Lens Diaphragms

A diaphragm or stop has three purposes - to limit the amount of light exiting the lens, reduce aberrations by limiting marginal rays, control the depth of field. Diaphragms were used in optical instruments prior to the invention of photography and versions where the aperture could be altered were fitted to some camera lenses from the early 1840s. They are usually present on early landscape lenses from the wet-plate period and before but less common on portrait lenses. Portrait lenses that were convertible for landscape use by removing the rear lens group and switching the front group to the rear were sold with a removable washer stop for use in landscape mode.

Washer Stops

Or Pillbox stops. These consist of individual discs with holes punched in them. They are found on early lenses especially on landscape types where they fit at the front of the lens behind a retaining ring.

Pivoted Stops

These were fitted to early landscape lenses from the 1840s and 1850s, one or more metal plates containing an aperture mounted on the front of the lens barrel can be swivelled in front of the lens.

Wheel Stops

Wheel Stops or Rotating Stops consist of a circular disc mounted off-centre to the lens axis containing a series of apertures. The disc is usually mounted within the lens and can rotate to bring one of the apertures into use. A spring engages a notch in the disc to align the stops. It was used from the early 1840s and is described in Willats's catalogue of 1846 but became popular in the 1880s and 1890s especially on wide-angle lenses.¹ A wheel stop formed part of Dancer's patent of 1856, they are also found on microscopes where they are known as Dancer diaphragms.²

Waterhouse Stops

These are roughly oblong plates with a hole punched in the centre of the plate and with a small tab added to grip the stop. They generally fit at the centre of the lens between the optics but on landscape lenses they are at the front of the lens mount. They were introduced by John Waterhouse in 1858 and were common from that time until the early 1900s when they were largely replaced by the iris diaphragm, though they were still found much later on less expensive and very large lenses. Lenses fitted with Waterhouse stops are advertised as early as September 1858 by the opticians T. Slater of Euston Road London. Slater's advertisement shows the stops to be in oblong plates with rounded ends, these slide into a holder which then slots into the lens barrel, the holder has a wide plate at the top to exclude light from entering the slot.³

The hole in the plate was circular except for half-tone and process work when square and others shapes were used to control the shape of the dot. Penrose patented a series of stops with lobes to spread the dot at the corners, adjustable stops where the dimensions of the aperture could be set were also in use.⁴

Lever

Three stops are mounted in individual strips of metal which are able to pivot in front of the lens axis. The end of the strip passes outside the lens mount to form a lever. Found on lenses by Darlot.

Iris

An iris comprises a number of curved leaves able to form an opening of varying size. The usual form is for each leaf to have a stud mounted at each end and on opposite sides of the leaf. On one side the studs fit into sockets in a ring on the other side the studs fit into slots in a second ring, as one ring is turned relative to the other the leaves move inwards or outwards so forming a varying sized and roughly circular aperture. A less common arrangement was for the leaf to have a single stud and at the other end a slot. The leaves were normally made of vulcanite (or similar material) or metal which was recommended for use with enlarging and projection lenses.

When introduced the iris was a more expensive option, as the cost reduced the iris became the common diaphragm from the early 1900s. Early examples often have the mounting rings in a housing outside of the lens tube later they are contained within the lens tube. When completely within the lens tube the aperture is sometimes set by a pointer moving in a slot in the lens barrel. Later, a moveable ring covered the slot. Angular rather than curved leaves came to be used especially on lenses with high apertures. Click-stops were fitted to some models to indicate designated apertures, an early example of this is on Wray lenses from around 1893.⁵

Early examples were demonstrated by Chevalier and Jamin in the 1840s and 1850s. It was much later though that the iris became commonly available. One of the first to commercially fit irises to their lenses was Lancaster when, in 1886, they were fitted to the Rectigraph series of lenses.⁶ They were in common use from that time.

A second form of iris was patented by Harrison and Schnitzer in 1858 where the leaves were broader and covered only half the diameter of the lens. Each leaf had two pins at the same end one acting as a pivot the other moving in a curved track to open and close the diaphragm. Not much use was made of the arrangement at the time but the idea was later used on combined shutters and diaphragms.⁷

The image on the right shows an unusual and early iris where the slots are in the leaves.

Other Forms

Sliding Stops

Consists of a number of holes punched in a strip of metal that slides across the lens opening. Found on cheaper cameras but also popular on continental hand cameras.

