Thursday, 21 November 2013

Using Teleconverters (Extenders or Multipliers) on your DSLR

Canon Teleconverters
Canon Teleconverters

A Teleconverter or tele-extender or multiplier is like a magnifying glass used between the camera lens and the camera body (like an extension tube). Teleconverter magnifies the image created by the lens in front of it and transmits the magnified image to the camera sensor. The major difference between a teleconverter and an extension tube is that teleconverters have optical elements in them to magnify the image where as extension tubes do not; they just act as spacers between the camera body and lens moving the lens farther away from the image plane

Uses of Teleconverters

Nikon Teleconverters
Nikon Teleconverters

Teleconverters are primarily used in situations where you can’t get close enough to a subject to attain the required magnification. Typical uses are Wildlife Photography (when it’s either impossible or is dangerous to get near your subjects), Sports Photography (when one is confined to the sidelines) and Macro Photography (when shooting live insects which are easily scared off if approached very near).

Attaching a teleconverter to your lens lets you get closer to your subject without physically moving closer.

Teleconverters and Magnification

Tamron Teleconverters
Tamron Teleconverters

Typically each teleconverter delivers a fixed amount of magnification; they are available from major camera manufacturers like Canon, Nikon, Olympus etc and also from third party manufacturers like Sigma, Tamron, Kenko etc. They are commonly available in the following magnifications.

1.4X Teleconverter :- These increase the magnification by 40% and causes a 1 Stop loss in light.

1.7X Teleconverter :- These increase the magnification by 70% and causes light loss equivalent to 1.5 Stops.

2X Teleconverter :- These increase the magnification by 100% and causes 2 Stop loss in light.

There are 2.4X and 3X Teleconverters available from third party manufacturers but the important thing about teleconverters is that they have optical elements in them and so unlike extension tubes they tend to degrade the image quality, higher the magnification greater the quality loss; so a 1.4X tele-converter provides least magnification but best image quality where as a 2X tele-converter provides more magnification at the cost of image quality. Most professional photographers are of the opinion that loss in image quality is apparent from 2X onwards and at even greater magnifications the amount of aberrations makes results unacceptable. It is better to shoot with a tele-converter of lower magnification and crop the image to get the print magnification rather than compromise on image sharpness.

Teleconverter Vs. Extension Tube

Pentax Teleconverters
Pentax Teleconverters

  • Teleconverters have optical elements in them while extension tubes are just hollow tubes.
  • Teleconverters deteriorate image quality while extension tubes do not affect image quality in any way.
  • The optical elements in the teleconverter actually increase the focal length of the lens attached; extension tubes do not.
  • Teleconverters do not change the working distance while extension tubes reduce the working distance to achieve magnification.
  • Both teleconverters and extension tubes reduce the amount of light transmitted to the sensor.
  • With both teleconverters and extension tubes the camera’s TTL meter automatically compensates for the light loss to attain a proper exposure.

Using Teleconverters for Macro Photography

Using Teleconverters for Macro Photography
Using Teleconverters for Macro Photography

In the illustration given above you can see how the teleconverter magnifies the image created by the lens. It can also be observed that the working distance (distance from the front of the lens to the subject) remains the same even with a teleconverter attached. In fact with a teleconverter attached to the lens, the photographer is slightly farther from the subject than without it (the length of the converter). This ability of the 
teleconverter to achieve magnification while allowing more working distance is what makes it the favourite tool of macro photographers.

Tips for Using Teleconverters

Olympus Teleconverters
Olympus Teleconverters

  1. To use teleconverters first attach the converter to the lens and then mount the combination to the camera body.
  2. Tele-converters provide best results when used with fixed-focal-length telephoto (Prime) lenses and fast professional level tele-zooms.
  3. Using tele-converters with cheaper zoom lenses or those superzooms  (which covers lot of focal length from wide angle to tele photo) will result in very low image quality.
  4. There are teleconverters designed and manufactured to be used with specific group of lenses or lenses of certain focal lengths, they are to be used with matching lenses for best results.
  5. It is not recommended to use teleconverters on wide angle lenses, do check the teleconverter’s instruction manual for details on compatibility with the camera model and lens type and also mounting instructions. Trying to use incompatible lens/camera/converter combination could even result in physical damage to your camera.

Teleconverters and Auto Focus

Sigma Teleconverters
Sigma Teleconverters

We have mentioned earlier in this article that teleconverters reduce the amount of light passing through the lens. This reduction in f stops also affects the performance of the camera’s auto focus systems. Most DSLR cameras, except the top of the line models have AF systems designed to work at apertures of f/5.6 or larger. With slower lenses the width of the light beam transmitted to the AF module isn’t enough to focus accurately and hence the cameras AF system gets deactivated. Attaching a teleconverter to an otherwise fast lens has the same effect on the AF system and you will lose AF function when the lens + converter combination is slower than f/5.6.

For example:

Any lens slower than f/4 will lose autofocus when a teleconverter (of any power) is used. Similarly to retain AF functionality when using a 2X teleconverter one should be shooting on a lens whose maximum aperture is f/2.8 or wider.

Apart from losing the AF function totally when the maximum aperture exceeds f/5.6 attaching a teleconverter also significantly slows down the performance of Auto Focus systems. The lens will take longer to focus than normal with fast tele photo lenses which have very fast focusing systems the reduction in AF performance will not be much of a problem, but cheaper lenses will struggle to attain focus even in brightly lit situations.

Advantages of Teleconverters

  • Small size: they are reasonably light and portable.
  • Cost effective: they are a relatively cost effective solution to buying lenses.
  • Vesatile: can be used for a wide range of applications.

Disadvantages of Teleconverters

  • Deteriorates image quality
  • Provides only Fixed Magnification: to change magnification you need to change the teleconverter with one of different power.
  • Loss of Light 
  • Renders the viewfinder dimmer.
  • They increase the focal length of the lens and also causes light loss, thus it becomes difficult to get sharp pictures handholding the camera. You will need to either raise ISO or use additional lighting (flash).
  • Slows down the performance of Auto Focus