I’ve had a number of questions about teleconverters, so let’s get to it!
Basically, a teleconverter is a magnifier for your existing lens.
It’s a specific type of lens designed to be mounted in between the camera and a normal lens, and the amount of magnification you get from it depends on the teleconverter.
Typical teleconverter magnifications are 1.4x, 1.7x and 2.
To figure out the effective focal length of your lens when using a teleconverter, just multiply the teleconverter’s magnification strength and the lens focal length.
So a 70-200mm zoom lens used with a 2x teleconverter gives you an effective focal length of 140-400mm.
That increased effective focal length is the principal benefit of using a teleconverter, but that’s not all a teleconverter does.
In addition to changing the effective focal length of the lens, it also changes the effective maximum aperture, decreasing it depending on the strength of the teleconverter.
This effective decrease in the size of your maximum aperture results in an actual loss of light transmitted through the lens and teleconverter to the camera.
A 1.4x teleconverter causes a 1 stop change in aperture and loss of light. A 1.7x causes a 1.5 stop loss, and a 2x teleconverter causes a 2 stop loss.
This means that when you use a 1.4x teleconverter with a 70-200mm lens, the effective focal range of the lens becomes 98-280mm, and the effective maximum aperture is f4.
When you’re using a teleconverter, you’ll see this reflected in your camera’s settings.
So with that 70-200mm f2.8 lens and a 1.4x teleconverter, the maximum aperture you’ll be able to set on the camera will be f4.
And if you were to use a 2x teleconverter with that lens, the maximum aperture you’d be able to set would be f5.6.
The obvious drawback to the loss of light is that depending on the lighting you’re shooting in, it may be more difficult to use the shutter speed you need for the photo you’re trying to take.
However there are other drawbacks to using a teleconverter.
Adding a teleconverter can slow down or completely disable your auto focus performance.
Most cameras can’t auto focus at apertures of f8 or smaller. This means that generally speaking you can only use teleconverters with lenses that have large apertures.
Specifically, you can use 1.4x and 1.7x teleconverters on lenses with maximum apertures of f4 or larger, and you can use 2x teleconverters on lenses with maximum apertures of f2.8 or larger.
Beyond that, even if you’re using a teleconverter on a lens with a large enough maximum aperture, using the teleconverter will likely slow down your auto focus speed.
How much depends on the camera, teleconverter and lens combination you are using, and generally speaking, you’ll get the best results by using the teleconverter from the same manufacturer as your lens.
In addition to the focusing drawbacks, using a teleconverter can degrade your image quality.
This happens for two reasons.
First, it’s magnifying the image being projected by the lens, which will magnify any flaws created by that lens.
Second, the teleconverter itself, depending on how well it’s made, can introduce it’s own flaws.
How much if any impact it has depends on the camera, teleconverter and lens combo you use, and on your subjective evaluation of the image quality.
Now these drawbacks might seem like deal breakers and convince you that teleconverters are not worth the money, but they do have a use and benefit.
If you need extra focal length for your photos, maybe for sports, maybe for wildlife, maybe for something else, but you don’t have the budget for a telephoto lens like a three hundred millimeter f4, you can get a teleconverter for a significantly lower price and get the focal that you need.
Certainly there are compromises in getting the teleconverter instead of the lens, but it’s a compromise many people are happy to be able to make.
Your camera is an amazing tool, but it’s no good to you if you don’t know how to use it!
If you want to take control of your camera and use it to take amazing photos like a pro, check out my Guide to Shooting in Manual Mode video course.