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All About Tele-Converters

A Tele-converter (sometimes called a Tele-extender) lens is a small, relatively inexpensive lens that fits between a Digital SLR (DSLR) camera and the lens. It increases the effective focal length of the lens that it's attached to, thus having the effect of magnifying the image that your camera captures.

They typically come in 3 "strengths": 1.4x, 1.7x and 2.0x.

Each "strength" magnifies by the amount indicated. For example, using a 1.4x teleconverter on a 200mm lens will effectively change the focal length to 280mm; using a 2.0x on a 200mm lens will change the effective focal length to 400mm.

Here is an example of using a 1.7x tele-converter on a 300mm lens:

(click on the image below to enlarge)
Exposure Compensation
Teleconverter comparison

The Benefit
The main benefit of a Teleconverter is obvious: getting a "longer" lens for not too much money.

If you want to shoot wildlife, it's a relatively inexpensive way to convert a lens you may already own into a "longer" lens, that will get you closer to your subject, without adding much weight.

In addition, the lens you attach it to maintains the same minimum focusing distance - which means you can focus relatively close.

But, teleconverters have their drawbacks as well.

The Drawbacks
There are several drawbacks to using a teleconverter. These include:
  • Reduced maximum aperture
  • Difficulty in shooting (camera shake)
  • Reduction in quality

Reduced Maximum Aperture
Teleconverters reduce the amount of light that a lens can transmit. This reduction can be significant. Here's how much reduction you can expect:

TeleconverterLight Loss
1.4x1 f-stop
1.7x1.5 f-stops
2.0x2 f-stops
Let's say you have a 300mm (non-zoom) f4 lens. When you add a 2.0x teleconverter to it, it becomes a 600mm lens. But, the maximum aperture is reduced to f8 - thus you effectively have a 600mm f8 lens.

It is unlikely that your camera will be able to auto-focus with an f8 lens, as most autofocus systems need at least an f5.6 maximum aperture to achieve focus.

If you have a zoom lens that has a variable aperture, you'll most likely be using the teleconverter when zoomed to the maximum focal length. Let's use the Nikon or Canon 18-200mm f3.5-5.6 as an example.

When zoomed to 200mm, these lenses have a maximum aperture of f5.6. Putting a 2.0 teleconverter on this lens will turn it into an effective 400mm f11 lens. This will make this lens very difficult, if not impossible, to use. At f11, the viewfinder will be VERY dark and the auto-focus system will not be functional. With the viewfinder so dark, it will make manual focusing almost impossible.

Putting a 1.4x teleconverter on this lens effectively turns it into a 280mm/f8 lens. The light loss isn't as great, but autofocus is still rendered ineffective, and the gain (200mm to 280mm) isn't that great.


Difficulty in Shooting
Whenever shooting with longer focal lengths, your ability to hand-hold your camera steady enough to take a non-blurry picture becomes more difficult. Using an effective focal length of more than 300mm, almost always requires some form of external stabilization: a tripod, bean bag, etc.

Reduction in Quality
The reduction in quality can be significant. Teleconverters ALWAYS reduce quality, often by a LOT. The more the teleconverter magnifies, the more the loss in quality. 2.0x teleconverters will reduce quality the most.

The most obvious reduction in quality will be in image sharpness. Images taken with teleconverters will always be "softer". This difference will be greatly exaggerated when low-quality (translated: less expensive) teleconverters are used. There will also be a reduction in contrast and color quality.

Teleconverters also work best with PRIME lenses - that is, lenses that don't ZOOM. The reduction in quality in a zoom lens will always be substantially more than with a prime lens. They also work best with LONGER telephoto prime lenses - e.g., 300mm or longer.

And when using a teleconverter, you never want to shoot at the lens' maximum aperture. For example, with an f4 lens with a 2.0x teleconverter, the maximum aperture is f8 - you will need to shoot with an aperture of f16, otherwise the loss of sharpness will be very high. This obviously makes shooting very difficult, especially in low light (cloudy day, etc.).

Here's an image of a tree trunk shot with a 300mm/f4 prime (non-zoom) lens. The image on the left was shot with a 1.7x teleconverter wide-open (f6.7). The image on the right was shot at the same aperture without a teleconverter, but up-sized to show the same scene area. You can see how much sharper the image without the teleconverter is, even when cropped.

(click on the image below to enlarge)
Teleconverter comparison

Cropping as an alternative
Modern digital cameras take very high resolution images. One of the great benefits of a high-resolution image is the ability to crop it without any significant loss in quality. You would crop your images in software (Photoshop, Elements, Lightroom, etc.). When you crop an image, it creates the same effect as if you had shot with a longer focal length lens, but you end up with a slightly smaller resolution image.

The following images show the CENTER portion of the images, thus show the highest quality (the edges of images are always lower quality). These images were taken with high-quality lenses and a high-quality teleconverter. Lower quality optics would yield significantly reduced quality.

The images on the left were taken with a teleconverter and the images on the right were not, but were cropped to show the same image area.

The top set was taken with a zoom lens and the bottom set was taken with a prime lens.

(click on each image below to enlarge)
Teleconverter comparison

Teleconverter comparison

You can clearly see the differences in the images. The prime lens with the teleconverter (bottom set) created a sharper image than with the zoom lens (top set), but both show a reduction in sharpness when the teleconverter was used. In each case the image taken without the teleconverter, but cropped, is much better.

Before using a teleconverter, you should consider whether cropping your images is a better alternative!

In Summary...
Teleconverters have a place, but for most photography, cropping can be a better, and less expensive, option.

Happy shooting!!

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