Teleconverters have a long and complex history in photography. In 1833 – six years before Louis Daguerre invented the daguerreotype process that launched a worldwide fascination with a new artistic medium, photography – Peter Barlow invented a negative lens that, when fitted to a telescopic eyepiece, extended the effective focal length of the telescope in which Mr. Barlow’s lens was used. In so doing, the magnification of the lens and the image scale of the subject were also increased. Known simply as the Barlow lens, this optical accessory is widely used by amateur astronomers. Commonly available in 2x and 3x versions, the modern Barlow is especially popular with lunar and planetary observers.
Nearly sixty years later in 1891, Thomas Dallmeyer and Adophe Miethe simultaneously developed nearly identical optical designs for photographic telephoto lenses. Both designs featured a front achromat doublet lens system and a rear achromat triplet grouping. The rear lens grouping acted much as Barlow’s negative lens, increasing the effective focal length of the front imaging elements. Dallmeyer and Miethe had independently invented the first photographic teleconverters.
Today’s modern teleconverters are also quite popular though not without their critics. No optical lens system is perfect and the teleconverter is certainly no exception. In addition to magnifying the subject of a photograph, a teleconverter also magnifies optical aberrations, making them more readily apparent. Commonly found in 1.4x, 1.7x and 2.0x versions, teleconverters typically magnify by as little as 40% (1.4x) and as much as 200% (2x). The biggest cost of this increased magnification is a loss of image brightness. By increasing the effective focal length of the lens while keeping the lens’ physical aperture constant, the maximum focal ratio of the lens increases by an amount proportional to the increase in effective focal length. For example, a 1.4x teleconverter increases focal length and focal ratio by 40%. A 200mm f/4 lens becomes a 280mm, f/5.6 lens. The teleconverter results in a loss of one stop of light.
This increase in focal ratio has a couple of potentially significant drawbacks. Compared to an f/4 lens, an f/5.6 lens will require an exposure twice as long to render a properly exposed image. Another option would be to increase the ISO (in-camera exposure brightening) or increase the brightness of the exposure during post-processing. Either approach will introduce some additional noise into the final image.
Another potential issue that results from an increase in focal ratio, is that of compromised autofocus performance. The brighter the image falling on the sensor, the faster and more accurate the camera’s autofocus system tends to be. As the f-stop used to make an image increases and image brightness on the sensor decreases, the camera eventually will not have enough light for reliable autofocus performance.
Because the function of a teleconverter (TC) is to extend the reach of a lens, to bring a photographer nearer the subject without having to physically move closer to the subject, it is a popular accessory for wildlife and bird photographers. With my growing interest in this type of photography and the recent purchase of a Nikkor 200-500mm f/5.6E VR telephoto zoom lens, I decided to give the Nikon TC-14E III 1.4x teleconverter a try. Attached to the new lens, the TC-14E III would have the effect of extending its zoom range to 280-700mm. The TC-14E III also facilitates communicates between the lens and camera, including effective focal length, f/-stop, shutter speed, AF mode, burst mode…the full suite of functionality one would expect of a Nikkor lens mounted to a Nikon camera body,
The main price to be paid for the extended reach achieved with a TC is an increase of the lens’s maximum f-stop. In the case of the 200-500mm f/5.6E, the focal ratio increases from f/5.6 to f/8. At f/8, the zoom would be operating at the very threshold of my Nikon D610’s ability to autofocus. This raised two issues of concern: would the lens be sharp at 700mm and would the f/8 maximum focal ratio allow for adequate autofocus performance?
One of the biggest technical challenges of bird and wildlife photography is capturing birds in flight. It is this aspect that makes bird photography so appealing to me, the challenge of mastering my equipment and expanding my knowledge of the animals to make good photographs. Bird photography also gives me an excuse to get out in nature and to be near these magnificent creatures. When the TC-14E III arrived, I couldn’t wait to run it through its paces by photographing the eagles, hawks and other birds found during winter in northern Arizona.
The above photograph of a juvenile bald eagle in flight illustrates the challenges I’ve been working to overcome. As you can see, the photo was made on a bright, sunny day. I used a shutter speed of 1/2000-second to freeze the action. The 200-500mm is at full zoom, which produces an effective focal length of 700mm with the 1.4x teleconverter attached. The maximum f-stop is f/8 but I chose to work at f/11 to produce an image with greater sharpness. In the photo’s caption, you’ll notice an ISO of 2500 for this exposure. That’s very high for a bright, sunny day. Now, if the above were a full 6,000 by 4,000 pixel image, the level of noise at that ISO would be quite acceptable. However, even at 700mm focal length, the raptor only covers about 1/5 the surface area of the D610’s sensor. The above image represents roughly a 2500 by 1700 pixel crop, which makes the noise more noticeable. In fact, I would judge the level of noise to be at the very threshold of what I consider, acceptable.
The above photo of Canada geese flying through northern Arizona’s winter sky is a roughly 1500 by 1500 square aspect crop. Notice the shutter speed is the same 1/2000-second exposure as used to make the previous image of a bird in flight. Also, please note the f-stop and ISO. The f-stop is f/9 or 2/3-stop brighter than the first image. As a result, the ISO is much lower. This was another bright, sunny day in northern Arizona so, lower the f-stop (increasing the aperture) allowed me to make an image with much less post-exposure brightening. At ISO 720, I was able to do an even more significant crop but without the noise penalty of the first image.
