Teleconverters look similar to extension tubes but they have a different function. They are inserted between the camera body and lens and provide magnification directly through optics. They are identified by the amount of additional image magnification that they provide. This is most commonly 1.4x and 2.0x, but are sometimes seen as high as 3.0x.
- Adds magnification while maintaining or increasing working distance.
- Will lose a little resolution and sharpness when added to a lens.
- Don't fit on all lenses, even of the same brand.
- Older ones may lose mechanical or electrical connection from the lens to the body.
Like extension tubes, teleconverters have a mechanical and/or electrical connection to the camera that may or may not allow for control of the lens from the camera body. I have a Sigma 1.4x teleconverter that is a newer model and allows full autofocus and aperture control by the camera. I also have an older Nikon teleconverter that only allows mechanical control of the aperture. The issues of electrical and mechanical connection are not a problem if you are using a teleconverter with a reversed lens as a reversed lens will have no direct electrical or mechanical connection to the camera unless a special adapter is being used. It does become a problem with the newest lenses.
One thing to watch out for is teleconverter-lens compatibility. Since teleconverters have internal lenses, there may be clearance issues when used with various lenses. My Sigma 1.4x teleconverter doesn't fit onto any of my Nikon macro lenses even though it is made for a Nikon mount. The only way that I can use the Sigma teleconverter is to put an extension tube between the lens and the teleconverter. This solution provides extra magnification but does solve the clearance issues.
My Nikon teleconverter is a different story. It will fit all of my lenses but doesn't have the electrical connection to the lens that the Sigma teleconverter has. Newer Nikon teleconverters have the electrical and mechanical connection. The older Nikon teleconverter works well with all of my lenses that have an aperture ring that can be set manually. My Nikon 18-70 zoom lens (no aperture ring) will take an exposure although the image will be severely underexposed, presumably related to the lack of an electrical connection to allow proper aperture control.
How do Teleconverters work?
The generally accepted dogma is that teleconverters increase the magnification, focal length, and aperture by the specified amount – such as 1.4x. A 100 mm f/2.8 1:1 lens will become a 140 mm f/4.0 1.4:1 lens with a 1.4x teleconverter. Once I started to think about how teleconverters work from an optics viewpoint, I quickly realized that the situation is not as clear or as simple as I would have hoped.
Close-up lenses work by shortening the focal length of the lens it is added to. They add magnification by increasing the effect of the extension that the lens already has. Teleconverters, as opposed to close-up lenses and typical camera lenses, are negative powered lenses. If you add one to a garden-variety camera lens you effectively create a telephoto lens out of the two, positive on the front (lens) and negative (teleconverter) in the rear. This combination creates a longer focal length lens in a relatively compact package, just like most long focal length lenses.
The added negative lenses cause the principal planes of the lens combination to move forward in proportion the increase in focal length. This allows the lens to retain the ability to focus at infinity, unlike close-up lenses.
The action of a teleconverter varies by how the lens is focused - far or near. When a teleconverter is added directly to a lens and that lens is used at infinite focus, the focal length of the combo will increase as predicted by the rated power of the teleconverter – such as 1.4x. This increase in focal length is done without changing the entrance pupil of the lens. This causes the f/number of the lens to increase proportionally. This means that my 100mm f/2.8 lens becomes a 140mm f/4.0 lens with a 1.4x teleconverter attached.
Where teleconverters get interesting (and complicated) for me is when the distance between the teleconverter and the lens elements is changing, i.e. you focus the lens closer than infinity. Camera lenses focus by either extending the lens or changing the focal length by moving lens elements in relation to each other. Most of the time you will see the rear lens elements recede into the lens as you focus the lens closer. This will create more distance from the teleconverter to the lens. The extra distance causes the focal length change to lessen (not 1.4x anymore) and even reverse at higher magnification. The combo of an older 100 mm lens focused at 1:2 with a 1.4x teleconverter actually has a focal length closer to 90 mm instead of the presumed 140 mm.
In addition to the focal length change that occurs with adjusting the focus, the principal planes are also in constant motion. The rear principal plane is at its farthest forward when the lens is at infinity focus and steadily recedes back into the lens as the lens is focused closer (in concert with the decrease of the focal length from its maximum). The interesting part about all of this focal length change and principal plane movement is that the amount of added magnification stays constant. To turn a complex explanation into a simple rule: A 1.4x teleconverter increases the magnification by 1.4x at all focus settings.
A secondary effect of the movement of the principal planes forward is that the working distance will improve. With our hypothetical 100 mm lens, the front principal plane was inside the lens without a teleconverter. When the teleconverter is added, the front principal plane moves forward significantly. This forward movement allows the working distance to increase. At the same magnification, the lens with a teleconverter will have more working distance than the same lens without a teleconverter.
A less prominent secondary effect of a teleconverter is that the exit pupil will decrease in size slightly, making the pupillary magnification decrease. This has no significant effect at far focus but does cause a slight decrease in its resolution capabilities at higher magnification. Counteracting the decrease in pupillary magnification is that the lens f-number will decrease somewhat as the lens is focused closer (related the the relative shortening of the focal length and no change in the entrance pupil). You will still worsen the effective aperture when you add a teleconverter, but you will lose a bit less at 1:1 than you would at infinity.
In my own experimentation in WinLens (optics software), I created a hypothetical 100 mm lens and added a 1.4x teleconverter to it. The resulting f-number went from 4.8 to 6.8 (1.4 x increase) at infinity, just as predicted. At 1.4:1 (1:1 on the lens with a 1.4x TC) the f-number went from 4.8 to 5.7 (1.2 x increase), not quite as much of a penalty. Overall, the effective aperture penalty is slightly worse than that of the same increase in magnification with extension tubes (but you do get more working distance).
An offshoot of the effective aperture penalty is that you will be better off with the teleconverter attached directly to the body instead of distant from the body. If you are using the teleconverter with extension tubes or a bellows, put the teleconverter on the body first, then add your extension. This will keep the aperture losses to a minimum and image quality at a maximum.
Finally, teleconverters cause a small loss in image resolution/sharpness related to the extra lens elements. Each of these lens elements will have their own lens aberrations added to the mix.
|Lens||Magnification||Working Distance (mm)||MTF50 (lw/ph)|
|55 micro + 25 ext||0.98:1||60||1188|
|55 micro + 1.4x TC||0.75:1||112||1110|
|105 micro + 25 ext||0.76:1||210||996|
|105 micro + 1.4x TC||0.72:1||273||854|
Take home points
- Teleconverters increase the magnification in proportion to their power rating. A 1.4x teleconverter will increase the magnification by 40% at all focus settings of the lens you are using.
- Teleconverter will increase the working distance at all magnification levels.
- Teleconverters will tend to slightly underperform extension tubes/bellows for resolution and sharpness at the same magnification with the same lens. Use them when you need the advantages of #2 above.
- Teleconverters are made to work best when added directly to the camera body. This placement will provide the best performance. If you are adding extension or a bellows, place the teleconverter first in line.