By Rob Sheppard
Recently, I had the chance to work with a set of tele-extenders from Kenko (THK) while doing an advertorial for THK and Outdoor Photographer magazine. They had sent Nikon mount extenders so I used a Nikon D3x and Nikkor 80-200mm f/2.8 lens. The results were, to be honest, really quite good. I was impressed with the excellent sharpness that I got with both the 1.4x and 2.0x teleconverter.
That said, it is important to understand that you could take these same tele-extenders with a different lens that you have and get terrible results. People will then say how terrible it was that I “sold out” and promoted crappy gear. Well, I never promote crappy gear, which isn’t hard today because digital camera equipment today is so very, very good. And I really, truly did get superb results from these extenders and would not hesitate to recommend them to anyone, regardless of doing an advertorial or not.
So how can it be possible someone could get poor results from the same extenders? Very easily, it turns out, and this has nothing to do with Kenko or THK, but everything to do with extenders. Extenders can only deal with the optics of the original lens. A good lens is more likely to give good results. The tele-extender is not only magnifying the view, but also any defects in the lens.
But it isn’t that simply. With an extender, you are adding complexity to the optical path by adding new optics. No matter how good those new optics are, they still have to work with a totally new set of optics in the original lens. Many low-priced zooms, for example, don’t adapt well to the addition of new optics from a tele-extender, so their results are not good.
Bottom line is that how well an extender will work for you depends on the lenses you use it with. Unfortunately, that can be difficult to predict ahead of time because few people put extenders on all lenses and test them. I can say that many zoom lenses, because of their complexity, don’t adapt well to extenders, but many do, which the Nikkor 80-200mm lens I used obviously did.
Extenders also drop the light to the sensor. The 1.4x cuts it by one stop, the 2.0x by two stops. In addition, extenders often work their best when the original lens is stopped down a stop or two from the maximum aperture. This can mean a serious drop in light that can result in a definite loss in sharpness due to no fault of the extender, but from DSLR camera movement. The extender will magnify the view, but also any camera movement during the exposure. Many people are disappointed in 2.0x extenders from any manufacturer and think they are not sharp. Often the problem is not unsharp extender or other optics, but an extreme sensitivity to camera movement, even when the camera is on a tripod. For that reason, I have actually gotten better results at times when using an image stabilization feature on camera or lens to help deal with that movement.
Another challenge is autofocus. Your digital SLR camera may or may not autofocus when using an extender, but again, this is not the extender causing the problem. What happens is that the loss of light to the sensor can make the light level drop to such a point that the camera does not have enough light to autofocus. It depends a lot on the camera, and I am not familiar with all cameras. When you use the 1.4x, you lose a stop of light. That may put your lens beyond the AF capabilities of your camera, or maybe not. It definitely will limit AF capabilities in low light. This is simply something we have to put up with for the use of any teleconverter.
More photo tips: Along with being Outdoor Photographer's editor-at-large, Rob Sheppard also teaches for the BetterPhoto digital online photography school courses. Rob's Internet photography tutorials includeBetterPhoto.com, including Creating Storytelling Photos and Impact in Your Photographs: The Wow Factor.
