For the majority of my bird photography I use the Canon EOS 1D Mark II digital SLR camera having a 1.3x crop factor. However, there are occasions when it is a definite advantage to use one of the Canon EOS dSLRs having a 1.6x crop factor. At the time of this writing (04/2007) Canon's most current 1.6x crop factor dSLRs are the EOS Rebel XT and XTi and the EOS 20D and 30D dSLR cameras. I will use the 30D when I think I will be photographing very small birds or, perhaps, birds at greater than normal distances. Using the 1.6x crop factor Canon bodies allows me to increase the image magnification of my telephoto lenses and, also, puts more pixels on the bird allowing for more effective cropping of the image, if necessary. Although the Canon 1.6x crop factor cameras have advantages in bird photography they do possess a common disadvantage. None of the 1.6x crop factor bodies allow autofocusing with lenses having a maximum aperture of f 8 or less. Adding a 2x teleconverter to my 500mm f 4 lens, for example, results in a maximum aperture of f 8. At f 8 my 30D doesn't even try to autofocus. If a bird is relatively slow moving, inactive, or stationary, it is quite easy to manual focus on the bird. Active birds, on the other hand, pose difficulties in attempting to keep the birds in focus without the obvious advantages of autofocus.

So, how do we get around this issue? I had read, with skepticism, on many Internet photography forums that some photographers were stacking two 1.4x teleconverters to maintain autofocus with cameras that do not allow autofocus at maximum apertures of f 8 or smaller. After a particularly frustrating day chasing small warblers with my Canon 30D with the 2x teleconverter on my 500mm lens, I decided to give two "stacked" 1.4x teleconverters a try. Oops! Two Canon 1.4x II teleconverters can't be "stacked" because of the protruding front lens elements in the teleconverters. To work around this issue I borrowed a friend's Tamron 1.4x teleconverter and attached the Canon 1.4x teleconverter to the 500mm lens first and then attached the Tamron 1.4x teleconverter between the Canon teleconverter and 30D camera body. After a quick test I found that the 30D camera would certainly autofocus with the two "stacked" 1.4x teleconverters, although a bit more slowly than just using one 1.4x teleconverter by itself.

Attaching the 2x teleconverter to my 500mm lens yields a total focal length of 1,000mm whereas stacking two 1.4x teleconverters yields a total focal length of 980mm. When you figure in the crop factors of the Canon bodies, the 1D Mark II (1.3x crop factor) yields a total adjusted focal length of 1,300mm while the 30D (1.6x crop factor) with "stacked" 1.4x teleconverters yields a total adjusted focal length of 1,568mm. This is a significant increase over using the 2x teleconverter on the 1D Mark II camera, allows the 30D to maintain autofocus, and leaves me more pixels for cropping.

So, how good are the results? The results can be very good if you use high quality teleconverters. My earliest attempts at using "stacked" 1.4x teleconverters yielded mixed results. I had borrowed a Tamron 1.4x teleconverter from a friend and teamed it up with my Canon 1.4x II teleconverter. The Tamron teleconverter was the least expensive teleconverter that Tamron produces. The results are shown below:

The American Avocet, above, was photographed in fairly contrasty lighting conditions. The Tamron 1.4x teleconverter "stacked" with the Canon 1.4x II teleconverter yields reasonably acceptable results.		The image, above, contains details from the American Avocet image to the left. This small crop from the image to the left is presented at 100% pixel size. Contrast is fairly good and fine details are rendered fairly well. All in all, a reasonable performance.
Images made in softer lighting conditions yielded disappointing results from "stacking" the Tamron 1.4x teleconverter with the Canon 1.4x II teleconverter. As the image above shows, tonal ranges are very compressed and the image lacks overall sharpness.		In this close-up view of the image to the left you can see how fine details are lost and how the contrast range has been compressed such that there are very few tones and colors from white to black.

The images, above, show the results obtained using the inexpensive Tamron 1.4x teleconverter "stacked" with the Canon 1.4x II teleconverter. As long as lighting conditions were of good to high contrast, this combination of teleconverters performed acceptably well. However, if the lightiing conditions changed to softer or lower contrast conditions, this combination of teleconverters failed to yield acceptable images. Beside sharpness and contrast issues, the inexpensive Tamron teleconverter appeared to add some uncorrectable red to the overall color balance of the image even though I saved these images as RAW files. At the time I made these images I suspected that these issues were the result of using an undercorrected teleconverter.

For my next attempt at using "stacked" 1.4x teleconverters I purchased a Kenko 1.4x Teleplus Pro 300 DG teleconverter to team up with my Canon 1.4x II teleconverter. This model of Kenko teleconverter is a highly corrected model and is considered an "APO" teleconverter that is corrected to focus all wavelengths of light on a single image plane. The results are displayed below:

This Long-billed Dowitcher was photographed under similar lighting conditions as the American Avocet in the top photograph. "Stacking" the Kenko 1.4x Teleplus Pro 300 DG teleconverter with the Canon 1.4x II teleconverter yielded an image with higher sharpness and a longer tonal range. Colors have recorded more clean and saturated and with less of a red bias.		A 100% pixel crop from the image at left illustrates nicely the fine detail that can be recorded from "stacking" the Kenko 1.4x Teleplus Pro 300 DG teleconverter with the Canon 1.4x II teleconverter.
I couldn't think of a more low contrast lighting condition in which to test the Canon 1.4x II teleconverter "stacked" with the Kenko 1.4x Teleplus Pro 300 DG teleconverter than to photograph a subject in deep shade. The Vermilion Flycatcher, above, was photographed against the blue sky (which has turned almost white due to overexposure) while sitting in the deep shade of a mesquite tree. Although not readily apparent in this small image, the image detail is extremely good.		Above, the image detail recorded under flat lighting conditions using the Canon 1.4x II teleconverter and the Kenko 1.4x Teleplus Pro 300 DG teleconverter "stacked" is simply amazing. There is more than adequate image details recorded for professional use and the contrast range has not been inordinately compressed.

The results obtained from "stacking" two 1.4x teleconverters certainly appears to be dependent upon the quality of the teleconverters used. I am very impressed and extremely pleased with the results of "stacking" the Canon 1.4x II teleconverter with the Kenko 1.4x Teleplus Pro 300 DG teleconverter. Besides the obvious good sharpness and better contrast there is no additional uncorrectable color cast to the images. I will be using this combination of teleconverters more and more to produce the type of bird photography I would like to produce. Here are a couple more images to illustrate the quality of "stacking" the Canon and Kenko teleconverters...

Female Vermilion Flycatcher

Least Sandpiper

* Although this article 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.