Canon EOS Digital Rebel White Paper - Page 12

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meters at the other 8 AF points. When an external EOS Speedlite is used, the AF-assist

beam of the external unit is used.

Speedlite

AutomaticSelection

580EX

Yes

550EX

Yes

430EX

Yes

420EX

Yes

380EX

Yes*

220EX

Yes*

ST-E2

Yes

* Focus can be achieved only with the center AF point.

Some of Canon's competitors have chosen to use in-body image stabilization. The tech-

nique involves moving the image sensor in a controlled fashion, based on signals from

movement detecting sensors in the camera body. The obvious advantage of this system

is that users have some sort of stabilization available with almost any lens they connect

to the body. Short focal length lenses require smaller sensor deflections; 24 or 28 mm

lenses might need only 1 mm or so. Longer lenses necessitate much greater movement;

300 mm lenses would have to move the sensor about 5.5 mm (nearly 1/4") to achieve

the correction Canon gets with its IS system at the same focal length. This degree of

sensor movement is beyond the range of current technology. Short and "normal" focal

length lenses need stabilization much less often than long lenses, so the lenses that

need the most help get the least. Further, in cameras with smaller than full-frame, 35

mm film size sensors, equivalent focal lengths become longer, by a factor of 1.5 or 1.6,

exacerbating the problem by making all lenses longer.

Less significant but still worth mentioning is the fact that in-body stabilization is not vis-

ible through the finder, whereas Canon lens-based stabilzation definitely is. Also, while

in-body stabilization works for many lenses, it does not presently work for all; high mag-

nifications and macro lenses have caused it difficulties.

In the Canon IS system, the Image Stabilizer has an actual

lens group that can be moved up and down, or side-to-

side, in parallel to the imaging sensor or film plane. A pair

of sensors in the lens can detect horizontal or vertical

shake. Signals from these sensors are sent instantly to a

microprocessor in the lens and analyzed. The micro-

processor then causes a group of lens elements, held in

place by a device called a coil, to move at the same ampli-

tude and frequency of the shake to cancel it effectively.

Canon is able to generate excellent image quality by stabilizing the image optically

before it ever reaches the imaging sensor in a digital SLR.

With the optical IS used by Canon, each lens with IS has a stabilizer unit designed for

that lens's needs. The unit in a lens such as the EF 28-135 f/3.5-5.6 IS USM or the EF-S

17-85 f/4-5.6 IS USM is vastly different from the powerful, broad movement stabilizers

III. FEATURE DISCUSSION

Manual Selection

Center

Top/Bottom

Yes

—

Yes

—

Yes

—

Yes

—

Left/Right

Mid-left/Mid-right

Yes

—

Yes

1: No camera shake

Group 1 (focusing group)

Group 2 (image-stabilizing group)

To Object

Focal

Group 3 to 6

plane

2: Lens front shakes downward

3: Image-stabilizing group counteracting

downward camera shake

Direction of Group 2

movement

How the Image Stabilizer works