Shutter-speed and motion

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Shutter Mechanism in Photography

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.

slow-shutter-speed catches Hawa Mahal at Jaipur

f/8, 2.5 sec., ISO-64, FL-24 mm eqv., Nikon d810, Tamron 24-70mm f/2.8, Jaipur, Rajasthan, India 2015.

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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 1500th. 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.

Shutter for one-forth second and half second duration SHUTTER-MECHANISM OF A CAMERA A ¼ sec. shutter mechanism above A ½ sec. shutter mechanism above Sensor exposed to light Sensor blocked by curtain Sensor exposed to light Sensor blocked by curtain

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.

Shutter mechanism of a DSLR Shutter mechanism (DSLR) OVF Normal state, image to OVF Shutter released, mirror up, OVF blocked Front curtain moves, exposure begins Light reaches sensor Rear curtain moves, exposure ends Mirror down : image to OVF Curtains reset Cycle complete for one click OVF-Optical View Finder subject mirror pentaprism lens curtains sensor

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.

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.

Shutter mechanism of a Mirrorless camera Shutter mechanism (Mirrorless) EVF/LV Normal state, image to EVF & Live view After shutter is released, Lower curtain moves up, EVF & Live view blocked Lower curtain moves down, Light reaches sensor, Exposure begins. Exposure ends, Upper curtain moves down, Upper curtain moves up (reset to normal). Image to EVF & Live view, Cycle complete for one click. EVF - Elecronic View Finder, LV -Live View curtains sensor subject lens

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.

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