pentax focal plane shutters User Manual
Section 11—Focal-Plane Shutters
ou've seen that the blade-type shutter sits behind the
lens or between lens elements. By contrast, the focal-
Y
plane shutter sits just in front of the camera's film aperture.
Some early focal-plane designs, such as the Speed Graph-
ics, use a single curtain with multiple openings ("slits" ),
Fig. FP1. Selecting a shutter speed determines which opening is used to expose the film.
Moving the curtain in the cocking direction also tensions a
spring—a spring located within the lower roller, Fig. FP2
(A). The tensioned spring is now ready to draw the curtain
in the release direction.
When you release the shutter, the spring pulls the curtain in
the direction of the arrow, Fig. FP2 (B). The film gets exposed as the curtain opening moves across the film opening, Fig. FP2 (C). At the end of the exposure cycle, the
curtain opening rests below the focal-plane aperture, Fig.
FP2 (D).
But notice that the entire frame doesn't get exposed at the
same time. First the top of the film is exposed, Fig. FP2
(D). Then, as the curtain continues moving in the release
direction, the rest of the film frame is exposed, Fig. FP2
(C). In effect, the slit wipes the exposure onto the film.
Fig. FP1 A curtain with multiple slits.
A
SLIT
FILM
LOWER
ROLLER
c
B
EXPOSED
AREA OF FILM
Fig. FP2 Exposure sequence with a multiple-slit curtain (view from front of camera).
D
108/FOCAL-PLANE SHUTTERS
Two factors determine the actual exposure time (shutter
speed):
TAPE
1. the width of the opening used to expose the film
2. the speed at which the curtain travels.
For the slowest shutter speed, the curtain opening is as large
as the focal-plane aperture—the top slit in Fig. FPL As the
curtain moves in the release direction, the film is completely
uncovered—now the entire film frame sees the maximum
amount of light. But the curtain continues moving in the
release direction until it completely recovers the film.
As the curtain recovers the film, the top of the film frame
gets cut off first. The bottom of the film frame continues to
see light until the curtain opening moves below the aperture.
The fastest shutter-speed setting uses the smallest open-
ing—the bottom slit in Fig. FP1. Here, only a small section
of film sees light at a given moment.
Selecting a smaller slit decreases the amount of time that
any given film section receives light. Similarly, moving the
curtain at a faster speed decreases the length of time that
the film sees light. Either or both techniques may then be
used to provide different shutter speeds—providing differ-
ent curtain openings or changing the speed at which the
curtain travels.
The way most focal-plane shutters vary the exposure is by
changing the size of the curtain opening. Very few cameras
change the exposure time by speeding up or slowing down
the curtain. Varying the curtain-travel time (curtain speed)
requires changing the tension on the spring—the spring that
draws the curtain in the release direction.
1st CURTAIN
BAR
2nd CURTAIN
TENSION ROLLER
FIG. FP3 Curtains in a two-curtain focalplane shutter.
1st CURTAIN
A—CURTAINS IN RELEASED POSITION (BACK)
COCKING DIRECTION
1st CURTAIN
COVERS FILM
2nd CURTAIN COVERING FILM
A single-curtain design has a limited range of shutter
speeds—the number of shutter speeds equals the number
of openings in the curtain. To achieve a full range of shutter
speeds, most focal-plane shutters use two curtains—the 1st
curtain and the 2nd curtain.
Each curtain connects to a spring-loaded roller—the ten-
sion (or take-up) roller, Fig. FP3. Each curtain has a pair
of thin cloth tapes—the tapes of the 2nd curtain are ce-
mented to the 2nd-curtain tension roller. The 1st curtain,
however, is cemented directly to its tension roller.
The other end of the 2nd-curtain is cemented to the 2nd-
curtain winding roller (not shown). Similarly, the tapes of
the 1st curtain are cemented to the lst-curtain winding roller.
The curtains and tapes are also cemented to the thin, metal
curtain bars, Fig. FP3.
