Focal reducers are relatively rare compared to teleconverters, though they work based on the same idea (simply employed in reverse). If you have not seen them in use before, this awesome video tutorial will show you how they work and why they are useful.
Coming to you from CVP, this great video tutorial will introduce you to focal reducers. While teleconverters are fairly common in photography, especially genres like wildlife or sports, where you can't get enough reach, focal reducers (also known as telecompressors or speed boosters) are generally only used in video. This is because they reduce the size of the image circle, making them suitable for using something like a full frame lens on micro four thirds bodies, which are popular for filmmaking. On the other hand, there simply are not enough medium format lenses out there to really justify a large market for focal reducers for full frame mounts. However, when you do use them, you can expect a reduction in the effective focal length of the lens, and unlike a teleconverter, you get the benefit of a wider maximum aperture instead of a loss of light-gathering power. Check out the video above for the full rundown.
