Teleconverters and Extension Tubes
A simple way to increase the
magnification of your telephoto lens is to use a teleconverter (which we’ll often
abbreviate as TC).
These are compact optical devices that attach to your lens (between the
lens and the camera) and magnify the resulting image by a factor of
either 1.4×, 1.7×, 2×, or even (in rare cases) 3×. An example of a 1.4× TC and a 2× TC are shown below.
When attached to a lens having a
focal length of F, a 1.4× TC results in an effective focal
length of 1.4×F,
whereas a 2× TC results in an effective focal
length of 2×F.
Thus, a 400mm lens with a 1.4× teleconverter attached provides
the same magnification as a 560mm lens, since 1.4 × 400 = 560. Similarly, a
400mm lens with a 2× converter produces the same
magnification as a true 800mm lens.
Fig. 3.4.1: Two
teleconverters: a 1.4× (left) and a 2× (right).
These can be attached between a lens and a camera body
to provide higher magnification, though at the cost of reduced light.
For birders, teleconverters can be a godsend.
For warbler-sized birds foraging high in the trees, even a 500mm or
600mm lens will generally prove inadequate for capturing significant
feather detail in the bird. By attaching a 1.4× TC, these 500mm or 600mm lenses
become 720mm or 840mm lenses, respectively. As we’ll see below,
teleconverters can even be stacked—meaning that you can attach
several TC’s in tandem to achieve even higher magnification. And
because TC’s are fairly easy to put on and take off, they give you some
flexibility in the field for varying your focal length even if you’re
using a prime lens (i.e., not a zoom).
Unfortunately, in photography (as in many things)
there is no free lunch.
Any teleconverter that results in an increased focal length will also
result in a reduced aperture,
and the reason for this is simple: since the f-number of a lens is defined as F/d,
for focal length F and
objective diameter d,
increasing F without also
increasing d will result in a
higher f-number, which
corresponds to a smaller
(and therefore darker images, or slower shutter
speeds, or higher ISO...). To continue with the examples above, a
400mm f/5.6 lens fitted with
a 1.4× TC will behave like a 560mm f/8
1.4 × 400mm = 560mm
1.4 × 5.6 ≈ 8
Similarly, with a 2× TC the 400mm f/5.6 lens becomes
an 800mm f/11 lens, since 2 × 5.6 ≈ 11. Thus, if your
teleconverter provides a magnification factor of M (where M may be 1.4,
1.7, 2, etc.), and your lens has a focal length of F and an f-number of N, then the result
of attaching the TC to the lens is to produce an
effective focal length of M × F
and an f-number of M × N.
One practical consequence of this is that if your
lens has a maximum aperture of f/5.6
or smaller (meaning that it has an f-number
of 5.6 or
higher), then you most likely won’t be able to use
autofocus when using a teleconverter with that lens. This is
because virtually all consumer-grade and pro-sumer camera bodies refuse
to autofocus at apertures smaller than f/5.6—and this means the effective aperture, after taking
into account the TC. Thus, for non-pro bodies, you’ll lose the
ability to autofocus if you attach a 1.4× TC to any lens slower than f/4,
or if you attach a 2× TC to any lens slower than f/2.8. For
pro bodies, which typically autofocus at f/8 or
faster, you can use a 1.4× TC with any lens that’s f/5.6 or
faster, or use a 2× TC with any lens that’s f/4 or
In practice, if ambient light levels are very low,
autofocus may become slow or inaccurate or even refuse to operate even
at effective apertures that normally would retain autofocus
capability. A well-known trick, known as taping the pins, is to put a piece
of tape or any other thin insulator over several of the electrical pins
on the TC which allow the camera to communicate with the lens.
For some camera systems and some TC’s, taping the appropriate pins will
trick the camera into thinking that the TC isn’t present, and that the
effective aperture is wider than it really is. Some third-party
TC’s don’t even provide all the internal electrical connections between
camera and lens, so that taping the pins isn’t even necessary with
these TC’s. As a result, the camera will attempt to autofocus
even if the effective aperture is smaller than f/5.6 (or smaller than f/8 on a pro
camera). In practice, since the aperture really is
smaller than what the camera thinks, autofocus accuracy and speed can
be significantly impacted, and in some cases taping the pins might even
lead to inadvertent damage to your lens or camera, due either to
electrical short-circuiting or to over-use of the lens’ focusing motor
as the camera tries in vain to find a proper focus.
