- Aperture Range: f/2.8 to f/22
- Three Glass Moulded Aspherical Elements
- Three SD Super-Low Dispersion Elements
- Multi-Layer Lens Coatings
- Silent-Drive Module AF System
- Internal Focus; One-Touch Focus Clutch
The Tokina AT-X 24-70mm F2.8 PRO FX lens gives photographers with full frame camera bodies a 24mm wide-angle to 70mm moderate telephoto lens with a fast F2.8 aperture at an affordable price.
The new groundbreaking proprietary optical design uses three precision molded all-glass aspherical lens elements to achieve excellent contrast, sharpness, and corrects for spherical aberrations. One of these elements is very special and difficult to manufacture. G09, is made from Super-Low Dispersion (SD) glass and is a large sized aspherical element.
These lens elements also yield even illumination across the whole image sensor.
- The Tokina 24-70MM lens also uses three SD (super-low Dispersion, “FK01” and FK03”) glass elements in the rear groups to control chromatic aberrations.
- All these specialized lens elements work in consort to yield maximum resolution to match the 50-megapixel DSLR cameras entering the market.
- The fast constant F2.8 aperture makes viewing and auto focusing possible in very low light situations while keeping some flexibility for setting the shutter speed.
In addition to excellent optics, the lens incorporates a fast moving SDM (Silent Drive-Module) motor to greatly reduce AF noise making it one of Tokina’s quickest and quietest auto-focusing lenses to date.
Tokina’s exclusive One-touch Focus Clutch Mechanism allows the photographer to switch between AF and MF simply by snapping the focus ring forward for AF and back toward the lens mount for manual focusing. The lens is also designed to stand up to the rigors of daily use by photographers in a wide variety of shooting conditions and environments.Aspherical Optics
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 moulded all glass elements with a better aspherical shape than any other lens manufacturer. This technique is unparalleled in its technological sophistication and precision.
This lens, the AT-X 24-70mm F2.8 PRO FX encompasses Tokina's new F&R aspherical moulded glass elements. These give higher performance with very even illumination in the corners and correction of spherical aberration across the image area.
SD Super Low Dispersion
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 colours 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 colours, 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, minimising the secondary spectrum or optical noise caused by chromatic aberration.
Tokina's wide-angle and standard zoom lenses feature a higher quality of optical glass known as Tokina HLD (High-refraction, Low Dispersion) glass. Having higher refractive index and lower dispersion properties, HLD glass is far less than standard optical glass to create lateral chromatic aberration, which is often a problem with conventionally designed wide-angle lenses.
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 enhanced a coating technique for all of its optics so that they will maintain faithful colour reproduction and render clean, sharp images.
Floating Element System
When designing a lens, Tokina calibrated its astigmatism at all points between minimum focus distance and infinity so that it will give the higher 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.
Internal Focus System
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;
- Faster focusing: Improved handling due to fewer movements near the centre of gravity
- More compact lens designs: Superior use of filters because the barrel with the filter thread does not rotate.
- Focus Clutch Mechanism: 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 focusing distance, 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.