AG Photo luxury
Shutter
Introduction
In photography the shutter is a device that is placed on the focal plane and allows or does not allow the sensor to be exposed to light that arrives from the lens. This mechanism is what then determines the exposure time or the shot time of a photograph and therefore, together with the diaphragm, allows us to choose the correct exposure. When one presses the shot button, it opens for the time selected in order to allow the passage of light and then closes again immediately after (figure 01).
01 The shutter is in front of the sensor. At the moment of the shot, it opens and exposes the sensor to light for the time specified.
Side view of shutter
Camera body
Sensor
Back in the day, in place of the shutter on some of the earliest cameras, a simple object suited for covering the lens was used and then removed only during the exposure time. It was possible to do this because back then film was not very sensitive and required an extra long exposure time.
Later, as the sensitive of film increased, it became necessary to automate the process and make blocking the light more precise.
Various kinds of shutter have been used: a fabric shutter curtain, rounded blades (like those seen on a diaphragm), and timed electronic ones. Today, there are two types of shutters used, both of which are time electronically:
- Electronically controlled mechanical shutters
- Electronic shutters
The first type is used in all reflex digital cameras while these second type is more commonly found in compact digital cameras and digital cameras in mobile phones.
Mechanical shutter electronically controlled (shutter curtain)
Focal plane mechanical shutters that we find on modern reflex digital cameras consist of a mount across which two ‘curtains’ slide. In the majority of shutters, the ‘curtains’ slide vertically, driven by a motor operated electronically. In figure 02 we see a shutter of this type which is closed and in figure 03 it is completely open.
02 Anatomy of a focal plane shutter that is closed.
Opening mechanisms
Curtain blades
Shutter rear-curtain
Shutter front-curtain
03 Anatomy of a shutter completely open permits to see the sensor.
Sensor
Printed circuit
Opened shutter curtains
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In figure 04, we can see the phases of a shot with long exposure times where the second shutter curtain moves after the first one has already opened, leaving the entire sensor exposed to light. During the brief exposure, though, as seen in figure 05, the second shutter curtain is already starting to move up while the first one just starts moving (after the time of chosen exposure), exposing the sensor only to the fissure that is created by the movement of the two shutter curtains. In this way, we are able to obtain an exposure time on the order of 1/4000 to 1/8000 of a second, exposing each zone of the sensor to light for the same period of time. The shorter the period of time, the slimmer the slit will be.
04a In this figure, we see the rising phases of a shot with the exposure time quite long.
In the starting phase, the shutter is closed.
04b In this phase the upper shutter curtain starts to fall back, rising and leaving a part of the sensor exposed to light.
04c In this phase the upper curtain is completely opened and the lower curtain has still not raised in order to close the shutter. The sensor is completely exposed to the light.
04d The lower shutter curtain starts to rise in order to close the shutter and does it at the same speed that the other opened at.
04e Now the lower shutter curtain gets closer to closing after which each zone of the sensor will be exposed to the light for the same time.
05a In this figure we examine the opening sequence linked to a short period of exposure time. We start with the shutter closed.
05b In this phase the upper shutter curtain starts to rise up in order to leave the sensor exposed.
05c The shot time is so short (a fraction of a second) that while the upper shutter curtain is rising, the lower one has already started to move. Between the two, a fissure forms that remains always the same distance that rises exposing only part of the sensor to the light for the exposure time selected.
05d The fissure created by the two shutter curtains rises with its and its size depends only from the exposure time chosen.
05e In the final phase, the shutter closes.
Upkeep
The mechanical shutter, like all mechanical parts, is subject to use of the components that it is made up of and then is tested by the manufacturers for a certain number of cycles. On reflex digital cameras, they are tested with quality controls up to 100,000 to 200,000 cycles (depending on the model). Once such a number of cycles have been reached, and perhaps a bit before, the components should be inspected. However, it is all about the precision of the components, and, if it is not able to guarantee the exact exposure time anymore, the photos might not have the correct exposure time. Also, in some cases, this can happen if the shutter is broken. Therefore, it is a good rule to send the equipment for service after intensive use.
High-end reflex cameras also possess an electronic check that accurately verifies that the time selected corresponds exactly to what the shutter “curtains” should be doing so that the correct exposure times are produced.
Every reflex has a shot counter that keeps track of the shots taken since the camera has been acquired. To find this information, all you have to do is look at the metadata of the last shot taken.
Electronic shutter
Electronic shutters are much simpler from a conceptual point of view because they function like an on-off switch. In practice, a true shutter does not exist: there is simply an electronic control that is driven by the sensor. The sensor’s data is then read for the time selected and then it is as if the sensor is turned off.
These types of shutters are currently present on smaller digital cameras and mobile phones and are economical.
Recently, manufacturers tried to discontinue mechanical shutters with an electronic control in order to make greater use of the electronic version. This took place through various pilot projects that tried to combine both systems. The result was a return to the mechanical shutter with the “curtains” due to the technical limitations that still exist that have not been able to keep pace with performance.
The electronic shutter seems to have some backlighting problems as well as problems linked to image quality when the photodiodes of the sensor are saturated.
These setbacks aside, manufacturers are continuing to invest in research for new solutions that would finally allow them to abandon the mechanical shutter and allow them to make an electronic version that could close within fractions of a second gradually lower.