A guided tour
through the inner workings of the EF-28-105 USM
|These are the main
parts of 28-105 USM. Almost everything I have written here is based on my own
understanding and taking apart the lens. I used the Canon Lens Work II book for some
For a more technical
explanation, please refer to Canon's Lens Work II book or the newest EF lenses
the focus key
I was surprised to see metal inside this lens when I took it apart. There really
is more metal in here than most people think. The focus key is made of metal,
presumably because plastic would wear down with all of the key's channels. There are also
two cams inside of the key and one outside (not visible in the picture), but the metal one
has the most channels and the greatest travel during zooming. Aout half of the
elements are contained within this metal focus key. The other half are in the
focusing groups, directly in front.
It is hard to
believe that this is a motor. Well, it is. This is the M-1 ring-USM, which is
pretty much the standard. Many common lenses have this very motor, from the 28-105
f/3.5-4.5 and 85 f/1.8, to the 300 f/4L and 70-200 f/2.8L.
The 62mm M-1 is dwarfed by its older brother, the 77mm L-1,
used in such monster lenses as the 50 f/1.0 and 1200 f/5.6. The M-1 is much easier and
cheaper to make than the L-1, but they both use the same basic technology.
Although the M-1 might seem puny compared to the L-1, there
is a smaller version yet. The Micro-USM is newer and much cheaper than the even the M-1.
It, however, uses different technology that makes it operate differently than
the M-1and L-1, which are both ring-type designs. For example, the micro-USM lenses
like the 28-80 II, II, & IV do not have Full Time Manual (with only one exception, the
|the Stator &
The notches edges of the USM stator
fit snugly against the USM rotor. A spring-loaded bayonet mounting plate maintains the
pressure which ensures that the stator is firmly in place against the rotor. The
stator does not move during operation, hence the name stator, which is very
similar to stationary.
rotor is the part that rotates. The stator fits above it, with the notched surface against
the a small lip on the top surface of the rotor.
This is the only part of the motor that moves. What
could possibly be more simple than a motor which is comprised of only two metal
|Put it all
how it works
The stator vibrates as a current is applied, but does not rotate. The vibrations
of the stator, combined with the pressure against the rotor, cause the resulting
frictional forces to rotate the rotor, which turns the output ring, moving the elements
and focusing the lens.
|the Output Ring
After the stator and rotor come the
output ring. A motor is useless without some kind of transmission, and this is it.
The black output ring contains three symmetrically positioned metallic
rollers. It receives input from either the rotor or the manual focus ring, and then moves
the focusing group at the front of the lens. If you take a close look at figure 1,
you can see the "transmission" from the output ring at the bottom to the
focusing group at the top.
don't leave home without it
The output ring is sandwiched between the M-1 USM and the manual
focus ring (under mild pressure). Either the USM rotor or the manual focus ring
turns (while the other remains stationary), rotating the three rollers, which move the
The FT-M mechanism, like the USM assembly, is so amazingly
note: The L-1 motor does not
have FT-M, but rather Electronic Manual Focusing (E-M)
It is very interesting to note that this lens is capable of returning distance
information to the body. Canon won't comment, but there is a pattern of contacts arranged
under the USM rotor, directly behind the distance scale.
Removing the distance scale and motor, it is easy to see
this. As the output ring and distance scale rotate, so does a set of contacts,
completing the circuit. When the lens is focused and the distance changes, the
pattern of the contacts is different. There is a unique pattern of the contacts for
each distance capable of being returned.
Canon EF and TS-E lenses all use deformation stepping motors to
control the aperture blades. The aperture is controlled by an electronic pulse, and the
result is very quiet and precise operation. Other than the initial signal, control over
the aperture is handled entirely within the lens.
As this picture shows, the 28-105 USM has a five bladed
aperture. This is probably a fairly standard Canon EMD unit.
explained by Chuck Westfall
If you look closely at figure
10, you'll notice a small mechanism positioned above a
semi-circular strip. This is a photocoupler, in other words a device consisting of an LED
and a photo-transistor. The semi-circular strip is an encoder ring covered with
alternating reflective and non-reflective strips.
[sw: the alternating strips are
very small, I had to use my loupe (see figure
12) to see them]
This assembly is used during the autofocus drive process as
1. The user activates autofocus detection by activating SW-1 (switch-1). [in other words,
by pressing the shutter button halfway if CF4 is set to 0 for example.]
2. Assuming successful autofocus detection, the camera outputs a focus signal (consisting
of a discrete number of pulses and a directional signal) to the lens through the
electronic mount. The lens CPU translates the focus signal to a USM drive signal. The USM
is driven in the direction determined by the AF detection mechanism.
3. As the USM turns, the phototransistor of the photocoupler detects changes in the amount
of light reflected from the LED, which occur as the encoder ring moves. The
phototransistor converts this data to pulse signals which are relayed to the lens CPU.
4. The CPU counts the pulses to determine when correct focus has been reached. When the
correct focus is reached, the CPU stops the USM.
the part that broke
This 28-105 USM was dropped from a relatively low altitude onto a
hard surface. Optically and mechanically, the lens seemed to be fine. However, there are
three plastic tabs which basically hold the plastic barrel near the mount and distance
scale onto the rest of the lens. Those three plastic tabs shattered as shown in the
picture, ruining an otherwise good lens.
I have been told by a visitor to this site that even the
higher quality zooms still have this same type of design (although with more connection
points). This person dropped a 28-70L, and it broke in a similar manner.
The lens probably could have been fixed, but I was looking
for something to take apart for this tour, and I was willing to pay for it.
It makes a good loupe!
Near the 105mm setting, without the focusing groups, this part of
the lens can focus on the plane directly in front of the metal barrell. Fine focus
adjustments can be made by zooming (turning the black ring at the top). This is currently
at the 28mm setting; the rear element (shown protruding on top) is flush with rest of the
lens at 105mm.
The only drawback is that it does not cover the full frame.
It does, however, have high magnification. I don't know what the power
is, but it is more than 8x.