A significant differentiator
between birding lenses today is the ability to apply real-time image stabilization, or IS. Different manufacturers
have their own, proprietary implementations of IS, with different
names, but they all seek to improve image sharpness by reducing the
effects of camera/lens shake.
The figure below illustrates the effect of image
stabilization at slow shutter speeds. Using a 560mm lens wide
open at f/5.6, I shot this
highly cooperative House Owl (species: Owlus plasticus) with IS turned on
(left image) and with IS turned off (right image). Both photos
were taken hand-held, without a tripod and with nothing to support the
lens but my arms.
Image stabilization reduces blur.
Left: a common house-owl (Owlus plasticus) photographed
with image stabilization. Right: the same subject, photographed
with IS turned off. Both photos were taken at 1/40 sec,
560mm, f/5.6, ISO 3200, hand-held. No flash.
At this size it’s difficult to see much difference, though if you look
at the feathers on the bird’s breast, you can see that the image on the
left looks sharper. In the image below I’ve zoomed in on the
bird’s left eye, and also applied some sharpening in Photoshop (the
same amount to both images). The IS version (left) clearly has
less blur than the non-IS version (right). If you look at the
white eye ridge at the 2 o’clock position above and right of the eye in
both versions, you can see that in the IS version (left) there is some
fine detail that is largely obliterated in the non-IS version (right).
The lack of sharpness in the
non-IS version of the image above is due to my inability to hold the
camera and lens perfectly still while taking the shot, resulting in
slight image movement while the shutter was open. That slight
movement resulted in the light rays from each point on the bird being
scattered over a small area on the image sensor, rather than striking a
single point on the sensor. This smearing of detail information
results in image blur, which
even a sharpening filter in Photoshop can’t fix.
Fig. 3.5.2: A
magnified view of the previous figure.
Left: with IS turned on. Right: with IS turned off.
Note that, for the non-IS image above, I resisted
the temptation to exaggerate the problem by intentionally shaking the
camera slightly while taking the shot. Because I’ve spent many
hundreds of hours shooting with this lens, hand-held, I’ve gotten quite
good at holding it very, very steadily. Even so, at 1/40 sec it’s
very difficult to get tack-sharp images with any non-IS lens, when
shooting hand-held. Although I very rarely shoot at speeds as low
as 1/40 sec, for crepuscular
subjects (meaning: active often at dawn and dusk) like the one above, without flash there’s little
choice but to shoot at low shutter speeds. Note that for the
photo above I opened my lens’ iris all the way to f/5.6 (the maximum aperture of this
lens) and cranked the ISO up to a whopping 3200, and still wasn’t able
to get my shutter speed up past 1/40 sec without underexposing my
subject. It’s in precisely these situations that IS becomes a
real godsend (especially if you don’t have a tripod with you).
It’s important to note that IS becomes more useful
the more you zoom in on your subject. That means that lenses with
larger focal lengths have a correspondingly greater need for IS.
And as illustrated by Figure 3.5.2, above, it also means that the more
aggressively you crop your
image (i.e., the more “digital zooming” you apply in Photoshop), the more
you’ll notice the effects of camera shake, and the more you’ll wish
your lens (or camera) had IS. The reason larger magnification
factors are more susceptible to camera shake is due to what’s known as optical leveraging, as illustrated
by the figure below.
Fig. 3.5.3: The
concept of optical leveraging.
The idea of optical leveraging is
simple. Imagine a see-saw in which one end of the see-saw is
longer than the other (see the figure above). Then if you move
one end of the see saw by a fixed amount, say 12 inches, then the other
end of the see-saw will move more than 12 inches, because that arm of
the see-saw is longer. In the figure above, the three see-saws
differ in the length of their right-hand arms. If we shake the
left-hand end of all three see-saws by the same amount, the right-hand
ends shake by different amounts, and the longer the right-hand arm of
the see-saw, the larger the shake. In an optical system, this
corresponds to greater image blur, because if the scene is moving while
the shutter is open, light rays from individual points on the bird
won’t collect at individual pixels on the imaging sensor, but will
instead be smeared out over a certain radius on the sensor.
Focal length works like a see-saw with unequal
arm lengths. At higher and higher focal lengths,
small vibrations in the camera or viewer (at left)
translate into larger movements in the image, and
thus blurrier photographs.
In practice, I find that for shutter speeds of 1/200
sec or faster, I typically don’t need IS. That’s true both when
I’m shooting at 400mm hand-held, and when I’m shooting at 800mm
tripod-mounted. Because of the effects of optical leveraging,
even tripod-mounted setups can benefit from IS at low enough shutter
speeds. If the wind is blowing (or there’s an earthquake or
nearby vocanic eruption), your need for IS will increase, especially
for large lenses that offer a larger surface to catch the wind.
