Exposure Meters

Until the arrival of commercially available dry plates in the late1870s there was little requirement for a meter to measure the light. Variations in the preparation of the plate, chemicals used, age of the chemicals, and how the plate would be developed would all affect the exposure required. Judging how sunny it was was not the photographer's biggest problem. Even when pre-prepared plates were made available commercially there was no recognised measure for the speed of the plate which at the time were sold as 'fast', 'extra fast' or something similar. The speed would also vary between one batch of plates and another as well being dependent on their age and how they were stored. Through experience photographers would have built up a rule-of-thumb for the exposure based on their own working methods.

Some early meters provided a way of gauging not only the light level but other factors as well; taking a meter reading was similar to making a test exposure, using the same sensitive material, preparation and development. One commercially produced meter was Claudet's Photographometer of 1848.¹ In 1876 Brice patented an instrument that tested the chemicals that were used, as well as the light conditions, it consisted of a glass plate overlaid with strips of semi-opaque material, this was placed in front of the photographic plate and a test exposure was made.² Another meter sold commercially, based on a graduated wedge, was patented by Bing. This was an actinometer using sensitive paper that provided a numeric value for the light rather than a relative comparison between one sensitive plate and another.³

Types of Exposure Meter

Tables & Calculators

These have various forms ranging from simple printed tables to instruments with dials or sliding scales. Usually they combine at least:

• Time of day and time of year,
• Weather, sky or light conditions,
• Type of subject e.g. open landscape, seascape,
• Plate speed.

Unless the latitude is included as a variable they will be intended for one particular region only. Tables were available from the earliest period but not commonly used until the dry plate period, they have remained in use pretty well until today (an exposure guide is usually included with a box of film). The earliest calculator, certainly of any consequence, was the Actinograph by Hurter and Driffield (1888) which was an outcome of their work on light levels and emulsion characteristics.⁴ From then on calculators were popular especially with the occasional photographer as they were both cheap and easy to use, later models were often incorporated into diaries e.g. Wellcome. A particular format was the dial calculator that originated in the 1920s, these were usually made of plastic and had small notches in the rim to move the discs (fig. E1).

Cameras often included a reduced form of table. If the variables are reduced to two - light conditions and subject, then they can be directly set on the shutter and aperture scales. Many Kodaks had this arrangement called Autotime (introduced in 1909). The light condition was set on the shutter dial and the subject was set on the aperture scale. Fig. E2 shows an Autotime fitting, the shutter has settings for Clear and Brilliant, the iris settings include Cloud, Marine View and Portrait.

Actinometers

In an Actinometer a small test exposure is made on sensitive paper. The usual arrangement is for the paper to be exposed until its density matches that of a comparison density. A second form was for a set exposure to be given to a strip of paper under a variable density wedge. Actinometers became popular during the 1890s and remained so through to the 1920s.

An unusual variation was the Chronoscope which produced a small paper negative of the subject. Medium densities in the negative were compared to a set of standard tints.

In Britain the watch pattern where the meter is in the form of a pocket watch became very popular, the Wynne (1893) and later Bee (1902) are two examples.

Early, commercially available actinometers, were by Woodbury (1879, primarily for carbon printing), Green & Füidge of 1884 and the Watkins Standard of 1890. The Standard was the first of a number of meters from Watkins, it included a subject scale and on the enlarging model a scale for bellows extension.⁵

The second form, density wedge actinometers, came to be used as darkroom Print Meters.

Extinction Meters

These are commonly found in two forms:

• The eye looks into an eyepiece and focuses on a screen, the light reaching the eye is gradually reduced by an iris or density wedge, the meter is set when figures on the screen or part of the scene becomes indistinct. A blue filter is often used to eliminate colour from the screen. A problem with extinction meters was the eye's ability to adapt to changing light levels. When looking into the eyepiece the eye will start to compensate for the lower level of light, the adaptation time varies but could take 30s or more. To overcome this part of the finder could be left bright thus controlling adaptation.
• The open pattern is simply a set of figures of varying opaqueness set in a housing, the meter is held away from the eye. Dunn⁶ is very critical of this type of meter and points out that the eye is constantly adapting to the changing ambient light, when using this type of meter the extinction wedge is judged relative to the ambient light to which the eye has already adapted. Any accuracy comes from setting the weather or sky condition scale which is usually present.

