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Understanding Teleconverters


On this extensive video wildlife and nature photographer Steve Perry (from backcountrygallery) takes a look at teleconverters and what they can do for you as well as some of their inherent disadvantages.

Let’s start with the basics – a teleconverter is basically a lens which you add between your camera and lens to extend the focal length of your lens. Since there are no free meals in optics you will lose some light in the process resulting in a slower effective max f-stop (and as we shall see in a moment there are other potential drawbacks as well).

Here are a few simple examples to get you to understand the basic idea:

  1. If you have an 70-200mm f/2.8  and you use a 1.4x teleconverter you will get a focal range of 98mm-280mm and your max aperture will be reduced to f/4 (simple to understand = 1.4x the focal range and 1-stop less in light).
  2. If you have a 1.7x teleconverter which some manufacturers do you will gain 1.7x the focal length and lose 1.5-stops of light. In this case, you will get 119mm-340mm f/4.8 out of the original 70-200mm f/2.8
  3. Finally, a 2x teleconverter gives you 2x the focal length but you lose a whole 2-stops of light. So your 70-200mm f/2.8 will turn into a 140mm-400mm with an f/5.6 aperture.

Just to make things clear if you add a teleconverter you will be getting the depth of field of the resulting “lens” – that is your 70-200mm f/2.8 with a 2X teleconverter will turn into a 140mm-400mm with an f/5.6 depth of field not f/2.8 (again no free meals in optics).

There are other disadvantages of using a teleconverter. First, you might lose some image quality. This isn’t as exact as with the focal length and aperture calculation we mentioned above – with some of the modern 1.4x teleconverters on high-end glass the image degradation might be so minimal that you will not notice it – hower we have not seen a 2X  teleconverter that does not degrade the IQ even on high-end lenses to some extent (although it might still be useful for some applications).

Another aspect that can take a hit when using a teleconverter is AF. This has to do with all sorts of factors including the loss of light. It will depend not only on the specific lens that you use and how fast it focuses (and what Aperture it has to begin with) but also on your camera and how well it can focus with very slow apertures (for example – if you start with say a 500mm f/5.6 lens and use a 1.4X teleconverter you will be shooting at most with an f/8 lens and you need a camera body that can AF well with such a slow aperture – don’t take this for granted – some new bodies do – some don’t and might even lose AF completely with a 2X teleconverters).

As Perry mentions in the video – if you are using a teleconverter and get an effective “lens” with a max aperture of f/5.6 or less – you should consider using the center AF points on your camera as it is possible that other might not function as well if at all.

Here is an interesting positive aspect of using a teleconverter. The min focus distance of your lens will not change – so for example, if you are using a Nikon 300mm f/4 PF lens with a min focus distance of 1.45m it will still have the same min focus distance even with a 1.4X teleconverter – although it is now effectively a 420mm lens. You can even use this to increase the magnification of macro lenses – which is actually pretty interesting.

Here is a nice tip to remember – if you use a teleconverter which effectively changes you f-stop say from f/5.6 to f/8 and then you remove it and go back to the lens without the teleconverter – some cameras (Nikon for example which Perry uses) keep the f/8 number so you need to remember to change it back to f/5.6 if you want to shoot wide open.

Finally – when working with a teleconverter just think of your lens as if it is a real longer/slower lens – that means that you might need to increase your shutter speed to prevent shake for example. You might also want to calibrate your teleconverter+lens combo separately from your lens only calibration (this mostly applies to DSLRs).

This is not Perry’s first teleconverter related video we have published here – you might also want to check out his interesting comparison: “Which is Better – Crop Camera Or Full Frame Camera With A 1.4 Teleconverter?” we have covered here a few years back.

You can find many more previews and reviews on our Photography review section here on LensVid. We have a special page dedicated to all of Perry ‘s videos which you can find here.
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