Noton

Consists of two overlapping plates each with a diamond shaped opening. By moving the plates a variable sized aperture is formed. First proposed by Noton in 1856, a similar idea was used on the Sands & Hunter shutter of 1881.

Maugey

Pierre Maugey patented (1858) an arrangement where the diaphragm was made by an elastic membrane placed across the lens, this had a small hole in it, the size of the hole could be adjusted by stretching the membrane. Maugey lenses fitted with this diaphragm were sold by Bland & Co.⁸

Setting the Iris Diaphragm

Manual

The iris is closed to its working aperture by moving a ring or pointer on the lens barrel. Click stops may be used to indicate set positions.

Pre-Set

The f-number is set on the lens but the iris remains open, immediately prior to exposure a ring, or similar, is turned which closes the iris to the pre-set position. The need for a pre-set device came from the use of reflex cameras where focusing needed to be at full aperture, the photographer would then need to look away from the focusing screen to set the aperture. Despite the advantage of a pre-set device they only came into common use in the late 1940s. An early example was the 'Iristop' fitted to the Beck Neostigmar lens in 1920.⁹

In 1950 Schneider introduced a pre-set device where the iris was set, a ring was then turned that opened the iris and tensioned the 'pre-set' spring, before exposure a button on the lens was pressed that released the spring and closed iris. The button was equipped with a thread enabling a double cable release to be used giving something like a semi-automatic operation.¹⁰

Semi-Automatic

Here the diaphragm is kept open by a spring latched in the tensioned position, the latch is released when the camera's shutter is fired, the spring then contracts and closes the diaphragm to its pre-set position. After exposure the spring has to be manually re-set. The arrangement was patented by Treitschke in 1916.¹¹ Later examples are the Contax F (c. 1957), late models of the Praktina FX (c. 1954/55), late models of the Praktica FX2 and Pentax K (1958).

The image on the right shows the setting lever on a lens from a Praktina FX.

Automatic

A improvement on semi-automatic operation was to use the action of advancing the film and setting the shutter to provide the power to re-set the iris. This type of iris is very common on cameras with leaf shutters. Examples are the Contaflex (1953), Bessamatic (1958/59), Praktina 11A (1958), Contarex (1958).

Fully Automatic

With fully automatic diaphragms (FAD) the iris is only closed for the duration of the exposure and automatically opens following exposure. Typically the iris is pulled open by a spring, a pin at the rear of the lens engages with a plate in the camera. Normally the plate is lowered which allows the iris to remain open. At the time of exposure the plate is raised which depresses the pin and closes the iris to its pre-set position, immediately after exposure the plate is lowered releasing the pin and opening the iris. The reverse arrangement is also found where the iris is pulled shut by a spring and kept open by the plate in the camera.

Fully Automatic - Lens Release

Several cameras from the 1960s had a 'shutter' release on the lens which fitted in front of the normal release on the camera body. This had the advantage that no mechanism was needed in the camera to operate what was, in effect, a fully automatic diaphragm. The diaphragm is kept open by a spring and closes to its pre-set value while the release is pressed. A problem was that on slow speeds finger pressure on the release might be removed before the exposure had completed causing the diaphragm to open; a lock was usually fitted to hold the release in the down position.

Diaphragm Calibration

For many years there was no commonly used method of comparing the speed of lenses; lenses were sold as having a certain diameter, focal length and able to cover a particular plate size. The lack of an arithmetic value for the speed of a lens would not have been a hardship at the time given the uncertainties introduced when the sensitive plate was made by the photographer and the absence of exposure meters to measure the light. The normal measure of lens speed - focal length divided by diameter (f/d) - probably came into general use in the 1870s. This value, the F/No. or f-number, is properly expressed as, for example, f/4 indicating an opening one quarter of the focal length. Alternatively it may be expressed as 1:4 or in recent years simply f4.

Iris Scales

On early lenses the values were not uniformly distributed along the scale, the settings are bunched at the end of the scale showing smaller stops. This did not present a problem at the time, except that the scales were difficult to read, but it prevented the coupling of exposure meters and the transference of Exposure Values (EV) to the camera. Later, scales were used where equal divisions represented aperture values changing in geometric progression, typically with a factor of 2. An early suggestion for this arrangement was made by Alfred Watkins (early 1900s) and later by Deckel in a series of patents from 1952.