Extinction meters were very popular in the 1920s and early 1930s and remained on sale into the 1950s.

One of the earliest produced was by Decoudun (1887), this was unusual in that the meter was placed against the focusing screen of the camera to take a reading.⁷ Much later Franke & Heidecke produced an attachment where an iris was placed on the viewing lens of the twin-lens Rolleiflex camera, the extinction point was judged on the focusing screen.⁸ A variation was to record the intensity of light on a phosphorescent tablet and then place the variable density scale over it, this was proposed by Warnerke.⁹ An early tubular form of the meter was Tylar's Pickard of around 1889.

Photoelectric

An electrical property associated with a photoelectric cell (resistance or current) varies according to the light falling on it and so can be used to measure the strength of light. Early suggestions were made for the use of selenium cells to be used.¹⁰ With these the variation in current was shown on a galvanometer. The first commercial use was not until 1931 with the Rhamstine Electrophot. Photoelectric meters increased in popularity throughout the 1930s. They also became smaller allowing them to be built into camera bodies. In more recent times cadmium sulphide cells (CdS) which are more sensitive have replaced selenium cells, with these the variation in resistance is used to indicate the strength of light.

On more recent meters a diffuser can be placed in front of the cell for measuring incident rather than reflected light.

Readout on Photoelectric Meters

The light level is generally shown by a needle moving over a scale, a calculator is then used to combine the reading with the film speed to give the result of shutter speed and aperture. Some common arrangement were:¹¹

• Direct reading. The simplest arrangement is for the needle to move over a scale of shutter speeds and indicate directly which value to use. This would be correct for one film speed and aperture, for other values a set of printed tables would be used. Rather than shutter speeds the aperture could be indicated by the needle. The Blendux and Ombrux meters are of this type (fig. E3).
• Light numbers. With this type the needle moves across a range of arbitrary numbers, the number indicated is manually transferred to the calculator. On some examples the film speed can be pre-set on the calculator, in which case setting the light number on the calculator, will indicate the shutter speed and aperture pairs. The popular Weston Master range is of this type. On others the light number was set against the film speed (fig. E4).
• Match needle. A moveable pointer is lined up with the meter needle, in moving the pointer the calculator dials are moved and show the shutter speed and aperture pairs (fig. E5).
• Align value. A value (perhaps the film speed, shutter speed or aperture) on the calculator is moved to align with the meter needle. The calculator will then show the correct shutter speed and aperture pairs. Coloured bands or channels were often drawn between the calculator and the scale over which the needle moves. This solved dimensional differences and, since the channels could have different widths, the problem of the needle not moving a uniform amount for a uniform change in light level (fig. E6).
• Align needle. The needle is brought to a fixed point usually by introducing a variable resistance or altering the light reaching the cell. In moving the needle the calculation dials are moved. Used on the Zeiss Helios and many cameras with built-in meters (fig. E7).

Depending on the configuration of the meter the result may simply be a single shutter speed and aperture pair, more useful is when the complete range of pairings is shown allowing the photographer to choose the most suitable. Also helpful is when the film speed can be pre-set on the meter rather than having to set it for each reading.

Ideally movement of the needle across the scale will be uniform for uniform changes in light level. This was not always achieved and many meters show bunching of values at the high and low end of the scale. Different ways were used to overcome this, one being to shape the ends of the magnet surrounding the galvanometer coil.¹²

Comparison

In this type of meter the available light is compared to a standard source, usually an electric bulb. The earliest, commercially, was by H.D. Taylor (1885) which used a candle as the comparison light source, in other respects it was modern in its design, the scene was viewed through a blue filter surrounding this was a ring of light that was reflected from the candle, a set of sliding stops was used to adjust the light from the candle.¹³ Phosphorescent material was used as the standard source in Ballard's meter of 1890.¹⁴ It was exposed so as to be fully excited and attain a constant value. It was not used to record the level of light as when used in an extinction meter. A phosphorescent ring was used that, after being fully exposed to light, was shut into a tube and compared to the subject, both being seen through an eyepiece and blue filter. The time taken for the ring to dim to the same brightness as the subject gave a value for the light level. A meter sold as the Chromophot which looked very much like an extinction meter also used phosphorescent material to provide a comparison tint.¹⁵ Materials that emit light when acted upon by a radioactive source were also proposed.¹⁶

The standard source could be reduced until it matched the subject by varying the voltage, introducing filters etc. Or the subject could be viewed under a variable density filter. These designs were never popular with amateur photographers but were used professionally as spot meters from the 1950s.