Although autofocus may become disabled, you can
still use a teleconverter as long as you don’t mind focusing your lens
manually. For the eagle photo below, I was using a 600mm f/4 lens
with a 2× TC, resulting in an effective
focal length of 1200mm at f/8.
Although my pro camera body
readily agreed to autofocus this configuration, I decided to manually
focus instead, since I was afraid the autofocus would incorrectly lock
onto one of the small twigs in front of the birds.
3.4.2: Bald Eagle family (Haliaeetus leucocephalus) photographed with
teleconverter. 1200mm effective focal length (600mm f/4 lens + 2× TC),
focused manually, with image stabilization. Image was heavily
in post-process from a 10 megapixel RAW file.
As the image above demonstrates, the use of extreme focal lengths
(1200mm in this case) can be highly useful, even essential, when
photographing sensitive species that you can’t get very close to.
In this case, not only was I manually focusing (which often leads to
suboptimal sharpness in bird images) and using an extreme focal length
(at which minute vibrations in the camera or lens will be magnified to
produce image blur), but I was using a 2× converter, which typically
involves a nontrivial loss of image sharpness. All teleconverters
degrade image quality, though high end 1.4× TC’s (such as Canon’s 1.4× Extender II, which runs about US
$380) when used with high-quality prime lenses typically produce very,
very good image quality. 2× converters, on the other hand,
regardless of make or price, typically degrade image quality quite
For the image below, I was again using a 600mm f/4
lens with a 2× converter, though this time I
relied on autofocus, and didn’t crop the image quite as aggressively in
post-process. It’s remarkable that, though I was about 200 feet
away from these hawk chicks, you can see the individual filaments of
downy feathers standing up on the birds’ heads. This is partly
due to the strong contrast imparted by the overhead light and the dark
background, as well as to the application of a sharpening filter in
post-process, but it demonstrates that detailed images can be obtained
with 2× converters at fairly long
distances with premium-quality optics.
Red-shouldered Hawk chicks (Buteo lineatus)
Even so, you’ll note that some
distortion is present in the lower half of the image above, particular
lower corners. With a lower quality TC or lower quality prime (or
even zoom) lens, the overall image quality could be drastically worse,
and as a general rule I try not to use any 2× TC on any lens unless it’s absolutely
in the nest, photographed with a 2× teleconverter
effective focal length). Image is heavily cropped from RAW.
For the truly fearless photographer working at
extreme distances to small birds, it’s even possible to obtain
reasonably sharp images when stacking
teleconverters—that is, when using multiple
teleconverters attached to the same lens at the same time. For
the image below, I use a 400mm f/4
lens with a 1.4× TC and a 2× TC attached, resulting in an
effective focal length of 1120mm (1.4 × 2 × 400) and an effective aperture of f/11 (1.4 × 2 × 4 ≈ 11).
Slate-colored Junco (Junco hyemalis)
photographed using stacked teleconverters. Effective focal
length was 1120mm (400mm lens + 1.4× TC + 2× TC).
Although the above image may appear reasonably sharp, it’s been
sharpened in post-process, and hasn’t been cropped much from the
original image. The use of teleconverters (even just one at a
time) often reduces your ability to effectively crop an image, since
image degradation introduced by the TC becomes more evident when
zooming in on the image during post-processing. For the above
photo, further cropping (i.e., digital zooming) resulted in an
awful-looking image, and I’d guess that prints of this image any larger
than 5"×7" would look pretty terrible.
For high-quality prime lenses, a good 1.4× TC should produce reasonably
sharp images when used properly. The warbler image below was shot
at 840mm using a 1.4× TC on a 600mm f/4 lens.
With some sharpening in Photoshop I was able to bring out quite a lot
of minute feather detail in this typically restless bird.
Prairie Warbler (Dendroica discolor) photographed with
teleconverter. Effective focal length: 840mm (600mm × 1.4),
tripod-mounted with image stabilization.
