When I am out photographing birds I almost always have the Canon EF 2x II teleconverter attached to my Canon EF 500mm f 4L IS lens. I can only think of a handful of images I have made with the 500mm lens without any kind of teleconverter attached to the lens. One issue I have found while photographing birds is that seldom do I have too much focal length! Birds are tiny. I think we subconsciously enlarge in our minds the birds we view. Even with the 2x teleconverter attached to the 500mm lens I have come up short on focal length. If you are a Canon "Big Lens Shooter" you may find, as I have, that "stacking" (attaching) the Canon EF 1.4x II teleconverter to your lens along with the 2x II teleconverter can provide just enough extra reach for you to photograph that "must have" image.

The image of the Verdin, above, was made with a Canon EOS 20D dSLR and Canon EF 300mm f 2.8L IS lens with both the Canon EF 2x II teleconverter and the Canon EF 1.4x teleconverter attached to the lens, f 22. Verdins are very tiny birds requiring long focal lengths and a close approach. This is about a 60% crop from full-frame.

The technique is fairly simple but there are some "rules" I have found that need to be followed to be able to produce acceptable images with stacked teleconverters.

• This technique only works with the very best, most highly corrected single focal length lenses.
• I have to be photographing in good, bright lighting conditions. This is definitely not a low-light technique. When attached to my 500mm f 4 lens, the teleconverters combine to yield a maximum aperture of f 11. That's a "loss" of 3 f stops of maximum aperture.
• The lens must be stopped down at least 2 f stops but 3 f stops would be better. This means that the apertures I will be shooting with will be f 22 to f 32. You need a lot of light and a high ISO rating to shoot with stacked teleconverters. Typically I am shooting at ISO 800 with my Canon EOS 1D Mark II dSLR.
• Shutter speeds of 1/200 sec or faster need to be used. Adding the 1.4x and 2x teleconverters to my 500mm lens yields a new focal length of 1,400mm. Even though I always have the IS (Image Stabilization) turned on, the slightest bit of vibration will cause unsharp images. Image stabilization alone cannot cancel out all of the minor vibrations. Shutter speeds slower than 1/200 sec is not enough.
• I have to use my very best "long lens" technique. This includes pushing the camera tight to my face, placing my free hand on the lens above the tripod collar, and locking down the tripod head adjustments.
• I must manually focus images made with stacked teleconverters. It is imperative that the diopter adjustment on the viewfinder of the camera be adjusted properly to allow the sharpest view.

If all of the above conditions are met then there is a good chance that I will obtain an acceptable image. I keep emphasizing "acceptable image". There are drawbacks to using stacked teleconverters. 1) Image sharpness may not be as good as if only one teleconverter was attached to the lens and 2) contrast will suffer somewhat. There is a lot of additional glass the image has to pass before being recorded on the film or ccd/cmos sensor. This additional glass does affect sharpness and contrast.

Below is a recent image I made of a Neotropic Cormorant drying its wings. I stacked both the 1.4x and 2x teleconverters on my 500mm lens, stopped the lens down 3 f stops, and kept my shutter speed at 1/200 sec. The inset image is a 100% pixel crop of the raw image. You make the comparison...

Neotropic Cormorant photographed with a Canon EOS 1D Mark II dSLR and Canon EF 500mm f 4L IS lens + Canon EF 1.4x teleconverter + Canon EF 2x II teleconverter. I manually focused on the cormorant's neck and kept my fingers crossed that the depth of field at f 32 would be sufficient to render the head sharp. This resulted in a 1 stop underexposure for which I compensated in my RAW conversion software.

To paraphrase the old FedEx commercial, "When you positively, absolutely have no other way to make the image...stack the teleconverters".

* Although this essay is geared to Canon "Big Lens" photographers, I beleive that this technique may be used by Nikon photographers, too. It is my understanding, however, that one of the teleconverters may need to be modified so that it may be stacked with other teleconverters. I'm sure this information may be found on the Internet when visiting many of the Nikon user groups.