Lens Evaluation
The technical factors ensure great ease and convenience of use, but how does this lens perform in terms of image quality in macro photography? Remarkably well, actually, despite the fact that it does not contain any low dispersion or aspherical elements. Images made with extreme close focusing--at any aperture from f/5.6-f/11--are razor sharp, with the most intricate detail crisply defined. Contrast is snappy, too. I found no need to stop down to f/11 for high edge sharpness as with some other macro lenses.

By f/16 and f/22, center sharpness is still high, but corner sharpness does begin to suffer, as with many lenses, due to diffraction: the bending of light rays as they enter a very small aperture. However, the greater depth of field at tiny apertures produces an impression of higher sharpness, because all parts of a three-dimensional subject can be within the range of sharp focus. Hence, I certainly did not avoid shooting at f/16 when necessary for extreme close-ups of a praying mantis cavorting on a gerbera daisy blossom.

At the more typical 6 ft to infinity distances, image quality is excellent at all apertures from f/4-f/11; this consistency is a sign of well-designed optics. Even at maximum aperture, central sharpness is high, an important consideration unless the primary subject is small and located near a corner of the frame.

Final Assessment
Regardless of the great optical performance of this macro lens, technical proficiency is a prerequisite for sharp images at high magnification. I made over 250 images with the Tokina AT-X 100mm f/2.8 PRO D and feel it is suitable for both serious nature photography and as a conventional short telephoto. This multi-platform lens is a very fine performer. Quite affordable for a 100mm macro with the latest multilayered coating technology, it offers exceptional value. Aside from the high image quality, it's mechanically solid and likely to provide long-term reliability even during extensive field use.

In order to illustrate the different effect produced by the same lens on digital SLRs of different formats--with full frame vs. APS-C size sensor--I made an image with two relevant EOS cameras at the Tokina lens' minimum focusing distance. That's 11.8", measured from the film or sensor plane. The EOS Digital Rebel generated Photo A that includes less of the subject area than the (16-megapixel full-frame) EOS-1Ds Mark II produced Photo B with more of the subject area, for reasons discussed in the text. It was easy to achieve equally high apparent magnification with the image made by the camera with 35mm size sensor by cropping in Photoshop. The resulting cropped TIFF file, Photo C, is now 18MB in size vs. the original 48MB, while the Digital Rebel image file (not cropped) remains at 18MB. (At f/8; Manfrotto tripod.)

Magnification Issues: 35mm Vs. Digital "Macro" Photography
Owners of digital SLRs may wonder about the effect produced by this 100mm macro lens when used on a camera with a sensor that's smaller than the 35mm film frame. That's understandable, because the "focal length magnification" factor is a well-known concept. Does the longer equivalent focal length produce greater than 1x magnification when the lens is focused to 11.8"? And can 1x magnification be achieved without moving so close to the subject? Finally, what is the difference in depth of field? These are important issues, well worth addressing, in a step by step manner.

The Basic Concept
When a lens of any focal length is used on a camera with a sensor smaller than the 36x24mm standard, it produces an effect that simulates a longer lens. The factor varies depending on the exact size of the sensor. Most affordable digital SLRs employ a CCD or CMOS chip that falls into the so-called APS-C format. The exact size varies slightly depending on the brand, and sometimes, on the specific model within a brand.

For example, a Canon EOS 20D employs a 22.5x15mm sensor while a Nikon D70 incorporates a 23.7x15.6mm sensor. Consequently, the "focal length magnification" factor varies from 1.5x (Nikon, Konica Minolta's Maxxum, Pentax) to 1.6x (Canon) and to 1.7x (Sigma digital SLRs). According to a simplified definition of the factor, a multi-platform 500mm telephoto, for example, becomes equivalent to a 750mm, 800mm, or 850mm focal length lens. That's a compelling explanation because of the effect that we see in the viewfinder. The focal length does seem to be longer in comparison to the effect that we get in 35mm photography because the subject appears larger in the frame.

This effect occurs because only a portion of the large image circle projected by the lens is recorded by a small sensor. In other words, the field of view--or "picture angle" in Nikon terminology--is being cropped, and that's reflected in the viewfinder. The subject appears larger because less of the scene is included in the image area. That effect is not produced by higher magnification but the subject does appear larger; hence, we can refer to this effect as greater "apparent magnification."

