Photographic Shutters

06.2018 - The shutter limits the exposure time of the film or sensor, i.e. the time that is allowed for the exposure of the image window or sensor to take a picture. Basically, a differentiation is made between leaf shutters and focal-plane shutters and, in the case of digital cameras, the electronic shutter, which does not require mechanically moving parts. Hybrid solutions are also available for CMOS sensors. The exposure of the sensor is started with the shutter open and the exposure of the sensor is ended with the second shutter curtain.

With the first cameras, whose light-sensitive layer required a longer exposure time, the lens cap was often sufficient to control the exposure time by removing and replacing it. With increasing sensitivity of the recording materials, the first mechanical shutters were used. In digital photography, a dark phase is required to read out the sensor and to create a noise reference.

Terms and categories for a better understanding
• Mechanical leaf shutters (Copal 0/1/3)
• Electro-magnetic leaf shutters (Sinar/Leica eShutter 125, eShutter 250, H-lenses, certain Phase One lenses)
• Electronic shutters (sensor-based rolling shutter, as currently available in IQ3 100/Trichromatic and Hasselblad H6D backs; flash light not feasible)
• Global shutters (sensor-based "global shutter" are currently not yet available for large format sensors, flash light not feasible)

From left to right: Copal 0, ALPA HR Alpagon 4.0/32 mm with Sinar/Rodenstock eShutter 250 (without control box), ALPA 12 FPS with Focal Plane Shutter, Phase One IQ 3 100 with electronic (rolling) shutter via sensor

Leaf Shutters
A leaf shutters consists of several lamellas/blades and are similar in shape and construction to an aperture. In contrast to the usual aperture systems, however, the leaf shutter can be closed completely. Classically, thinly rolled steel is used as the material for the shutter blades. Carbon lamellas have also been used for some years. Hybrid material mixes are also used in some cases. These have proven themselves especially at high speeds for short exposure times. Most leaf shutters are used as so-called intermediate lens shutters. The shutter is installed at the point in the lens where the beam path is most concentrated. There, the required aperture requires the smallest diameter in each case. The smaller this diameter, the shorter the distance that the closure blades have to travel when opening and closing. With the smaller shutter blades, the moving mass can be kept smaller and a higher speed of the shutter itself can be achieved. The faster the overall sequence can be achieved, the faster the shutter speed that can be realized.

An important advantage of the leaf shutter is their short synchronization time. Sync speed is the fastest shutter speed at which a shutter opens the entire image field for exposure. A flash system that is triggered during this time window enables uniform illumination of the entire image.

For example, the synchronization time of current Phase One and Hasselblad central shutter systems reaches times of 1/1600 or 1/2000 second. Today, leaf shutters are mainly used for large and medium format camera systems in the field of image-based photography. In addition, leaf shutters are also installed in compact cameras such as the Fujifilm X100F. In practice, the disadvantage of having to install a central shutter as an intermediate lens shutter in each individual lens has no effect, as the lens cannot be changed. The shortest synchronous shutter speed for the X100F is 1/4000 second.

COPAL specification
Manual No. 0/1
All information without guarantee.

Important care instructions for Copal shutters

• Only adjust times in uncocked status and never when the shutter has already been cocked (wear, damage to the escapement possible).
• Only full time increments can be set; never try to set split times (e.g. 1/45 second).
• Today's high-performance optics are NOT designed to exchange shutters freely and to screw in and out front and rear lens elements.

Who produced leaf shutters?
Mechanical leaf shutters were manufactured in Europe by Deckel/Compur/Prontor and until the 1990s by Gitzo in France. In Japan, Nidec Copal and Seiko/Seikosha were the most important manufacturers of leaf shutters. The last independent manufacturer of purely mechanical leaf shutters was Nidec Copal. The production of classic mechanical leaf shutters has since been discontinued. A transfer of the production by third parties failed due to the condition of the tools. Mechanical leaf shutters are no longer in production. Remaining stocks, however, are still available for some time.

