The next major type is the diaphragm shutter, as epitomized by the "Compur" shutters so popular from the mid 1920s until the 50s. One of the earliest incarnations of the diaphragm shutter was the Bausch & Lomb "Unicum" shutter patented in 1891 and commonplace on a wide variety of equipment by 1897. The "Unicum" was a fairly simple design using two overlapping vulcanite blades, and as such doesn't strictly fit the diaphragm definition, as it's function is closer to an eyelid rather than an iris. Built into a handsome brass circular case the shutter leaves were opened by a pre tensioned spring, triggered by a small lever or rubber bulb attached to the pneumatic release piston. The delay between opening and closing was governed pneumatically, i.e.. by air escaping from a another piston. Though popular at the time, this was not the ideal method of controlling shutter speed as the shutter would have been susceptible to changes in temperature, humidity, altitude, air pressure, contamination and wear, maintaining accuracy at speeds faster than 1/125th sec. would have been problematical when they were new. When any wear became apparent it would be impossible. At the time this wasn't too much of an issue - as film speeds dictated the use of fairly long exposures. Although these shutters gave sterling service, (many still function well today) - their days were numbered as film speeds increased to the point that the accuracy offered by a pneumatic governor was inadequate.

The problem was solved with a mechanical system that used variable spring tension for the shorter exposures and a brake system for the slower speeds. This design was perfected in the Compur rim set shutter of 1928, this shutter mechanism was the truly state of the art when it was introduced. These shutters are beautifully made, with mechanisms akin to Swiss watches, shutter leaves increased to three, opened and closed in a circular movement resulting in an action that looks rather more like an iris. Although expensive, mass production led to it being adopted by many camera manufacturers and thousands are still to be found today in perfect working order, such was the quality of the workmanship. The slowest speeds (generally those longer than 1/25th sec.) were governed by a mechanical brake that worked in a similar manner to the escapement of a watch or clock, it is this mechanism you hear buzzing when you release one of these shutters at one of the slow speeds. This design was used as the inspiration for several other makers, but the principle behind all is very similar. Common diaphragm shutters are Compound c1907, Compur (dial set c1912 and rim set c1928), Compur Rapid - with a shutter speed of 1/500th sec. c1935, Compur (Synchro) c1951 (flash synchronisation), Epsilon c1949, Kodak Ball Bearing shutters c1909, Prontor-S c1948, Vario c1950 - although this brief list is a long way short of exhaustive. Shutters prior to the mid 1950s tend to be calibrated in the older scale, starting at T, B, 1sec, 1/2sec, 1/5sec, 1/10sec, 1/25sec, 1/50sec, 1/100sec, 1/200sec and sometimes 1/300sec. After this the scale adopted was B, 1sec, 1/2sec, 1/4sec, 1/8sec, 1/15sec, 1/30sec, 1/60sec, 1/125sec, 1/250sec, 1/500sec and sometimes 1/1000sec. The T setting of the earlier scale was for timed exposures and also for focussing on a ground glass screen, a largely redundant exercise in popular cameras after the 1930s. When using the T setting the shutter will remain open until the release is pressed a second time so remained useful for very long timed exposures. The B setting was still used on modern equipment up until the digital revolution took hold, and stood for 'Bulb', an archaic term dating from when the shutter was opened pneumatically using a rubber squeeze bulb, remaining open until the pressure was released. This could be used for the shorter timed exposures or for early flash photography, in which case the shutter was opened, the flash powder ignited, then the shutter closed.

Some common diaphragm shutters - please note, not to scale.

Compound c1907

Compur Dial Set c1912

Compur Rim Set c1928

 Epsilon c1949

 Vario c1950

 The diaphragm shutter continues to be employed with modern studio cameras due largely to it's ability to synchronize with flash photography at all shutter speeds which is it's principle advantage over focal plane shutters. As we move into the 21st century the diaphragm shutter is in it's swan song as digital photography is now moving progressively into the studio photography market.

In addition to these high quality mechanisms many millions of mass produced cheap shutters were produced to satisfy a growing demand for cameras from a picture hungry public. Most of these shutters were very nicely finished to mimic the more expensive models - but generally had limited shutter speeds, in the order of 1/25th to 1/100th sec. They were common in cheap cameras up until 1960s, when changing fashion meant they started to die out. Generally these are identified by a lack of the need to cock the shutter prior to release, the shutter being powered by the press of the release itself - or self energizing. Although there is no danger of forgetting to cock the shutter with this system, the mechanism is more prone to camera shake as a result of the jerk imparted at the moment of release.

Here are a few from 1909 up to 1963.

The Kodak Ball Bearing shutter introduced in 1909 on the extreme left is a bit of an oddity as although it is technically part of this group, it is very nicely made and was intended as a "serious" shutter.

 In the 1950s a trend of encasing good quality shutters within the lens housing produced cameras that looked increasingly homogeneous, nevertheless they still had what amounted to ring set shutter buried within, with the name proudly printed on the casing.

Here are two Prontor shutters as buried within a Voigtländer Vito B on the left and a Zeiss Ikon Contessa LBE on the right.