With the shutter released, the 1st curtain wraps around the
1st-curtain tension roller. The tapes of the 2nd curtain wrap
around the 2nd-curtain tension roller. The 2nd curtain covers the film aperture, Fig. FP4 (A). As you cock toe shutter, both curtains move from the released position, Fig. FP4
(A), to the charged position, Fig. FP4 (B). Now the 2nd
curtain wraps around its winding roller. The tapes of the 1st
B—CURTAINS IN CHARGED (COCKED) POSITION
| RELEASE DIRECTION
1st
CURTAIN
C—1st CURTAIN RELEASED, 2nd CURTAIN LATCHED
d—2nd CURTAIN RELEASED
FILM
APERTURE
RELEASE DIRECTION
2nd CURTAIN
FIG. FP4 Exposure sequence with a twocurtain focal-plane shutter.
FOCAL-PLANE SHUTTERS/109
curtain wrap around the lst-curtain winding roller. Notice
that the curtains overlap one another during the cocking
movement. This curtain overlap prevents light from reach-
ing the film.
Now, with the shutter charged, the 1st curtain covers the
film aperture. The springs insided the tension rollers have
been tensioned—both springs want to pull their respective
curtains in the release direction. But each curtain is now
latched—the 1st-curtain latch holds the 1st curtain, and the
2nd-curtain latch holds the 2nd curtain. The latches prevent the springs from pulling the curtains in the release
direction.
RELEASE
DIRECTION
1st CURTAIN
FILM APERTURE
2nd CURTAIN
Releasing the shutter disengages the lst-curtain latch. Now
the 1st curtain moves across the aperture in the release direction, Fig. FP4 (C). Since the 2nd curtain remains latched
by the 2nd-curtain latch, the shutter opens—the film sees
light as the 1st curtain moves.
The right-hand edge of the film in Fig. FP4 (C) sees light
first, The film then gets progressively uncovered as the 1st
curtain moves. Finally, the 1st curtain reaches the other side
of the aperture, completely uncovering the film.
Next the speed-control mechanism of the camera disengages
the 2nd-curtain latch. Now the 2nd curtain moves across
the aperture. The 2nd curtain progressively covers the film,
ending the exposure to each part of the film in turn. Finally,
the 2nd curtain reaches the end of its travel at the other side
of the aperture, Fig. FP4 (D).
The sequence we've just described is called the full-aper-
ture shutter speed—the 2nd curtain is released when the 1st
curtain reaches the closing side of the aperture. For a moinenl, the entire film frame receives light. The full-aperture
shutter speed depends on how fast the curtains travel and
the exact moment that the 2nd curtain is released. In the
classics and antiques, the full-aperture shutter speed is typically around 1/30 to 1/60 second.
For a faster shutter speed, the speed-control mechanism
releases the 2nd curtain before the 1st curtain reaches the
end of the aperture. Consider that the 2nd curtain releases
when the 1st curtain reaches the position shown in Fig. FP5.
The 1st curtain is then still in the aperture—still partially
covering the film—when the 2nd curtain starts its release
movement.
As a result, a slit formed between the two curtains moves
across the film, Fig. FP6. The sooner the 2nd curtain is
released, the narrower this slit becomes—and, as a result,
the faster the shutter speed.
At the fastest shutter-speed setting, the 1st curtain just starts
to move when the 2nd curtain releases. A very narrow slit
then moves across the film. The shutter speed depends on
the actual slit width and the speed that the curtains travel.
Changing the slit width can provide a wide range of shutter
speeds. The classic cameras may provide speeds of 1/60
second (the full-aperture speed), 1/125 second, 1/250 sec-
FIG. FP5 1st curtain released, 2nd curtain
latched.
SLIT
FIG. FP6 Both curtains released and
traveling across film.
ond, and 1/500 second (or 1/1000 second) just by
changing the release point of the 2nd curtain. Modern cameras rely on the same principle to provide
shutter speeds as fast as 1/8000 second.
The two-curtain design also allows shutter speeds slower
than the full-aperture speed. Here, the 1st curtain completely
uncovers the film aperture—just as with the full-aperture
speed. A mechanical speeds governor—similar to the speeds
governor in blade-type shutters—then delays the release of
the 2nd curtain. As a result, the film aperture remains completely uncovered for the length of the exposure time.
