What are teleconverters and how do they work?

Tele-converters are handy lens add-ons that multiply the focal length of the lens they are attached to. They can be mounted in between the camera body and lens (typically a telephoto lens) and increase the focal length of the lens which they are attached to.

By how much is dependent on their power, which is stated as a factor. E.g. a 1.4x teleconverter multiplies the original focal length of a lens by 1.4. As a result a 100mm lens would turn into a 140mm lens, a 200mm lens would turn into a 280mm lens, and so on. Most common are 1.4x or 2x converters, but occasionally converters as strong as 3x can be found as well.

Why am I telling you this in a macro blog? Well, the beauty of these converters is that their power not only applies to the focal length of regular lenses, but to the magnification ratio of macro lenses just as well!

Tele-converters for Macro Photography

Mounting a 1 macro lens to a 1,4x teleconverter will increase its magnification to 1,4x.

Unfortunately, the same factor will apply to the (effective) aperture that is being used, in terms of light loss and in terms of diffraction softening as well. A 1.4x converter will lose you one stop of light, a 2x converter reduces it by two.

The reason for this effect is, that such converters simply enlarge the aerial image that an existing lens is projecting, and because we are only using the center portion of this enlarged projection only a portion of the light, that enters the lens, reaches the sensor; hence the light loss. The slides beneath illustrate this effect:

On the flipside, this means there are good news, too; since the converter merely enlarges an existing image, there is no effect on its Depth of field, nor on the working distance, which can be incredibly useful out in the field or with reflective subjects…

Teleconverters Vs. Extension Tubes

But teleconverters are not the only way to enable macro magnification on a given lens, so why would one choose a n expensive piece of extra glass over hallow tubes that can be purchased for a fraction of the price a good converter costs?

Of course extension tubes are much more common among macro photographers, more affordable and therefore usually the first choice. But extension tubes will only get you so far, and every tube added is less effective than the previously one. Especially when we are using all our resources to get as close to the subject as possible, they make for an incredibly useful tool, that allows us to multiply the magnification ratio by a relatively large factor.

E.g. when we’re photographing something as tiny as a snowflake for example, or when we need a large magnification ratio and a reasonable working distance at the same time. To increase the magnification ratio even further we can combine a teleconverter with extension tubes as well. When doing so it is important to mount the converter first in line and the tubes to the converter – this will maintain a noticeably better image quality.

However, due to the fact that we add additional optics to an existing lens, teleconverters will decrease the image quality further than extension tubes would. The extra glass elements are additional obstacles in the path of light and no matter how well they are engineered, they will decrease quality to some degree.

However, both, teleconverters as well as extension tubes, will exaggerate chromatic aberration and limitations in sharpness, as we expand the image circle.

Because of the inherent loss of image quality, mounting multiple converters in sequence is not really recommendable. Of course this is dependent on a variety of factors such as the specific converters in use, their individual quality, and how well they work together but more often than not a sequence of multiple converters will ruin your images, either by simply decreasing their quality or due to diffraction.

An exception to this “rule” can be found in the images beneath, for where I combined two 2X converters to achive a magnification ratio of 16X. To get away with this I had to shoot my lens wide open at f/3.5 and accept slight diffraction softening.

Time to sum it up!

• Benefits:
  • Can be combined with tubes for even more magnification.
    (Always mount the extender first in line, closest to the camera.)
  • Increased working distance compared to a lens of equal magnification and focal length without a teleconverter. (The working distance will not change by adding a converter)
  • No loss of DOF. (generally the depth of field at a shrinks as we increase the magnification ratio; not so with teleconvertes.)
  • They are versatile, small and easy to carry. Work with a wide variety of lenses. (Including all Macro lenses, except for few specialty lenses.)
  • and even if you don’t own a macro lens, a 2X converter might just be enough to catapult you into the range of 1:1 lifesize magnification. Many zoom lenses have a close-up function, that will sometimes bring you up to 1:2 lifesize. Take that times 2 and you’ve got yourself a macro lens.
• Downsides:
  • Reduced image quality (degree varies, acceptable results are well possible)
  • Expensive (depending on the model and source. Good deals on 3rd party converters can be found on platforms like eBay. I have had good experiences with most renowned brands, like Tamron, Soligor, Kenko and Yongnuo.)
  • Light loss (1 stop 1.4x-converters or 2 stops at 2x-converters)

… and that’s pretty much it.

Just in case you’re interested, I’ll link my YouTube Video on this topic below.

For more future videos and macro tutorials, please subscribe to my channel, it’s a quick and easy way to support my content and let me know that you enjoy posts like this.

About the Author

Maximilian Simson is a macro photographer based in London, Ontario. You can find more of his work and creative ideas on his website and YouTube channel, and follow him on Instagram and Facebook. This article was also published here and shared with permission.