Meters Built into the Camera

Tables

of exposure settings are often printed on the back of cameras, even advanced models such as the Rolleiflex. Calculators with moving dials were also included on some models.

Actinometers

were produced in the 1900s for fitting into the wood of a camera body.¹⁷ An early example where the meter was a more integrated part of the camera was the Actino Midge by Butcher (1900).¹⁸ The single value from the actinometer reading was set on the shutter control, the type of subject was set on the diaphragm control.

Extinction

meters were often built into cameras, the Voigtländer Prominent (1932) was an early example (fig. E8). The Argus K (1939) had a semi-coupled meter which was an exception.¹⁹

Photoelectric

meters attached to the camera offered, at first, little advantage over a separate meter; they were uncoupled and no smaller than a hand-held meter. In the 1950s meters became smaller and so could take up unused space in the camera housing, they also started to be coupled to the camera controls. The earliest camera with a built-in photoelectric meter was the Contaflex (1935) (fig. E9), the Eumig cine camera having a meter coupled to the diaphragm was also produced around this date²⁰ . The Super Kodak Six-20 (1938) had a semi-automatic coupling (automatic aperture setting). Zeiss, Leitz and others were investigating coupled meters in the 1930s for which period several patents were issued, included were methods of coupling the meter to the camera controls and reflecting the meter needle into the viewfinder.

An interesting and early electronically controlled shutter regulated by a selenium cell to give automatic adjustment of exposure times was patented by Carl Eisner in 1902. The mechanism describes the current flowing through a selenium cell varying the speed of an electric motor which drives a rotary shutter, i.e. the greater the light intensity the faster the shutter will rotate and lessen the exposure time. Attached to the shaft of the motor was a breaking device (a coiled spring acting on a brake disk) used to adapt the shutter to the speed of the lens.²¹

Through-the-Lens (TTL)

As meter cells became smaller placing them so as to measure the light coming through the lens became a practical option. The first to have this arrangement was the Mec 16SB (c. 1960), this was followed by the Topcon RE Super or Super D 35 mm camera (1963) and Asahi Spotmatic (1964).

Two common arrangements for setting the exposure were to align the meter needle, visible in the finder, to an index, the other was the 'match-needle' method where a second pointer is moved to coincide with the meter needle.

Readings were 'stop-down' i.e. when taking the reading the lens diaphragm operates in manual mode and adjusts the light exiting the lens or 'full-aperture' where adjusting the diaphragm is simulated in the meter and the diaphragm remains open. The last type requires extra coupling between the lens and the camera body; where the diaphragm ring is turned to adjust the meter reading its value has to be communicated to the meter, the meter also needs to know the maximum aperture of the lens as this determines the level of light seen by the meter.

Where the finder was removable manufacturers would often supply an accessory pentaprism with TTL metering.

There were other specialised methods of TTL metering such as the meter produced in 1958 for the Exakta which was intended for macro work.

Accessories

Many meters were produced to attach to cameras, these generally fitted into to the accessory shoe or other part of the body, on to the viewing lens of a twin-lens reflex, or replaced the pentaprism. When coupled to the camera controls they usually connect to the shutter speed dial, the Leica M3 (1954) is an example of this. Schneider (c. 1960) produced a set of lenses having a detachable meter that coupled to the iris.

Types of Coupling

Uncoupled

The light reading is taken, the resultant shutter speed and aperture are then manually set on the camera controls. This covers nearly all built-in meters prior to World War II.

Coupled

The shutter and aperture dials on the camera are connected to the light meter. The reading is taken by altering these controls.

Semi-coupled

Only one dial is connected to the meter, the other is set manually.

