HOW TO USE A TELECONVERTER
Teleconverter (English sec. TS) is an additional optical system, which is fixed between the camera body and the main lens. The goal of the teleconverter is to increase the effective focal length of the primary lens. TSs are characterized by multiplicity of increase, which varies from 1.2x to 3.0x (the most common are 1.4x and 2.0x). For example, for a telephoto lens with a focal length of 300 mm f / 2.8, a 2.0x teleconverter will double the focal length and reduce the maximum aperture by two full feet, resulting in a focal length of 600 mm f / 5.6. Teleconverters have the same effect on the zoom lens – the entire zoom range will be increased, and the maximum aperture will be reduced. For example, a 1.4x fold TS turns a lens with a focal length of 70-200 mm / 2.8 into a lens with 98-280 mm / 4.0.
Teleconverter consists of several optical elements. Usually, the larger the multiplicity of the increase, the greater the physical size of the vehicle. Its optical design usually includes standard lenses without optical corrections, which leads to an increase in optical aberrations. Manufacturers are trying to minimize their impact by incorporating more complex lens elements, such as aspherical lenses, into a TC design.
Teleconverters have limited use of extra-low dispersion lenses, in part because of possible compatibility issues with primary lenses. But sometimes manufacturers make vehicles specifically for one lens, and in such cases they can include any suitable elements of optical lenses. One of these known cases is the NIKKOR AF-S TC800-1.25E ED teleconverter, which was developed specifically for the exotic Nikon 800mm f / 5.6E FL ED VR lens, and because of this the Teleconverter TC800-1.25E ED cannot be purchased separately, as other common teleconverters.
NIKON D700 + 300 mm f / 4D AF-S @ 420 mm (1.4x TC), ISO 250, 1/2000, f / 5.6
Each lens manufacturer also produces teleconverters, the most common models with multiplicity of 1.4x and 2.0x. Some manufacturers produce teleconverters with different multiplicities, but their use and efficiency can vary greatly depending on the lens. For example, Nikon and Hasselblad produce 1.7x teleconverters, Kinko create vehicles with much larger multiples of 3.0x.
Unfortunately, teleconverters have a pretty strong influence on the performance of the lens both in terms of overall sharpness and in terms of autofocus speed, so you need to be very careful when choosing a device with a multiplicity of more than 1.4x. To get closer to an object, it is sometimes better to crop the image in post-processing than to do the same with a teleconverter.
Teleconverter and lens compatibility
Although teleconverters are usually designed to work with multiple lenses, there are no teleconverters on the market that work with any lens. Both Nikon and Canon have fairly small lists of lenses that are compatible with their teleconverters. Most lenses are not designed to be connected to teleconverters. Some have physical limitations, such as a rear element that goes too close to the camera mount, while others have optical limitations. Since most teleconverters are specially designed for professional super-telephoto lenses, most wide-angle, standard and tele-lenses are not compatible with them.
However, there are exceptions: some macro lenses, such as the Nikon 105mm f / 2.8G VR, work quite well with Nikon teleconverters. It is important to note that, with very few exceptions, made by one manufacturer of the vehicle are designed to work only with lenses of the same manufacturer.
Using TS with zoom lenses
Teleconverters usually work much better with super-telephoto lenses than with zoom lenses.
There are several reasons for this:
Zoom lenses are slower than normal, i.e. They get less light for the camera’s autofocus system. As a result, this affects both the overall autofocus speed and its accuracy. Teleconverters can significantly reduce the maximum aperture of the lens, and even completely disable the autofocus capabilities of the camera.
It is very difficult to optimize a zoom lens for uniform operation at all focal lengths, which, when adding a teleconverter, makes the sharpness uneven and irregular throughout the entire zoom range.
With the increase in the number of lenses moving in groups when scaling, lens decentering and other optical problems become even more apparent.
However, there are cases when teleconverters work well with zoom lenses. For example, it is known that the Nikon 70-200mm f / 2.8G VR II works well with the TC-14E II / III, very well with the TC-17E II, and if someone is ready to stop at f / 8, then even the TC- 20E III can be a very useful combination.