Lens extenders are nice, they give you the extra reach without having to buy another lens. All this at the cost of some image quality and loss of light.

What is a lens extender? (Also known as a teleconverter) A teleconverter is an inexpensive lens supplement for detachable and fixed-lens cameras. Most commonly used with single-lens reflex (SLR) cameras, the device is designed to increase the magnification of any compatible lens. Due to the brand-specific design of lens mounts, the converter must also be brand-specific. The term “lens extender” is used by the Canon brand to refer to its line of teleconverters. Read more: What Is a Camera Lens Extender? | eHow.com

Basically, you take the magnification factor from the extender (usually 1.4 or 2.0 for example) and multiply your lens focal length by that number… it even works for zoom lenses. for a 100mm, the 1.4 extender would make it a 140mm equivalent. So instead of buying 2 lenses, you use the extender on what ever lenses you currently own.

Is it worth it?

With the increased low light/high ISO performance of new DSLRs, it is easy to compensate for the decreased light sensitivity caused by extenders.  On the other side, newer camera have higher resolution which has the side effect of making the softening more apparent when looking at the 100% crop.

It’s about target media

Deciding if you should or not buy an extender is now more of a question of how the image will be used. If you expect to do some serious cropping or use the image at 100% crop, you should stay away from extender (if possible). On the other hand, if you plan to shoot video or mostly showcase your picture on the net, the decreased image quality will not be visible.

Of course the decrease in sharpness is relative to the quality of the extender. High end extenders from camera manufacturers carry a higher price tag but their impact on image quality is minimal and often unnoticeable.

Note: Some of the newer generation extenders claim to not affect image quality by their manufacturers. I have yet to confirm this having only used those from Sigma and the old generation from Canon.
Note: if the extender is the only way for you to get your image, there is no need to ponder if you should or not get one!
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