Semi-automatic

The meter is coupled and additionally either the aperture or shutter does not have to be set. When the exposure is made the correct setting is made e.g. only the shutter speed need be set, providing the light is sufficient the aperture will be set at exposure time. Examples are the Super Kodak Six-20 (1938), Agfa Automatic 66 (1956), Savoyflex Automatic (1959), Ultramatic and Contaflex Super B both of (1963).²²

Fully-automatic

A coupled meter where neither the speed or aperture are pre-set, the values are determined and set by the meter at the point of exposure. Examples are the Agfa Optima series (1959) and the Rolleimagic (1960).

Scales, Values and Systems

The scales on the calculation part of a meter are usually logarithmic so that divisions on the scale show the ratio of the underlying values and movement between the scales indicates a geometric change in exposure e.g. they are scaled in 1 stop increments.

Cameras have two settings to control the exposure - shutter speed and aperture - exposure meters supplied these two settings. There were exceptions though, the Watkins Snipe simply indicated if an exposure was possible.

Methods of expressing the light level as a single value were tried, but required some level of coupling or integration with the camera. These had the advantage of being simpler to use; one number transferred from the meter to the camera set a workable shutter speed and aperture. The EV system is an example of this. An early proposal, not commercially realised, was from Watkins in the early 1900s. His ideas were mainly concerned with integrating the calculation part of the meter and the camera but included coupling the shutter speed and aperture and the requirement for logarithmic scales.

Watkins

Watkins worked on proposals, summarised in a paper read before the Royal Photographic Society (RPS), for integrating the meter reading with the coupled controls of the shutter and diaphragm.²³ The proposals include having the shutter speed and diaphragm controls interlinked so that changing the value of one would cause the other to change in a complementary way. The initial value for the settings would be provided by measuring the light value on a separate meter, this value would be set against the plate speed, once this is done indices would point to a usable shutter speed and diaphragm value. Either index could then be moved, the other moving appropriately. Diagrams show four scales the outer two are for shutter speed and diaphragm and are fixed, the inner two are for plate speed and light value, these move independently of each other. The initial setting is made by sliding the inner scales so that the light value matches the plate speed, indices on these two scales will then point to a usable shutter speed and diaphragm value pairing on the outer two scales. To change to a different pairing the inner scales can be moved together. For this, the outer two scales must be logarithmic i.e. equal movements along the scale must change the shutter speed or diaphragm in a constant geometric ratio. Watkins points out that generally this is not the case, for instance the f numbers on lenses are typically bunched towards the smaller apertures. Suggestions on how to overcome this are included in the patents.

The proposals did not result in a commercial product but trial pieces were made, a modified Volute shutter is shown. It would be some years before logarithmic scales for the shutter and diaphragm were adopted and the two coupled together. The single value transferred to the camera was the light value, the speed of the plate was part of the camera setting, which is different to the EV system where the plate speed was part of the meter reading.

Compass Units (CU)

The Compass Camera (1937) used a single value system called Compass Units. The meter provided a single value which if set on the shutter would give the correct exposure when the aperture was fully open. The aperture control was not coupled to the shutter dial but a simple additive system was used, as an aperture was set a number was displayed in a window and this was added to the meter reading before setting on the shutter. The film speed and filter factors were handled in the same way, setting a filter showed the CU value to be added and films were marked with the CU value. A Compass Unit was equal to half a stop.²⁴

Exposure Value (EV)

The EV system was introduced in the mid 1950s. With this system a single value for the light level and film speed (EV) could be set on the camera controls, once set, the shutter and aperture dials were inter-connected and either could be moved to the required value, the other would move appropriately. Many meters of the period provided read-outs in EV. EV 0 corresponds to an exposure time of 1s and an aperture of f/1.0, values on cameras generally ranged from 2 to 19. The EV is calculated as log₂ (A²/T) where A is the f number and T is the Time in seconds, (1 EV = 1 stop). EV was used extensively on cameras with Compur shutters.²⁵ Fig. E11 shows the scales of a Gossen meter, the EV scale is in red. It also shows a type of 'match needle' system where an angled line is moved to coincide with the needle and a ruled horizontal line.

The APEX system of around 1960 was similar but more comprehensive and used additive terms.²⁶

Taking the Reading

Integration

The most common way of taking a reading was to aim the meter at the subject the reading then indicates the light reflected from the subject.

