Pixiq

Simon Stafford explains how to use and get the best out of the Nikon AF teleconverters. Pictures of the teleconverters courtesy of the Nikon Corporation, all other pictures by the author.

 Nikon TC-E Teleconverters

Nikon teleconverters are an inexpensive and convenient way to gain greater subject magnification with certain Nikon Nikkor lenses. Currently, Nikon produce teleconverters in three magnifications: 1.4x, 1.7x and 2x. A teleconverter magnifies the image formed by the principal lens, so a 1.4x teleconverter increases the effective focal length of a lens by a factor of 1.4, while a 2x teleconverter increases the effective focal length by a factor of 2.

The current range of Nikon teleconverters comprises the Nikon AF-S TC-14E II, TC-17E II and TC-20E III. The earlier Nikon AF-I TC-14E and TC-20E have exactly the same optical construction as the AF-S TC-14E II and TC-20E II, with the only difference between the AF-I and AF-S versions of these teleconverters being cosmetic, as the external paint finish of the AF-S teleconverters was changed to match that of AF-S Nikon Nikkor lenses and AF-S TC-17E II teleconverter. The TC-20E II was replaced during early 2010 by the TC-20E III, which has a new optical configuration, including an aspherical element. 

Nikon TC-E Teleconverter Compatibility

The range of Nikon TC-E converters, as listed in the chart above, has been designed to only work with specific professional grade Nikon AF-S / AF-I type lenses, generally these include all variants of fixed focal length Nikkor lenses with a focal length of 200mm, or more and a maximum aperture of f/4, or wider (lower f/# number), plus all variants of Zoom-Nikkor lenses with a focal length range of 80-200mm, 70-200mm, and 200-400mm and a maximum aperture of f/4, or wider. The lens mount of a TC-E teleconverter has an additional, small flange that allows only compatible AF-S /AF-I lenses, which also have a modified mounting bayonet, to be connected together. 

The difference in the lens mount of these lenses does not affect their compatibility with Nikon camera bodies but does prevent non-compatible Nikon lenses being used with the range of Nikon TC-E converters. For the same reason it is not possible to connect two TC-E teleconverters together, a technique sometimes called “stacking”; even if this were possible the loss of optical quality would make the technique unworkable. 

Some photographers have the additional flange of their TC-E teleconverter milled away to allow it to be attached to a wider selection of Nikon Nikkor lenses; however, this will not only invalidate any manufacturer warranty but also requires that the lens is chosen with care, because unless the lens has a recessed rear element it will still not be possible to connect the TC-E teleconverter to the lens, the front elements of the TC-E teleconverter protrude too far forward. 

Always check the documentation to your lens and teleconverter, or consult a Nikon technical support web site to confirm compatibility.

Autofocus with TC-E Teleconverters

Even though a particular TC-E teleconverter may be compatible with a Nikon Nikkor AF-S / AF-I lens, autofocus operation may still not be possible. Autofocus systems used in Nikon AF cameras use the light passing through the lens to acquire focus; a small amount of the light passes through the reflex mirror and is directed down on to the AF sensor located in the base of the mirror box by a secondary mirror. If insufficient light is transmitted though the lens / teleconverter combination for the AF sensor to work properly autofocus operation will be impaired; at best AF speed / accuracy will be reduced, while at worst the AF will not function. 

Nikon states that for proper autofocus operation the maximum effective aperture of the lens / teleconverter combination must be equivalent to at least f/5.6. At a smaller effective aperture value (larger f/# number) autofocus operation may be affected, although the degree to which this occurs will depend in part on the level of ambient illumination. In bright conditions, using a cross-type autofocus sensing point (the number of cross-type autofocus points varies between different Nikon AF camera models but the central AF point is always of this type) it may still be possible for autofocus to work. Remember a lens / teleconverter combination will reduce the level of light transmission (see chart above), so for example a lens with a maximum aperture of f/2.8 combined with a 2x teleconverter will have a maximum effective aperture of f/5.6, allowing the camera to still perform autofocus but a lens with a maximum aperture of f/4 will have a maximum effective aperture of f/8, which is less than the threshold value of f/5.6, so autofocus operation is likely to be affected 

Shooting With Teleconverters

There are some photographers who continue to shoot under the misunderstanding that a teleconverter changes the focal length of the lens to which it is attached, and therefore also influences the depth of field characteristics of that lens, which is nonsense! The focal length of the principal lens remains exactly the same when a teleconverter is connected; it is the magnification of the subject that the teleconverter changes, and since depth of field is a function of subject magnification and the size of the lens aperture, it is these parameters that impact on the depth of field, while for all practical purposes the effective focal length has virtually no influence. 

This can be tested by taking a series of photographs of the same subject set against the same background, using a range of different focal lengths; provided you keep the subject magnification (size) the same in each image by altering the camera-to-subject distance, the depth of field will, for all intents and purposes, be exactly the same in each picture. Although the perspective in each image will be different since perspective is dependent on the camera-to-subject distance. If, however, you maintain the same camera-to-subject distance and use a range of different focal lengths, so the magnification of the subject alters, the depth-of-field will be different in each picture, while the perspective will remain the same.

