoid disappointment in any extremely high magnification work, use professional photographic techniques. That means a rigid tripod, cable release, and reflex mirror pre-lock at the 1/4 sec to 1/30 sec shutter speeds where the effects of internal vibration are most noticeable. And make sure the subject is not being moved by the breeze, even slightly. Finally, do recognize the effects of shallow depth of field. The zone of apparent sharpness is very limited in high magnification photography; the out of focus areas should not be interpreted as an indication of low resolution.

In a nutshell, the Sigma AF 105mm f/2.8 EX Macro produced image quality much higher than its moderate price would suggest. Aside from the factors already evaluated, there is virtually no vignetting by f/5.6, flare is well controlled, color rendition is very close to neutral, and pincushion distortion is minimal. Overall, this lens is a bargain. It's one that would suit the needs of the photo hobbyist who is interested in trying some extreme close-up work whether for nature or for stamps, coins or other inanimate subjects.

Technical Specifications

Construction: 10 elements in nine groups
Minimum Aperture: f/32 (Nikon D and Pentax); f/45 (Sigma SA and Canon)
Angle Of View: 23.3°
Minimum Focusing Distance: 12.3" (31.3 cm)
Maximum Reproduction Ratio: 1:1
Diaphragm: Eight blades
Coating: Multilayer
Filter Size: 58mm
Dimensions: 2.9x3.7" (74x95mm)
Weight: 15.9 oz (450 g)
Mounts: Nikon D AF, Canon AF, Pentax AF, Sigma AF