ISO, is the central issue when using a teleconverter with a moderately fast lens. Pro telephoto lenses offer maximum f-stops in the f/2.8 to f/4 range. The large apertures of these long lenses collect and deliver a lot of light to the sensor. As a result, even with a 1.4x TC in the mix, they still operate at f/4 or f/5.6, delivering enough light to the sensor to allow a camera’s AF system to be snappy and accurate. Using the TC-14E III with a lens such as the 200-500mm f/5.6E, a modestly slow zoom, immediately puts you right at the brink of acceptable performance.
The 200-500’s maximum f-stop (with the TC) is f/8. At f/8, the optical system captures images with noticeable softness and chromatic aberration. Closing down the aperture just by 1/3-stop to f/9 largely compensates for these aberrations and allows the lens to deliver crisp, true color images to the sensor. At f/9, the lens is operating outside Nikon’s official boundary for full AF performance. At f/9, you’ll no longer be able to work in AF-C, 3D mode. That option isn’t even available in the D610’s menu at f/9. However, I’ve been able to get good AF performance in AF-C, 9-point mode, my preferred autofocus setting for dynamic bird and wildlife situations.
The above photo illustrates the price one pays when losing focus even for a moment while doing photography with an f/5.6 (or slower) telephoto and a teleconverter. Again, this photo was made on a bright and sunny afternoon. I shot with the 200-500 and 1.4x TC combo wide open at f/8. Why? It was late in the afternoon. The sun was about an hour from setting, low on the western horizon and not quite as bright as during a midday exposure. Notice the shutter speed of 1/2000-second. That’s for a photo of a perched bird. OK, the bluebirds were flitting from plant-to-plant and not spending more than a few seconds on any one perch. However, when they’re perched, the birds aren’t moving…at least, not nearly as much as when in flight. By shooting at 1/2000-second in late day light, the ISO was jacked up to 1600. I probably could have used a shutter speed of 1/800-to-1/1000-second, which would have cut the ISO to 800 or less.
What saved this exposure was the fact that I’d noticed the western bluebirds flitting about from stalk to stalk and had pre-focused on this stalk, ahead of time. It’s still a cropped final image but at approximately 3350 by 2240 pixels, there’s enough real estate on the camera sensor to mitigate the noise. If this was shot with a 500mm f/4 telephoto and the Nikon 1.4x TC, I could have shot at f/5.6 and kept every other setting the same with the camera selecting and ISO of 800 or lower. Being a professional quality optic, the 500mm f/4 would probably be very sharp even wide open with a TC. With a consumer, telephoto zoom such as the 200-500, the margin for error is much more narrow. You’ve got to pay attention to the details and look for every opportunity to balance that f-stop/shutter speed/ISO triangle in your favor.
Here’s an image that’s a product of a collection of lessons learned during my first few weeks of ownership of the TC-14E III 1.4x teleconverter. It’s a photo that was made in good light on a clear day. The red-tailed hawk was perched atop a Ponderosa pine scanning the nearby shallow water lake. Anticipating the bird would launch within a few minutes (at most) of my arrival, I had selected a shutter speed of 1/1600-second…fast enough to mostly freeze the action of wings flapping but slow enough to catch a bit of motion and convey a hint of the dynamic action. I chose an f-stop of f/9 to noticeably sharpen the resulting image while still putting a bright image on sensor. The combination of these choices resulted in an exposure where the D610 chose an ISO of 720. In my experience, keeping ISO at or below 1000 is essential to producing noise-free images in exposures that will likely be significantly cropped.
After shooting with the TC-14E III on the Nikkor 200-500mm f/5.6E VR zoom lens for several weeks, I’ve learned the following:
- The TC-14E III is sharp. Comparing exposures made with the bare 200-500 and exposures made with the combo of the 200-500 and TC at equivalent focal lengths, any differences in image quality are subtle, at most, and only discernible at the pixel level.
- When shooting at 700mm, I prefer to stop down the combo to f/9. Even the 1/3-stop closure is enough to noticeably improve image quality. Beyond that, IQ does improve up to about f/11. However, the gain is so marginal as to be not worth (in my opinion) the associated loss of quality that comes from using a higher ISO or (for BIF) a slower shutter speed.
- For best image quality when photographing BIF (a scenario where significant cropping of the resulting image is likely), I target a shutter speed of 1/2000-second but will slow the shutter shutter speed to 1/1000 in low light and will slow the shutter speed to 1/500 for perched birds.
- I need to continue experimenting with shutter speed. At 1/1000-to-1/1600, the wing motion blur helps convey the dynamic action of flight. It’s not unlike prop blur in photographs of piston engine planes in flight. The prop blur conveys the power of the plane. Wing blur with a sharply focused face communicates the dynamic nature of the bird.
I’ll leave you with one last sample image. The 200-500/teleconverter combo is great for perched birds. In good light, I can close the aperture to ensure tack sharp detail, make exposures at relatively slow shutter speeds (under 1/1000-second), and still keep ISO under 1000. These settings deliver excellent detail in a properly focused image.
With all that potential awaiting you, there’s no excuse. Get out and shoot.
Bill Ferris | March 2016