As yet another example of what’s possible with a quality teleconverter
attached to a prime lens, the image below was again shot at 840mm
(600mm + 1.4× TC), but this time without any
tripod or other rigid support—in other words, this photo was
shot hand-held using an
enormous lens with a teleconverter attached
(!). Though there are some signs of artificial sharpening in this
image, it’s at least a testament to the amount of detail that a prime
lens with a good TC can capture. Note that this image was fairly
heavily cropped, so that the original RAW image contained quite a bit
more background, especially on either side of the bird (the image was
shot in landscape rather than
orientation, and then cropped to portrait).
Florida Scrub Jay (Aphelocoma coerulescens) photographed with
teleconverter. 600mm lens + 1.4× TC = 840mm,
Now to show you just how bad teleconverters can be,
the owl photo below was taken using the Nikon 80-400mm f/5.6 zoom lens
with a cheap ($59), third-party 2× teleconverter attached. The
image was taken with an older, 6 MP camera, and was cropped a bit in
post-process, so some of the problems with the image probably stem from
the camera rather than from the TC. But the overall lack of
sharpness and contrast (even after post-processing) is most likely due
to the poor optical quality of the TC.
Low-quality image of a Burrowing owl (Athene cunicularia).
Photographed with a cheap 2× teleconverter
attached to a zoom lens.
Effective focal length: 800mm (400mm × 2), hand-held,
vibration reduction. Cropped from 6 megapixel JPEG.
Another useful accessory for shooting birds with
telephoto lenses is the extension
tube. These small devices outwardly resemble a
teleconverter, but without any glass elements. Whereas a
teleconverter will typically contain 5-10 glass elements which magnify
the image, an extension tube is just an empty tube containing nothing
but air—basically like a plastic doughnut
with a camera mount on the one side and a lens mount on the
other. Just like with teleconverters, they mount between your
camera and your lens.
Extension tubes. Left: 12mm. Right: 25mm.
These allow you to focus on birds that are closer than
the minimum focus distance of your lens.
As you can see with the Canon
extension tubes shown above, these tubes contain the metal pins which
allow the camera to communicate electronically with the lens, to allow
autofocus and control of the lens’ iris
and (if present) image
stabilizer. Nikon’s extension tubes, on the other hand,
these pins, so that when using the Nikon tubes you’ll have to autofocus
manually (though third-party tubes for Nikon cameras are available,
which do allow autofocus).
The purpose of an extension tube is to reduce the minimum focus distance (MFD) of
your lens. Large-focal-length lenses in the 600mm to 800mm range
typically have an MFD of 20 feet or more, so that if you try to focus
on a bird only 15 feet away, the camera will refuse to focus. An
extension tube will reduce the MFD, sometimes by as much as 5 feet or
more, and since tubes can be stacked, you can sometimes reduce the MFD
quite drastically. Although I used to use extension tubes quite a
lot with my 800mm lens, I’ve stopped doing so, since they have several
drawbacks. Although an extension tube doesn’t affect the aperture
like a teleconverter (or does so by a negligible amount), they do stop
you from focusing on distant
objects. Thus, if you put on an extension tube to focus on a bird
that’s very close, and then soon thereafter encounter a bird high up in
a tree that you want to shoot, you’ll have to take off the tube before
shooting the more distant bird, because with the tube connected, the
lens loses the ability to focus on more distant subjects.
Another drawback of extension tubes is that they can
confuse the autofocus system in your camera, because they affect the
focal length of the lens (very slightly—not enough to provide any useful
magnification). Recall from section 2.6
that the autofocus
systems in DSLR’s use a clever trick to compute the exact amount by
which they need to move the focusing element in the lens in order to
achieve focus. The introduction of extension tubes will slightly
change the amount that the focuser needs to move, since they
effectively move the focal point of the lens by a few
millimeters. As a result, unless the camera knows the tube is
attached and compensates for this, the camera will tend to back-focus
slightly. In newer DSLR’s this can be compensated for by dialing
in a microadjust setting (see
section 3.11) to eliminate the back-focus
Instead of extension tubes, I now keep a second camera
with a shorter-focal-length lens (and much closer MFD) slung over my
left shoulder, so that if a bird approaches closer than what my
tripod-mounted lens can focus on (approximately 18 feet), I can quickly
switch to the other camera/lens combination, which focuses down to
about 11 feet. I’ve found this to work well in practice.
Note that some third-party extension tubes have a
reputation for spontaneously disconnecting from either the camera or
the lens. With a tripod-mounted setup, this can result in your
camera falling off the lens and possibly sustaining damage when it hits