Teleconverters are often considered as a panacea to increasing subject magnification; however, there are a number of factors to consider in terms of how they affect image capture and image quality in general:

• By reducing the light transmission the teleconverter causes the effective maximum aperture of the principal lens to be reduced, which has a direct influence on the range of available shutter speeds and ISO sensitivity settings in any given amount of light

• Most lens / teleconverter combinations work best at one to two stops down from the maximum aperture of the principal lens, which limits the available shutter speed and ISO range even more

• The introduction of the extra glass / air surfaces of the teleconverter into the optical path can reduce image contrast and colour saturation 

• Image sharpness is often reduced as the teleconverter not only magnifies any shortcomings of the principal lens but also introduces optical aberrations of is own

• Teleconverters work best with principal lenses that are prime lenses (a prime lens has a fixed focal length); very few zoom / vari-focal lenses produce an acceptable image quality with a teleconveter

A teleconverter is essentially a magnifying glass. It is designed to magnify a central portion of the image formed by the lens to which it is attached, the principal lens, so that it covers the full frame area of the sensing medium in the camera (film, or digital sensor). For example, used on a Nikon FX-format digital camera, where the frame area is 24 x 36 mm, a 2x teleconverter magnifies the central 12 x 18 mm area of the image formed by the principal lens to fill the 24 x 36 mm frame, thus the maximum reproduction ratio of the principal lens is doubled. Introducing the teleconverter into the optical path not only shifts the principal lens further away from the sensing medium reducing the amount of light reaching it but also reduces light transmission itself, which affects the exposure level, as well as the depth of field characteristics; however, the minimum focusing distance of the principal lens is the same when a teleconverter is attached.

A lens of focal length (L) and aperture (f/number) value (f) with a teleconverter of magnification (M) attached will behave, for all intents and purposes, as a lens with a focal length of (M x L), and aperture value of (M x f). If the aperture value (f/number) and the camera-to-subject distance remain unchanged, and then a teleconverter is attached, the lens will focus at the same distance; however, the image, including the diffraction effects and aberration effects of the principal lens, will be (M) times as large, the exposure (shutter speed) will need to be M x M times longer, or alternatively the ISO sensitivity (M x M) higher, the hyperfocal (HF) distance will be multiplied by (M), and the depth of field (DoF) will be divided by (M). Plus, the teleconverter will also introduce some optical aberrations of its own.

For example, a 200mm f/2 lens used with a 2x teleconverter behaves as a 400mm (M x L = 2 x 200) f/4 (M x f = 2 x 2) lens. In this case the focal length and aperture values are referred to as the "effective" values. If the base exposure established with the principal lens is 1/1000 second at f/2 (ISO 200), attaching the teleconverter and leaving the aperture value of the principal lens at the same setting, will require either the shutter speed to be increased by four times (M x M = 2 x 2) the original duration, to a 1/250 second, or alternatively the ISO value raised by a factor of four (M x M = 2 x 2), to ISO 800, to maintain the same exposure level. If the camera-to-subject distance is not altered the subject is magnified by a factor of two, while the depth-of-field for the lens/teleconverter combination will be half (DoF / 2) that of the principal lens alone, and the hyperfocal distance will be doubled (HF x 2).

Alternatively, if the aperture value of the principal lens is adjusted (opened to a lower f/# number) to maintain the same exposure level (shutter speed and ISO) when the teleconverter is attached, the image formed by the principal lens is magnified by (M); however, the depth of field will be divided by (M x M) and the hyperfocal distance will be multiplied by (M x M). In terms of optical aberrations these are likely to be increased, as the principal lens is now being used at a larger aperture, and the teleconverter will magnify these aberrations by (M), plus, as mentioned above, it also introduces some optical aberrations of its own.

For example, a 200mm f/2 lens used with a 2x teleconverter behaves as a 400mm f/4 (effective) lens. If the base exposure with the principal lens is 1/250 second at f/4 (ISO 200), and the lens aperture is opened to f/2 when the teleconverter is attached, so the effective aperture of the lens/teleconverter combination is f/4, to maintain the same exposure level, so the shutter speed and ISO value are not altered, the depth of field will be reduced by a factor of four (DoF / M x M = DoF / 2 x 2), and the hyperfocal distance will be increased by a factor of four (M x M = 2 x 2).

Summary

As the high ISO sensitivities of modern Nikon D-SLR cameras continues to improve the use of a teleconverter becomes ever more practical since it enables the user to offset the negative effects of reduced light transmission, since faster shutter speeds can be attained without compromising image quality. I have used all three of the current Nikon TC-E teleconverters extensively; in my opinion the Nikon TC-14E teleconverters have an almost imperceptible impact on image quality with all compatible lenses, while the TC-17E II is not far behind with most lenses. For a 2.0x teleconverter, theTC-20E II does not enjoy a particularly good reputation and I would concur with this; however, the new TC-20E III has changed all that and based on my testing so far represents a very usable photographic tool. It will be the subject of a full review by me here at Pixiq in due course.