With a mechanical speeds governor, the slowest shutter
speed is typically 1 second. The camera may provide shutter speeds of 1/2 second, 1/4 second, 1/15 second, and 1/30
second just by changing the engagement of the speeds governor. Modern cameras again rely on the same principle.
But the electronic controls of today may provide even longer
exposure times—perhaps 30 full seconds.
110/FOCAL-PLANE SHUTTERS
THE CURTAIN ROLLERS
ach curtain has its own spring-loaded tension roller—
when the curtain is released, its tension roller pulls it to
E
the released position. Each curtain also has its own winding roller to draw the curtain to the cocked position.
The drum design
Some focal-plane shutters combine the two winding rollers
into a single curtain drum, Fig. FP7. The classic screwmount and M-series Leicas use the curtain drum. Another
classic—the Nikon F—also uses the drum.
The center section of the curtain drum, Fig. FP7, turns independently of the end sections. But the end sections turn
together as one unit.
Fig. FF8 shows the construction of the drum. A shaft joins
the two end sections. But the center section can rotate freely
around the shaft.
One curtain—the 2nd curtain—is cemented to the center
section of the curtain drum, Fig. FP9. The tapes of the 2nd
curtain then cement to the 2nd-curtain tension roller.
The tapes of the 1st curtain are cemented to the end sec-
tions of the curtain drum, Fig. FP9. The 1st curtain is then
cemented to the lst-curtain tension roller.
Note that the two end sections of the drum are slightly larger
in diameter than the center section, Fig, FP9. The larger
diameter of the end sections provides a slight separation
between the curtains.
END
SECTIONS
FIG. FP7The curtain drum.
CENTER (2nd-CURTAIN)
DRUM
CENTER
SECTION
As you cock the shutter, the curtain drum rotates as a unit—
the center section is locked to the end sections. The 2nd
curtain wraps around the center section of the drum. At the
same lime, the tapes of the 1 st curtain wrap around the end
sections of the drum. As the drum rotates, it draws both
curtains to the cocked position—against the tension of the
tension rollers.
Fig. FP10 shows how the two sections of the drum are locked
together during the charge cycle. Note the pin on top of the
center drum, Fig. FP8. The pin passes through a cutout in
the top section of the lst-curtain drum, Fig. FP10.
As you cock the shutter, the cocking mechanism turns the
lst-curtain drum as shown by the curved arrow, Fig. FP10
(B). The lst-curtain drum then comes against the pin on the
center drum. So, as the lst-curtain drum turns counterclock-
wise, it turns the center drum in the same direction.
Both curtains now move to the cocked position. The cur-
tains overlap to prevent light from reaching the film. When
the curtains reach the cocked position, a latch—the 2nd-
curtain latch—engages and holds the center drum (you'll
see the 2nd-curtain latch in the next section). The 2nd-cur-
tain latch prevents the 2nd-curtain tension roller from pull-
ing the 2nd curtain to the released position.
2nd CURTAIN
1st-CURTAIN
DRUM
TAPES OF
1st CURTAIN
FIG. FP8 Construction of the curtain drum.
FOCAL-PLANE SHUTTERS/111
END SECTIONS
OF DRUM (1stCURTAIN
DRUM)
CENTER
SECTION OF
DRUM (2ndCURTAIN
DRUM)
FIG. FP9 Curtain drum viewed from front of camera.
PIN ON CENTER DRUM
1st-CURTAIN TAPE
2nd CURTAIN
1st-CURTAIN TAPE
CENTER DRUM
REMAINS LATCHED
A
A—SHUTTER
RELEASED
B—DURING
CHARGE
FIG, FP10 Top section of 1st-curtain drum.
The lst-curtain drum is also held in the cocked position—
the lst-curtain drum is still engaged to the cocking mechanism.
When you release the shutter, the cocking mechanism disengages the lst-curtain drum. Now the 1st-curtain tension
roller pulls the 1st curtain to the released position. The 2nd
curtain remains in the charged position—it's still held by
the 2nd-curtain latch.