Also note that while flash photography in dimly lit environments can
sometimes make IS unnecessary even at extremely slow shutter speeds
(because the flash’s 1/50,000 sec pulse freezes the bird), this doesn’t
work in well-lit scenes, since the ambient light contributes a
non-negligible amount to the overall exposure, resulting in visible
Canada warbler (Wilsonia canadensis) photographed
with a large, heavy lens (600mm f/4), hand-held, with image
Although I highly recommend buying a lens (or
camera) that has some form of IS built into it, it’s worth noting here
that there are situations where IS can be an impediment. First,
some IS implementations will actually increase
image blur when used on a tripod. The newest lenses from Canon
have fixed this problem, because they can detect when the camera is
tripod-mounted and will adjust the operation of the IS
accordingly. Similarly, for birds in flight, Canon and Nikon
stabilizers have an optional panning
mode (“mode 2” on Canon lenses) that you can
manually enable, since standard (non-panning) IS tends to work poorly
when panning, and may actually cause more image blur.
Second, because IS can slow down the initial focus
acquisition by the autofocus system, for restless birds like warblers
it’s typically best to turn IS off at high enough shutter speeds (say,
1/200 or 1/300 sec or faster). Otherwise, you may find that just
as soon as the camera has acquired focus, the bird has moved to a new
location and you need to re-acquire focus. With IS turned off in
these situations, you may gain a fraction of a second on the bird,
which can be just enough to take the shot.
As mentioned previously, different companies have
their own implementation of IS. Nikon calls theirs VR (“vibration reduction”), Sigma calls theirs OS (“optical stabilization”), and Tamron calls theirs VC (“vibration compensation”), while Canons refers to theirs
as IS. More important
than the names are the differences in the way these systems work.
The biggest difference is the one between in-the-lens IS and in-the-camera IS. In the case
of in-the-lens IS, the lens contains a small, highly sensitive gyroscope, which can detect tiny
movements in the lens housing. When movements are detected, the
lens counteracts these by rapidly moving an extra group of optical
elements in the opposite
direction, so as to keep the image stationary. As with all types
of IS, there’s a limit as to how much compensation can be applied, so
that large amounts of camera shake will still cause image blur.
In the case of in-the-camera IS, there are two
popular options: digital IS,
or mechanical IS.
Mechanical, in-the-camera IS again uses a gyroscope to detect small
movements in the camera, but then compensates for these by moving the
imaging sensor. Digital IS, on the other hand, doesn’t move
anything in the camera or lens, but simply detects image shake using
digital image analysis circuitry and then shifts the image (digitally,
via built-in software in the camera) in real-time. Digital IS is
mainly used in video cameras.
In DSLR’s, mechanical, in-the-camera IS is touted as
a cheaper alternative to in-the-lens IS, because you only pay for the
IS once—when buying the camera. Once you’ve got a camera with
in-body IS, you can use IS at any time, no matter what lens is
attached. With in-the-lens IS, you only get IS if the lens you’re
using has IS built-in, so you end up paying for IS many times—once for
each lens that you buy. Conventional wisdom says, however, that
in-the-lens IS should be more effective than in-body IS, and this claim
is somewhat supported by the fact that the two biggest camera makers
(Canon and Nikon) use only in-the-lens IS, though these companies
obviously have a vested interest in lens-based IS, since they can make
more money that way. Rigorous comparisons between in-lens and
in-body IS are very hard to find, and existing comparisons on the
internet are somewhat contradictory. While lens-based IS has been
around for at least 10 years, in-body IS is still under active
development by various camera manufacturers, so the final verdict will
probably not be in for a few years.
A few quick, additional notes on IS are in
order. First, while IS can be very effective at eliminating or
reducing the effects of “camera shake”, it does nothing whatsoever to
compensate for “bird shake”—that is, IS can’t reduce motion
blur due to movement of the subject, only movement of the camera.
Second, the effectiveness of a particular IS system
at counteracting camera shake is typically measured in “stops” of IS. Although we’ll defer
a complete discussion of stops of
exposure to Chapter 6, we’ll just mention here that a stop of IS
is equivalent to a doubling
or halving of the shutter
speed. Thus, if you can normally get tack-sharp images at 1/200
sec with your particular lens and shooting technique, then with a
3-stop IS system you should (in theory) be able to get similarly sharp
images at 1/25 sec, since 1/200 × 23 = 1/25.
Canon’s newest crop of IS-enabled super-telephoto lenses feature a
4-stop image stabilizer.
Finally, note that in-the-lens IS tends to drain
your camera’s battery fairly quickly. These are mechanical
systems which require lots of electricity—much more than is required to
open a shutter, for example. If you spend hours with your finger
on the shutter release button (meaning that the IS gyroscope is kept
spinning the whole time) while waiting for those rare occurrences where
an owl or eagle or other stationary subject strikes an interesting or
unusual pose, you can expect your camera’s battery level to drop quite
a bit (though how much it drops is highly dependent on the type of
battery in your camera, since some cameras have much better batteries