Incident Method

The reading is taken from the position of the subject with the meter directed towards the light source or the camera. The theory of this is that a reading should be taken of the light reflected from an area of standard tint rather than the subject itself. The reading can then be used to fix that tint to the appropriate point on the characteristic curve. Since the reflective properties of the standard tint are known this would be the same as measuring the light falling on the tint. Incident meters employ a diffuser in front of the cell such as opal having a known transmission value (which substitutes for the standard tint), it is from the back of this diffuser that the reading is taken. The diffuser is placed at the surface of the meter or even in front of the meter to collect light from the whole of the surroundings, this is in contrast to the integration method where the reading is restricted to a narrow angle. Incident meters gave a high-light reading and were most applicable in colour transparency and cine work. The first incident meter was the Avo Smethurst (1937), from then photoelectric meters were usually supplied with an incident light attachment.²⁷

Hight-Light Method

The exposure is anchored at the upper end of the characteristic curve.

Shadow Method

The exposure is anchored at the lower end of the characteristic curve.

What is Being Measured?

Some terms and definitions.

Light Source

the candle was long accepted as the standard light source, over time the definition of the make up of the candle changed and other light sources substituted whilst maintaining the same value.

Luminous Intensity

is a measure of the power produced by the light source. It was measured in candle-power - the light produced by a standard candle. Candela is the SI equivalent though numerically slightly different.

Luminous Flux

is a measure of the Luminous Intensity for the radiation angle of the source (the cone of light emitted from the source). It is measured in lumens, 1 lumen is the luminous intensity of a 1 candela source over a solid angle of 1 steradian. If the light source emits uniformly in all directions it would radiate 4π lumens. 1 candle-power = 12.56 (i.e. 4π ) lumen.

Illumination

refers to the amount of light falling on a surface (the subject). It is affected by the Luminous Intensity of the source, distance of the surface from the source and, in practical terms, by the medium through which the light travels (e.g. atmosphere). It is measured in lux, foot-candles or candle-metres. 1 lux is 1 lumen per square metre. Sometimes given as the illumination on a surface at one metre from a light source of one candle-power. 1 foot-candle = 10.764 lux. The light meter shown in fig. E14 is for industrial use rather than photographic, it is scaled in foot-candles and lux.

Brightness or Luminance

refers to the light reflected from a surface to the camera. It is affected by the type of surface and the angle the surface makes to the camera. It was measured in foot-lambert, a surface that emits or reflects 1 lumen per square foot has a luminance of 1 foot-lambert.

One way of bringing this together is to imagine a sphere of 1 metre radius centred on a light source of 1 candela. By definition a sphere subtends at its centre 4π steradians. It has a surface area (4πr²) or 4π in this case. The Luminous Flux inside the sphere will be 4π lumens, so the luminous flux within 1 steradian will be 1 lumen. The Illumination of the surface of the sphere will be 4π lux, so the illumination of a surface bordered by 1 steradian will be 1 lux.

Exposure

the exposure received by the sensitive surface is given by E = I×t where I is the illumination and t is time. This is sometimes expressed in C.M.S. - Candle-Metre-Second or lux-second. Illumination of one lux or one candle-metre falling on the sensitive surface for one second.

Sensitometry

Sensitometry is the study of how sensitive materials respond to light; their speed, contrast characteristics and tonal reproduction. The first scientific study was carried out by Hurter and Driffield resulting in the publication of their findings in1890. From their work came the H & D speed ratings, the characteristic curve and the analysis of how the tonal range of the subject is reproduced in the negative and print.²⁸

The sensitive material is given a controlled series of exposures of constant ratio and developed in a controlled manner. The densities of the differently exposed areas are then measured and plotted on a graph. The most common presentation is where the density is plotted against the log of the exposure, which gives the S or characteristic curve.

Sensitometers and Densitometers

The instrument used to provide the series of exposures was a Sensitometer, the term Densitometer came to be used for the instrument that was used to read the density though the more general term of photometer was often used. Sensitometry was mostly the province of the plate and material manufacturers who used specially built equipment, but simplified sensitometers or 'plate testers' were sold for use by photographers.

