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photo:teleconverters

teleconverters explained

Introduction

  • a teleconverter (or “teleextender”) is a small optical lens component which normally fits between the camera and the lens (although there are some that are built into the lens and can be moved in and out of the optics, and some which are designed to go on the filter thread at the end of the lens).
  • these teleconverters are designed to provide more magnification by effectively increasing the focal length of the lens
  • they generally come in two “strengths”: 1.4x and 2x
  • these should NOT be confused with “extension” tubes which do not have any optics but just increase the distance between the camera and lens to allow higher magnification via closer focusing for macrophotography

problems with teleconverters

  • effective aperture and light transmission becomes less, potentially requiring higher ISO or slower shutter speeds
  • degradation of image quality
    • may require stopping the aperture down 1 stop to attain sharpness if it is a high quality teleconverter
    • lower quality teleconverters can give poor image quality no matter what the aperture
    • if one needs to increase ISO to compensate for the aperture loss, then this will add to image degradation
      • one should decide whether the benefits of a teleconverter really gives better image quality than just cropping the image and keeping the benefits of a low ISO and wide aperture and accept the loss of pixels:
        • a 1.4x teleconverter effect can be achieved by a 1.4x image crop thus a 20mp image would become 10mp
        • a 2x teleconverter effect can be achieved by a 2x image crop thus a 20mp image would become 5mp
        • the “loss of pixels” may not be as big an issue as it may seem, especially if one is shooting through large distances of atmosphere creating a image degrading mirage effect
    • may affect quality of bokeh and degree of distortion, coma, astigmatism, spherical aberration and CA
    • TCs will also lower contrast by adding internal reflections and flare
    • image quality issues are more likely to be evident when used with zoom lenses rather than prime lenses
  • may affect AF accuracy
    • this is particularly likely with dSLRs, ypu may need to do a AF microcalibration with the TC on
  • generally result in slower AF due to less light getting to the sensors and failure of PDAF capability
    • most dSLRs have cross-type AF sensor points which revert to non-cross at apertures of f/5.6 or smaller
    • in fact, the decrease in aperture may make AF not possible in some cameras (especially dSLRs in which the AF points often do not function if the effective wide open aperture becomes less than f/8)
  • may alter the biomechanical ergonomics of larger lenses by pushing them further from the camera
  • may affect weathersealing
  • additional electronic lens communication pins means increased chance of failure of lens communication, especially if the pins are getting tarnished or misaligned
  • additional mechanical device means increased risk of optical misalignment and wobble of the lens
  • may reduce efficacy of image stabiliser
  • they may be designed ONLY for certain lenses
  • they may NOT be designed to be stacked - some do allow a 1.4x to be used WITH a 2x to give 2.8x effect but most modern ones do not

  • for still photography in low light, especially with moving subjects, you will probably be better off cropping rather than using a teleconverter (or use a more powerful telephoto lens)
  • in general, with the same lens shot wide open, you will get sharper images with a 20mp 1.5x cropped sensor camera than with a 20mp full frame camera and a 1.4x TC, plus you lose any ISO advantage of the full frame and potentially a much reduced PDAF region.

1.4x teleconverters

  • these effectively change the lens focal length and effective aperture to 1.4x the focal length so that when used on a 200mm f/2.8 lens it has the field of view and depth of field (DOF) of a lens 280mm f/4 lens
  • the price you pay for this benefit in telephoto reach is:
    • 1 stop less aperture which may require 1 stop higher ISO in low light levels, and,
    • perhaps 1 stop less sharpness (5-15% less with primes, 15-25% less with zooms) , so you may need to stop down by 1 stop which then requires a further 1 stop higher ISO in low light

1.7x teleconverters

  • these effectively change the lens focal length and effective aperture to 1.7x the focal length so that when used on a 200mm f/2.8 lens it has the field of view and depth of field (DOF) of a lens 340mm f/4.8 lens
  • the price you pay for this benefit in telephoto reach is:
    • 1.5 stops less aperture which may require 1.5 stops higher ISO in low light levels, and,
    • perhaps 1 stop less sharpness, so you may need to stop down by 1 stop which then requires a further 1 stop higher ISO in low light

2x teleconverters

  • these effectively change the lens focal length and effective aperture to 2x the focal length so that when used on a 200mm f/2.8 lens it has the field of view and depth of field (DOF) of a lens 400mm f/5.6 lens
  • the price you pay for this benefit in telephoto reach is:
    • 2 stops less aperture which may require 2 stops higher ISO in low light levels, and,
    • perhaps 1-1.5 stop less sharpness, so you may need to stop down by 1 stop which then requires a further 1 stop higher ISO in low light

more resources

photo/teleconverters.txt · Last modified: 2019/07/01 14:38 by gary1