For the full-aperture speed, the 1st curtain completely
crosses the focal-plane aperture. A disengaging lever attached to the lst-curtain drum then strikes and disengages
the 2nd-curtain latch. Now the center drum can rotate. Notice in Fig. FP10 (D) that the center drum can turn freely in
a clockwise direction—its pin no longer comes against the
upper section of the lst-curtain drum.
But for a slit-width speed, the center drum releases before
the 1st curtain has completely crossed the focal-plane aperture. The disengaging lever on the lst-curtain drum strikes
(the 2nd-curtain latch during the lst-curtain travel. The 2nd
curtain then follows the 1st curtain as shown in Fig. FP11.
C—CURTAINS
CHARGED
(1) OUTER SECTION OF DRUM RELEASES,
1st CURTAIN TRAVELS
(2) CENTER SECTION OF DRUM RELEASES,
2nd CURTAIN TRAVELS
D—1st CURTAIN
RELEASED
FIG. FP11 Curtains viewed from front of
camera.
112/FOCAL-PLANE SHUTTERS
Selecting a slit-width shutter speed then determines the release point of the center drum. If you set a faster shutter
speed, you select a smaller slit—the center drum releases
sooner during the travel of the 1st curtain. The sooner the
center drum releases, the narrower the slit—and the faster
the resulting shutter speed.
From this description, we can determine a general rule for
focal-plane shutters: The 1st curtain releases the 2nd curtain. Further, the slit width depends on how far the 1st
curtain travels before releasing the 2nd curtain. For a fullaperture shutter speed, the 1st curtain completely crosses
the focal-plane aperture; it then releases the 2nd curtain.
The sooner the 1st curtain releases the 2nd curtain, the narrower the slit—and, as a result, the faster the shutter speed.
The double-roller design
Other focal-plane shutters use two completely separate
winding rollers rather than the drum. One winding roller
sits directly in front of the other, Fig. FP12.
Fig. FP12 shows the mounting of the 2nd curtain—the curtain is cemented to the 2nd-curtain winding roller, while
the tapes are cemented to the 2nd-curtain tension roller.
The 1st-curtain tension roller has a free-turning small roller
at each end to route the 2nd-cutain tapes.
The 2nd-curtain winding roller also has a small free-turning roller at each end, Fig. FP13. The tapes of the 1st curtain wrap around the small rollers en route to the Ist-curtain winding roller. The tapes then cement to (the ends of
the lst-curtain winding roller. The 1st curtain is cemented
to the 1st-curtain tension roller.
To charge the shutter, the cocking mechanism turns the two
winding rollers together. But the two winding rollers turn
individually on the release cycle.
2nd-CURTAIN WINDING
ROLLER
1st-CURTAIN
TENSION
ROLLER 2nd-CURTAIN
1st-CURTAIN
WINDING ROLLER
TENSION
ROLLER
FIG. FP12 Position of the 2nd curtain in a
double-roller design.
1st-CURTAIN TAPE
1st-CURTAIN
2ndCURTAIN
WINDING
ROLLER
WINDING
ROLLER
There are several systems used to lock the winding rollers
together during the cocking cycle. You'll see examples in
the studies of actual cameras. Frequently, cameras use a
pair of interlocking studs, Fig. FP14. One gear in Fig. FP14
couples to the charge mechanism; the other gear couples to
the 1st-curtain winding roller.
As you cock the shutter, the wind mechanism turns the lower
gear in Fig. FP14. The stud on the lower gear then turns the
upper gear. And the upper gear turns the 1st-curtain wind-
ing roller. As in the drum design, the 1st-curtain winding
roller turns the 2nd-curtain winding roller.
Pushing the release button moves down the lower gear—
the interlocking studs disengage. The 1st curtain, now free,
moves to the released position. But the 2nd curtain remains
latched by the 2nd-curtain latch—just as with the drum
design.
FREE-TURNING ROLLER
FIG. FP13 Mounting of the 1st curtain tapes
in a double-roller design.
1st CURTAIN
WIND
MECHANISM
INTERLOCKING
STUDS
FIG. FP14 Gears that wind 1st curtain.
FOCAL-PLANE SHUTTERS/113
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