Like the 90mm, however, the new 100mm f/2.8 handles bellows extension in a different way than (almost) any other macro lens. Bellows extension, a phenomenon common in large-format photography, refers to the light falloff that occurs when the lens aperture moves away from the image plane during focusing. With most S LR lenses, the movement is negligible, but with 1:1 macro lenses, which by design require relatively long travel to go from infinity to maximum magnification, the bellows factor ( i.e., the amount you must adjust exposure to comoensate for the diminished light transmission) can be considerable. W ith this Tokina, for example, the image plane receives two stops less light at close focus than at infinity. When using most 1:1 macro lenses on today's AE SLRs; as you focus from infinity to 1:1, the AE system's aoerture readout remains constant as shutter speed lengthens to compensate for the light loss. This Tokina is the same, except for this: the aperture readout also changes as focus moves closer, going lroml /2.8 to f/5.6.l t's an indicationo f the effective aperture, not the actual aperture. Don't be alarmed. At maximum aperture, you're still getting the depth-of-field of an f/2.8, even though your camera's LCD says f/5.6.