Transmission Numbers

The f-number, and those others shown below, are arithmetic expressions of the geometry of the lens i.e. focal length and diameter, they do not say anything about the light transmitting properties of the lens. In cinematic use it is important to be able to switch lenses and maintain the same light level at the image. Cine lenses, then, are often marked with an additional scale of T-numbers or T-stops, these show the equivalent theoretical f-number e.g. T-8 is the same as f/8 on a lens that transmits 100% of the light, though it might be positioned at f6. The T numbers were obtained by actual measurement of each lens type. The inclusion of transmission values had long been part of a photographer's mental calculation and Hurter and Driffield included it as a factor in their work on exposure.

Comparison of Diaphragm Scales

The following table shows an approximate comparison of the different calibration systems in use.

Relative
Exposure	F/No.	F/No.	Dallmeyer	Goerz	Congress	Zeiss	Zeiss	US
Required		Continental				New	Old
256	64		400	384	40			256
		50	300			1	4
128	45		200	192				128
		36	150		16	2	8
64	32		100	96				64
		25	75		8	4	16
32	22		50	48				32
		18	40		4	8	32
16	16		25	24				16
		12.5	20		2	16	64
8	11		15	12				8
		9	10		1	32	128
4	8		6.4	6.4	.7			4
		6.3	5	4.6		64	256
2	5.6		3		.32			2
		4.5	2.5		.2	128	512
1	4		1.5					1
		3.3	1

F/No.

The ratio of the focal length to the diameter of the aperture. The numbering of the scale varies, the most common are:

• f2, 2.8, 4, 5.6, 8, 11. This was the standard used in Britain.
• f2.2, 3.3, 4.5, 6.3, 9, 12.5. This was widely used on continental lenses.

Other values used were: f10, f20, f30. Found on Lancaster lenses.

The values on the scale generally indicate an aperture half the size of the preceding one, and therefore a relative exposure requirement of x2, i.e. going from a larger stop to the next value requires twice the exposure. The numbers themselves, since they are calculated from a dimension and not an area, increase by a factor of √2 (square root of two).¹²

US - Uniform System

Adopted in 1881 by the Royal Photographic Society (RPS). The aperture ratio f4 was taken as a starting point of 1, other apertures in the system had half the area of the preceding. So the second aperture in the series - 2 - requires twice the exposure of 1. The system has the advantage that smaller apertures indicate directly the exposure factor. Used by Ross in c. 1890 for a few years but the scale was soon forgotten in favour of F/No's. The system remained in use in the USA for many years. The US value = F x F / 16, where F is the aperture ratio.

Congress

Adopted by the 1889 Paris Congress. f10 is taken as the unit figure, other figures, as in the US system, give the relative exposure required. The Congress value = F x F / 100, where F is the aperture ratio.

Zeiss Old

f100 was chosen as the initial value of 1, the following numbers indicate relative intensity of the image. This has the advantage that larger values represent larger apertures. Introduced in c. 1890. The Zeiss values can be calculated as 10000/F x F, where F is the aperture ratio.

Zeiss New

Similar to the Zeiss Old scale but the starting point was taken as f50 rather than f100.

Zeiss Millimetre

Some early Zeiss combinable lenses, such as the Protar, are marked with the actual diameter of the aperture in millimetres. This allowed the same scale to be used for several lens combinations, printed tables were issued to show the corresponding relative aperture for the lens in use. As well as the millimetre index lines, figures for 3, 4, 6, 8, 12, 17 and 24 millimetres were engraved, each of these giving roughly double the exposure of the previous.

Dallmeyer

f3.16 is taken as the unit figure. Other values give the relative exposure value. Introduced around 1886 in use until the late 1890s. This corresponds to being found on lenses with serial numbers from around 43000. f3.16 is derived from being the square root of 10.¹³

Goerz

Similar calculation to the Dallmeyer scale. In use from about 1889. A similar scale was used by Voigtländer.

Early Dallmeyer

The apertures were numbered sequentially from 1. Each higher numbered stop requiring twice the exposure of the previous. Half stops were marked with an 'X'. The serial number of the lens is usually present on the largest stop. In use from 1860 to probably the mid 1880s.

Effect of Close Focus

The calibration of the diaphragm is based on the image distance being the same as the focal length, when focused on close objects the image distance is increased and so the calibration is incorrect. For ordinary distances this can be ignored and for macro photography it is usual to look up the bellows extension in tables to get the exposure factor. Some close focusing lenses though incorporated a mechanism that adjusted the aperture as the lens was extended, this was used on both East and West German Zeiss lenses of the 1950s and 1960s.

References & Notes