Q) I have an excellent 300mm ƒ/4 lens and would like to increase the magnification using a tele-extender. I’ve been told that this will degrade the image quality and I won’t be happy with the results. What is your experience using these add-on magnifiers?
St. Louis, Missouri
A) Tele-extenders can be valuable tools if used properly and with the right lenses. The zoom lenses that are usually designed to work with extenders are 70-200mm or 80-200mm zooms from several manufacturers, and these typically give good results. For other zoom lenses, a 1.4X tele-extender likely will yield reasonable results, and a 2X tele-extender probably will disappoint you.
Your question concerns a single focal length 300mm ƒ/4 lens, and these are usually excellent candidates for use with either a 1.4X (yielding 420mm) or 2X (yielding 600 mm) tele-extender. The advantage of using this type of lens is that the quality generally is quite good and the close focus remains at the same distance even though you have the increased focal length. Other advantages are the lower price (rather than buying a new longer focal length lens), the light weight of just adding a small adapter and the versatility of having several focal lengths. Be aware that you’ll lose 1 stop of light with the 1.4X and two stops with the 2X tele-extender, and compensate for this in your exposure decisions.
Finally, try to purchase the converter from the same manufacturer as your prime lens. It may be more expensive, but it will be matched to the lens you intend to use it on and the quality of the images will be maximized.
The addition of a 1.4X Canon tele-extender made my 500mm ƒ/4 lens a 700mm ƒ/5.6 lens. The added reach was instrumental in capturing a Rufous hummingbird coming to a delphinium flower. A Canon EOS-1D Mark III was used with a Canon 580EX flash.