pentax focal plane shutters User Manual
Section 11—Focal-Plane Shutters
You've seen that the blade-type shutter sits behind the lens or between lens elements. By contrast, the focalplane shutter sits just in front of the camera's film aperture.
Some early focal-plane designs, such as the Speed Graphics, 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
B
EXPOSED
AREA OF FILM
c
D
Fig. FP2 Exposure sequence with a multiple-slit curtain (view from front of camera). 108/FOCAL-PLANE SHUTTERS
Two factors determine the actual exposure time (shutter speed):
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 different 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.
TAPE
1st CURTAIN
BAR
2nd CURTAIN
TENSION ROLLER
FIG. FP3 Curtains in a two-curtain focalplane shutter.
1st CURTAIN |
2nd CURTAIN COVERING FILM |
|
A—CURTAINS IN RELEASED POSITION (BACK)
COCKING DIRECTION
1st CURTAIN COVERS 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 tension (or take-up) roller, Fig. FP3. Each curtain has a pair of thin cloth tapes—the tapes of the 2nd curtain are cemented 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 2ndcurtain 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 |
2nd CURTAIN |
1st
CURTAIN FILM
APERTURE
C—1st CURTAIN RELEASED, 2nd CURTAIN LATCHED
RELEASE DIRECTION
d—2nd CURTAIN RELEASED
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 reaching 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.
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.
RELEASE |
FILM APERTURE |
DIRECTION |
|
1st CURTAIN |
2nd CURTAIN |
FIG. FP5 1st curtain released, 2nd curtain latched.
SLIT
FIG. FP6 Both curtains released and traveling across film.
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-
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
Each curtain has its own spring-loaded tension roller— when the curtain is released, its tension roller pulls it to the released position. Each curtain also has its own wind-
ing 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 sections 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 CENTER SECTION
FIG. FP7The curtain drum.
CENTER (2nd-CURTAIN)
DRUM
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 ofthe 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 counterclockwise, it turns the center drum in the same direction.
Both curtains now move to the cocked position. The curtains overlap to prevent light from reaching the film. When the curtains reach the cocked position, a latch—the 2ndcurtain 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 pulling the 2nd curtain to the released position.
2nd CURTAIN
TAPES OF 1st-CURTAIN 1st CURTAIN DRUM
FIG. FP8 Construction of the curtain drum.
FOCAL-PLANE SHUTTERS/111
1st-CURTAIN TAPE
END SECTIONS
OF DRUM (1st-
CURTAIN
DRUM) CENTER 2nd CURTAIN
SECTION OF
DRUM (2nd-
CURTAIN
DRUM)
1st-CURTAIN TAPE
FIG. FP9 Curtain drum viewed from front of camera.
|
CENTER DRUM |
|
PIN ON CENTER DRUM |
REMAINS LATCHED |
|
A |
||
|
A—SHUTTER |
B—DURING |
C—CURTAINS |
D—1st CURTAIN |
RELEASED |
CHARGE |
CHARGED |
RELEASED |
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 (the2nd-curtainlatch during the lst-curtain travel. The 2nd curtain then follows the 1st curtain as shown in Fig. FP11.
112/FOCAL-PLANE SHUTTERS
(1) OUTER SECTION OF DRUM RELEASES, 1st CURTAIN TRAVELS
(2) CENTER SECTION OF DRUM RELEASES, 2nd CURTAIN TRAVELS
FIG. FP11 Curtains viewed from front of camera.
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-turn- ing roller at each end, Fig. FP13. The tapes of the 1st curtain wrap around the small rollers en route to the Ist-cur- tain 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.
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 ofinterlocking 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 winding 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.
2nd-CURTAIN WINDING ROLLER
1st-CURTAIN TENSION
ROLLER 2nd-CURTAIN
TENSION
1st-CURTAIN ROLLER WINDING ROLLER
FIG. FP12 Position of the 2nd curtain in a double-roller design.
1st-CURTAIN TAPE
1st-CURTAIN WINDING
2ndROLLER CURTAIN
WINDING
ROLLER
FREE-TURNING ROLLER
FIG. FP13 Mounting of the 1st curtain tapes in a double-roller design.
|
1st CURTAIN |
|
WIND |
INTERLOCKING |
|
MECHANISM |
||
STUDS |
FIG. FP14 Gears that wind 1st curtain.
FOCAL-PLANE SHUTTERS/113
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