In the early 1880s when plates were often still made by the photographer the Warnerke Sensitometer provided an easy way to find the speed rating of a plate. Later, when plates were commercially produced, photographers would use 'plate testers' such as the Chapman Jones, for testing different processes or equipment and to verify the speed rating given by the manufacturer. Densitometers were in use by photographers largely to determine the density and contrast of plates and so the correct exposure and paper grades to use when making a print. An early sensitometer was by Mucklow and Spurge, it consisted of a number of tubes having a varying number of holes at their top to admit light, below the tubes was the photographic plate to be assessed.²⁹

Exposures in a sensitometer were made either for a fixed time and varying illumination (Intensity scale instruments) or with fixed illumination and varying time (Time scale instruments). Where the time varied it was either continuous or intermittent. Densitometers can be categorised as Transmission Densitometers for plates and films or Reflecting Densitometer for paper.

Density Wedges have several uses in sensitometry and provide an easy way to test the sensitivity or other characteristics of plates. There are various forms, one commonly used was a wedge of dark glass, in another an opaque substance such as carbon is held between two glass plates set at a narrow angle.

Hurter and Driffield

H & D's original work was made using a standard candle as the light source, this was soon replaced by other sources that were calibrated to a candle intensity. The different light sources, though, had different spectrum emissions and none conformed to daylight. This was of not much consequence with 'ordinary' plates that were sensitive to only the blue end of the spectrum. Problems arose with the introduction of orthochromatic and panchromatic plates that were sensitive to a wider spectrum. Plate manufacturers would sometimes quote two speeds for plates one for daylight the other for half-watt light, which would be higher than the daylight figure.³⁰ It was not until the 1928 International Congress of Photography that standards for the light source and blue filters to give a colour equivalent to daylight were agreed upon.

In describing their work Hurter and Driffield defined the following terms, which have remained in use:

• Transparency - The ratio of light transmitted by a material compared to the incident light. T = It/Ii.
• Opacity - The inverse of T, so O = Ii/It.
• Density - D = log O = log (Ii/It). This gives values for D ranging from 0, transparent, to around 3.³¹

S curve

The familiar S curve is obtained by plotting Density against the log of the Exposure. The key parts of the graph are:

• Toe - portion before the straight line section where the rate of increase in density is rising compared to increases in log E.
• Straight Line Portion - portion where the density increases proportionately to increases of log E.
• Shoulder - portion after the straight line section where the rate of increase in density falls compared to increases in log E.
• Gamma - the rate at which the density increases compared to log E, which is the tangent of the angle the straight line portion makes with the log E axis. In practice this gives a measure of the contrast of the material.
• Fog - the portion parallel to the log E axis where the density is the result of development of unexposed silver.
• Threshold - the first increase in density over the fog level.
• Inertia - the value of log E from the extrapolation of the straight line portion to the fog level. This point will be further to the left the higher the speed of the plate. For the same plate the inertia point was, at the time, largely stable for different development methods.
• Average Gradient - the angle made between a line drawn between the ends of the subject tonal range where they cross the curve and the log E axis. Since it generally includes part of the toe this is a better measure of contrast for modern material.

The tonal range of the subject will occupy only part of the curve, in Hurter and Driffield's view the tonal range should occupy only the straight line portion of the curve. In part this was to match the sensitive printing paper (P.O.P) in use at the time which had a very short toe. The importance of the toe region in rendering shadows came to be understood with the introduction of bromide paper which has a longer toe region. From then the subject tonal range came to occupy part of the toe region. If the tonal range occupies too much of the toe it is under exposed, the negative density range will be compressed and shadow detail will be lost. If it reaches into the shoulder region it is over exposed, the negative density range will again be compressed and highlight detail will be lost.^{32 33 34}

Plate and Film Speed Calibrations

Early suggestions for determining the speed of a plate, for example Claudet's Photographometer (1848), compared the relative speed of plates without providing a numeric rating or giving standardised conditions that would allow comparisons at different times.

Early dry plate

The speed was often given as being a factor of the speed of a wet-plate, Paget, for instance supplied plates labelled XXX indicating they were 30 times the speed of a wet-plate. Other manufacturers labelled their plates simply as 'Ordinary', 'Rapid' or 'Extra Rapid'.