Macro Lens Specifics
Moving back to specifics about extreme close-up photography, let's examine how field of view crop affects the images that we make. Let's say that you have mounted a Tokina AT-X 100mm f/2.8 PRO D macro lens on a 35mm Nikon F100 loaded with slide film. Subsequently, you'll mount the same lens on a digital Nikon D70. Using each camera in turn, you move in very close to a cooperative bee on a small blossom until you reach the 11.8" minimum focusing distance.

After processing the film and downloading the digital image to a computer, you'll be able to make the following observations:
· In the 35mm slide, the bee is exactly bee-size on the film frame, because the lens has provided 1x magnification, or a 1:1 magnification ratio.
· When the slide is projected, the bee appears to be massive, filling an entire wall. That's simply due to the magnification produced by the projector lens.
· In the digital image, the field of view is narrower because the camera's sensor is smaller than a 35mm film frame. Less of the surrounding vegetation is included in the image (reducing distraction caused by extraneous elements) and the bee appears much larger in the frame.
· You can achieve the same effect with the 35mm slide: a larger bee. Simply make (or order) a high-resolution scan of the film frame and crop the digital image until the bee appears to be larger and less of the surroundings are included.

The same concepts would apply if you substituted a digital SLR with full-frame (36x24mm) sensor for the 35mm camera in the earlier example. Only a few current digital SLRs employ the large sensor: the 12-megapixel Canon EOS 5D and the 16-megapixel EOS-1Ds Mark II. If you crop the digital image made by the full-frame sensor camera, the bee would appear to be just as large as it does in the image made with the 6-megapixel Nikon D70. Sure, you would be discarding pixels, but the cropped image would still exhibit high resolution because you started with far more pixels.

Conclusion
If using a digital SLR with a small sensor, you might want to take a different approach in extreme close-up nature photography. Instead of making an image where an insect is extra large in the frame--by moving in very close to the subject--shoot from a slightly greater distance. You can move back a bit, increasing the camera-to-subject distance by about 50 percent and still get the equivalent of 1x apparent magnification, with a few bonuses. Because you're not as close to the subject, there's less risk of spooking an insect or casting a shadow in the composition. As well, the extra working space allows for more room to position a small reflector or off-camera flash unit.

That leads to another benefit of using a digital SLR with a small sensor. Because you do not need to move in as close to the subject for high apparent magnification, the range of acceptably sharp focus will be greater. If you extend the camera-to-subject distance by 50 percent, the depth of field will increase by 50 percent.

That's a real benefit in macro photography with three-dimensional subjects, increasing the odds that the entire subject will appear sharply focused in the final image. It also reduces the need to use a very small aperture, such as f/22. Hence, the lens will produce better image quality while shutter speeds will be faster, reducing the risk of blur from subject movement.

And in the final analysis, information of that type is more valuable than details about technical differences between the various camera formats. Unless you plan to use two entirely different types of cameras with the same lens, simply take advantage of the characteristics that your own SLR offers. Instead of worrying about comparable focal lengths or magnification ratios, do your best to make extreme close-ups with great technical quality and high visual appeal.

Technical Specifications
Focal Length: 100mm
Maximum Aperture: f/2.8
Minimum Aperture: f/32
Lens Construction: Nine elements in eight groups
Focus Limiter: Limits focus to 1.28 ft to infinity
Diaphragm Blades: Nine to render specular highlights as circular at small apertures
Minimum Focusing Distance: 11.8" (30cm)
Maximum Magnification: 1x (1:1)
Filter Size: 55mm
Lens Hood: Bayonet type; included
Dimensions: 2.9x3.74"
Weight: 19 oz (540g)
AF Mounts: Canon EOS and Nikon AF
Street Price: $299

For more information, contact THK Photo Products, Inc., 2360 Mira Mar Ave., Long Beach, CA 90815; (800) 421-1141, (562) 494-9575; www.thkphoto.com.

A long-time "Shutterbug" contributor, freelance stock photographer Peter K. Burian is the author of "Mastering Digital Photography and Imaging" (Sybex) and a digital photo course instructor with www.betterphoto.com.