The leaf shutters installed in lenses for medium format cameras today come from exclusive productions for the respective manufacturers. Sinar produces the eShutter with a shortest shutter speed of 1/125 second and the eShutter 250 with a shortest shutter speed of 1/250 second, which are also used by Rodenstock under its own name in Rodenstock lenses. Sinar offers the eControl control unit for its electromechanical eShutter, which allows the eShutter to be used anywhere and without a computer. This makes it easier to use outside the studio. The Sinar eControl is the successor of the eShutter Control, which was introduced in 2012. Power is supplied by a rechargeable and replaceable lithium-ion battery. In contrast to the shutter, the control unit can be easily exchanged between different lenses. The respective lens is automatically recognized by the eControl and shown on the display. The desired aperture and exposure time can be set using a multifunctional wheel by turning and clicking. ALPA will introduce its own solution in 2018.

Focal-plane shutter
A technical alternative to the leaf shutter is the so-called focal-plane shutter. It is not installed in the lens, but in the camera body and therefore behind the lens. This was the reason for his popularity with the advent of camera systems with interchangeable lenses. The first focal-plane shutters were so-called cloth shutters, as they are still used in analog Leica M cameras today. Here a textile roller blind consisting of two closing curtains is moved horizontally. The sync speed is also in this case exactly the time in which the shutter is fully open. With faster shutter speeds, only one gap between the two curtains is released at a time ("scanning slit"). In newer focal-plane shutter models, the textile was replaced by thin metal foils. Later metal lamellas were used instead of roller blinds.

In the now mainly used metal lamella focal-plane shutters and their smaller space requirement, the flow direction of the closure elements changed from horizontal to vertical. This also shortened the distance to be covered by the shutter curtains for the mostly right-angled image formats. Seiko states 1/8,000 second as the shortest shutter speed for its 35mm shutters. The shorter distances also made it possible to shorten the synchronization time. With cameras in 35mm format, synchronization times of 1/320 second are now possible as standard.

In medium format cameras with their significantly larger image windows, the distances to be covered by the shutter curtains and the masses to be accelerated are significantly larger. This results in significantly longer synchronization times for focal-plane shutters compared to central shutters. Since this would have an even stronger effect on large format cameras, focal-plane shutters are not used there for this reason.

Changes to the requirements for a shutter
With the announcement of CMOS sensors for medium format cameras in January 2014 (Hasselblad H5D-50c and Phase One IQ250), the requirements for shutters also changed. In contrast to analog film and CCD sensors, CMOS sensors can be started with the shutter open (Life View). The shutter is only important here to stop the light stream to the sensor so that the complete sensor can then be read out without receiving new information. In addition, in the non-exposure state, the reference voltage of the sensor is determined, which is subtracted from the exposure data in order to take a correct picture.

The digitization of photography has brought another change in shutters. If one could simply unscrew the front and rear lens elements of a lens from the shutter at analogue times and then use the shutter in another lens, this is no longer possible today. One reason for this is the lack of housing stability of the shutters from the analogue period. Today the shutters are optimally adapted to the respective lenses and a shutter change would lead to a loss of the adjustment of the azimuth and the correct dimension of the aperture.

Electronic shutter
The CMOS sensors available today do not read out simultaneously over the entire sensor surface. With these sensors, one can imagine a line with simultaneous start of exposure, which usually moves across the sensor line by line. With current medium format sensors, this takes about half a second. If the camera and subject are stationary, all pixels are exposed at their correct positions even with CMOS sensors, regardless of when they were exposed during the said half second. However, if images are taken of a moving subject or with a moving camera, the objects are displayed at their current location when the lines are exposed one after the other. Since the mapping takes place line by line, the object has already moved from one line to the next to such an extent that, when all lines are combined, an object is displayed that was not mapped as a whole at once. Since the image is taken line by line at different times, a straight line in the subject can be bent or distorted. The use of flash units is usually not possible.

Extreme rolling shutter effect

The future
Even if future sensors can be read out faster and it is therefore possible that electronic shutters in the version of the global shutter can also be used in medium format, a mechanical shutter will very probably still be required. This unit prevents the sensor from being exposed to light exposure if this is not intended in connection with a photograph. However, the shutter is then no longer necessarily used for the purpose of forming exposure time and it now takes on a new function as a protective element.



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