A standard lens is made up of a combination of spherical lens elements. Individual "lenses" within the lens are commonly referred to as "elements". A spherical element has an even curve to the surface of the glass. However, there can be problems with such elements; light entering the center of the lens and light entering at the edge may not be perfectly focused at the same point. This is called spherical aberration. More advanced computer assisted optical designs are creating lenses with more spherical elements. More spherical elements within a lens means a greater risk of spherical aberration having a negative impact on optical quality.
Wide-angle zoom lenses and wide-angle lenses with large apertures are especially at risk for spherical aberration.
To eliminate spherical aberration, Tokina employs aspherical all-glass elements in many of its optical designs to correct this problem. The aspherical shaped surface of the lens element focuses light rays entering both the center and edge of the element correctly at the film plane for an accurately focused image. In addition to correcting spherical aberration, these elements fully correct light quantity and distortion at the edge of the image and provide excellent results when used in combination with a floating element design.
Through a close collaboration with Hoya Corporation, the world's largest optical glass manufacturer, Tokina has succeeded in producing high quality precision molded all glass elements with a greater aspherical shape than any other lens manufacturer. This technique is unparalleled in its technological sophistication and precision.
This lens, the AT-X 235 AF PRO encompasses Tokina's new F&R aspherical molded glass elements of an exceptionally large 50 mm diameter in the front and 20 mm diameter element at the rear. The 50 mm front element is one of the largest aspherical elements being employed in optical designs today. These give outstanding performance with very even illumination in the corners and correction of spherical aberration across the image area.
When standard optical glass is used in telephoto lenses, a phenomenon called chromatic aberration can occur. Chromatic aberration is the inherent tendency for glass to disperse (separate) a ray of light into the colors of the rainbow. The rainbow effect created by a glass prism is the most dramatic demonstration of chromatic aberration. In lenses, it is much less pronounced, but still creates slightly out of focus colors, akin to an "optical noise" that has a negative impact on the quality of the picture. To eliminate chromatic aberration, Tokina employs expensive, special glass material having super- low dispersion (SD) properties.
Lenses in the Tokina line-up with the SD mark incorporate these Super-Low Dispersion glass elements, minimizing the secondary spectrum or optical noise caused by chromatic aberration.
Reflections off the surface of lens elements are the enemy to any photographer and to every lens manufacturer. They are reduced or eliminated by bonding multiple layers of a transparent anti-reflection chemical to the surface of the glass. Tokina has developed and perfected a unique coating technique for all of its optics so that they will maintain faithful color reproduction and render clean, sharp images.
When designing a lens, Tokina calibrated its astigmatism at all points between minimum focus distance and infinity so that it will give the best image results at all settings. However, when there are large differences between the focus limits, effect calibration is not possible. A floating element system incorporates optical elements that move in proportion to the focus setting of the lens. This allows astigmatism to be corrected. Many Tokina lenses employ floating element systems to provide optimum correction of astigmatism from minimum focus distance to infinity.
The two most used methods of focusing a lens are either the complete straight forward movement of lens elements (used mainly with single focal length lenses) or the rotation of the entire lens barrel group (used mainly with zoom lenses). The internal focusing system used by Tokina move each element group within the lens, but does not change the overall length of the lens. This is especially useful with telephoto designs.
The internal focusing system has a number of advantages including:
By movement of the forward elements within the stationary main barrel: AT-X 235 AF PRO, AT-X 287 AF PRO SV, AT-X 280 AF PRO, AT-X 828 AF PRO
Tokina AT-X PRO series lenses all feature the patented "Focus Clutch" Mechanism for switching the lens between auto focus and manual focus modes.
The manual focusing ring can move (be snapped) back and forth between an AF and MF position. When the focusing ring is forward in the AF position, it is not engaged to any of the internal focus gearing and will turn freely. Without the added weight of the metal ring the camera can auto focus the lens more quickly and smoothly.
For manual focus, simply rotate the focus ring all the way to one side or the other on the focus travel, either infinity or it's closest focusingrldistance, then pull back (towards the mount plate) on the manual focus ring. While pulling back, rotate the ring from one side of the focus travel to the other. When the gears align, the focus ring will snap back into the MF position and the lens can be focused manually.
To return the manual focus ring to auto focus mode, simply snap the ring forward from any point.
A refinement of the focus clutch mechanism has been incorporated in the AT-X 280 AF PRO Nikon and Canon mount, so that now the lens focus ring can be snapped back to the manual focus position at any time, from any position. This improvement means it is not necessary to change the AF/MF switch (on the body with Nikon or the lens with Canon) every time the lens is changed from auto focus to manual focus mode.