The shutter mechanism inside a camera allows light to
reach the film or sensor for a pre-determined period of
time. The duration for which the film/sensor receives
light can be controlled by a switch or from menu.
f/8, 2.5 sec., ISO-64, FL-24 mm eqv.,
Nikon d810, Tamron 24-70mm f/2.8, Jaipur, Rajasthan, India 2015.
Imagine yourself inside a very dark room where all the doors
and windows are closed. There is day-light present outside
the room, but no light inside. Now you open a window
just for 1 second, then close it. The room will be exposed
to light for 1 second. This time period of 1 second may be
called as shutter-speed; and the size of the window may be
referred to aperture of a lens.
In this page, let us check the mechanism of focal plane shutters (also called
mechanical shutter) inside a camera.
In front of the film or digital sensor of
a camera, there remains a pair of curtains
(named as front curtain & rear curtain)
which block any light to reach
the film or sensor. In general, when we are in front of
a scenario to photograph,
we look through vuew finder to ensure good composition and framing;
then adjust focal-length, focus-point,
aperture, shutter-speed, iso, white-balance etc. When everything is
fixed, we press the shutter-release button which is the final step for
recording a photograph.
As soon as we press the shutter-release button, one of the
curtains folds itself, and allows
light to reach the film or sensor through the lens.
We skipped a mirror-mechanism in case of DSLRs,
which will be discussed shortly.
The curtain remains
folded for a moment, say for 1⁄500th. of a second,
or for ½ or 2 seconds
(whatever period we set it earlier). During this
period, the film or sensor remains exposed to the scenario/light, and
an image is recorded in digital format.
This time period, for which the film or sensor remains open to light,
is called shutter-speed
for that particular photograph.
Following is an effort to depict the focal-plane-shutter-mechanism
for a ¼ and a ½ second shutter-speeds. For simplicity, only one
curtain movement is shown.
Note: The second action above, which is executing a ½ sec.
shutter-speed, is able to gather double the light than the first system
(which is executing a ¼ sec. shutter).
Shutter Mechanism of a DSLR camera
In case of DSLRs, the film or sensor always remains blocked by the front
and rear curtains. Only when shutter-release happens,
the blocking curtains remove themselves from the path
of light. Then the sensor/film is exposed to the incoming
light, and an image is recorded. After that the sensor/film
is again blocked by curtains.
The following animation is related to DSLR-shutter-mechanism only.
For mirrorless cameras,
the technique is different. In a DSLR, a mirror sits in front
of the shutter-curtains and diverts light towards the optical
viewfinder. After the shutter-release button is pressed, first the mirror
flips upwards from the path of light; then the curtain mechanism begins.
Before releasing the shutter, the sensor is covered by the front
curtain; both the curtains sit behind a mirror.
The mirror remains slightly slanted and reflects the incoming
light (via lens) to a pentaprism or pentamirror located above
the main mirror. The pentaprism or pentamirror again diverts
light towards the Optical View Finder(OVF). Keeping our eye
on the OVF, we can see what is happening in front of the lens.
This is the time when we compose the frame, focus and set all the
parameters like aperture, shutter-speed, ISO, white balance etc.
through buttons and menus. Now we are ready to release the
shutter button.
When a DSLR's shutter is released :
● Mirror flips upward to allows incoming light
● Light no longer reaches OVF; rather falls on closed curtains. ✫
● Front curtain moves, light reaches the sensor/film; exposure begins.
● Light signals are converted to electrical signals, a digital image is recorded.
● Rear curtain moves, light is blocked; exposure ends.
● Mirror moves downwards; light is reflected towards OVF.
● Two curtains move to their own place.
✫ This is the reason that we can see nothing through the
view-finder when the sensor is exposed to light
and image is recorded.
Shutter Mechanism of a Mirrorless camera
Unlike DSLRs, in a mirrorless camera,
as the name suggests, there is no mirror in front of the sensor.
Also, instead of OVF(Optical View Finder),
an Elecronic View Finder is used
in mirrorless cameras. Such a viewfinders displays digital version
of the scenario in front of the lens.
So, any pentaprism or pentamirror is also
not required to divert light to the eye.
This is why mirrorless cameras are
thinner and lighter than DSLR cameras.
When a mirrorless camera is switched on, unlike DSLRs,
the sensor is continuously
exposed to light. The converted electrical signals
are sent to the Electronic view-finder (EVF) and the
Live-view screen in the form of a digital image.
After composing the scene, and setting all parameters,
when we press the shutter-release button, the 1st curtain
flips upwards to cover the sensor
(until now it was continuously exposed to light, opposite
to a DSLR mechanism)
.
This blocks the incoming
light to the sensor, resulting in blockage of light to the
EVF and Live-view too. The same curtain moves back to its
original position, and the sensor is exposed to light. The
exposure begins. At this point, the EVF or LV is still black.
At the end of exposure, the 2nd curtain moves
down to cover the sensor. Exposure ends here, and a digital
image is created in this duration. Now the 2nd curtain moves
upward to its original position. The sensor is exposed to light
again, and light also reaches the EVF and Live-view for next
composition.
In case of a DSLR, in switched on mode, the sensor is blocked by the mirror.
In case of a mirrorless, the sensor is open to light in switched on mode,
and continuously light signal is converted to digital signal for display
in the EVF and the Live view screen. This is why mirrorless cameras consume
more battery, and may collect more dust in the sensor if not taken care of.