Warnerke

This was the first generally used method that gave a speed rating to sensitive plates. It was devised by Warnerke in 1880 and used his Standard Sensitometer. It gave the threshold value of the plate as an index between 10 and 25 where 25 was the fastest and the average wet-plate had a speed of 10. Each number indicated a ⅓ increase in speed over the previous. The system remained in use for several years and was used by manufacturers such as Marion and Elliott when labelling their plates. The sensitometer was easy to use and often used by the photographer to find the speed of the plates he was making. The standard light source was a phosphorescent tablet excited by light from burning magnesium ribbon for a set time.^{35 36}

H & D

The Hurter and Driffield rating was introduced in 1890 and used commercially from 1892.³⁷ The speed is determined by the inertia point. With modifications the system remained in use until the mid 1940s, one early change was the replacement of the candle as the standard light source. The H & D speed is calculated as 34/inertia value (later 10/inertia value).³⁸ The nature of sensitive plates and papers continued to develop from the time of Hurter and Driffield's original work, in particular the toe region of the characteristic curve came to be used, this meant that the H & D speed rating was often less than it was in practice. H & D is an arithmetic scale, the values are proportional to the speed of the plate i.e. if the plate speed is doubled the speed number would double.

Watkins

Introduced in 1890. A Watkins value of 1 was for a plate that required 2 seconds exposure with full summer sun at f8.^{39 40}

Wynne

The Wynne speed was used on meters from the Infallible Meter Co. and is sometimes marked on boxes of plates. The speed is given as, for example, F56. It is defined as the size of the diaphragm through which the plate would require the 'Actinometer' time for exposing a normal subject. This system allows the plate speed and diaphragm to share the same scale on Wynne meters.

Scheiner

Introduced in 1899 (details published in 1894). The speed is determined by the threshold point. It has a logarithmic scale such that a doubling of the plate speed is shown by an increase of 3° Scheiner.⁴¹

DIN

Introduced in 1932. The speed is determined by the fixed density of 0.1 above the fog level. It has a logarithmic scale such that a doubling of the film speed is shown by an increase of 3 DIN. Originally written as a fraction e.g. 19/10°.⁴²
Modified in 1957 when the method of development was changed and the fractional form was discontinued.
Modified around 1961 to bring it in line with BS and ASA.

BS

Introduced in 1941. The speed is determined by the fixed density of 0.1 above the fog level. It has a logarithmic scale such that a doubling of the film speed is shown by an increase of 3 BS.
Modified in 1947. The speed is determined by the Fractional Gradient. Both logarithmic and arithmetic scales were used.^{43 44}
Modified in 1962. The speed is determined by the fixed density of 0.1 above the fog level. Arithmetic scale only.
The BS log value is also written as BSI or BS index, the arithmetic value is sometimes written as BS Arith.

ASA

Introduced in 1943. The speed is determined by the Fractional Gradient. Arithmetic scale.
Modified in 1947. Both logarithmic and arithmetic scales defined.
Modified in 1960. The speed is determined by the fixed density of 0.1 above the fog level. Arithmetic scale.⁴⁵

Weston (Old)

Introduced around 1933. Replaced by the new Weston ratings in 1957 which corresponded to ASA values.

Plate and Film Speed Comparisons

Obviously accurate comparisons between systems cannot be made as speed ratings have a different basis i.e. threshold, fixed density above fog, Fractional Gradient. The H & D comparisons to later systems (DIN, ASA etc.) are particularly flexible. Bellow are some very approximate comparisons.⁴⁶

Watkins	Wynne	Wellcome	H & D	Smith	Warnerke
2	9	16
3	11	12
4	13	8
6	16	6
8	18	4
12	22	3
15	25	2	10	6	15
20	29	1 1/2	14	7
30	35	1	20	8	18
45	43	2/3	30	9
60	50	1/2	40	10	21
90	60	1/3	61	11
120	70	1/4	82	12	24
180	86	1/6	122	13
240	99	1/8	163	14	27
360	121	1/12	245	15
480	140	1/16	326	16
720	172	1/24	490	17

H & D	Scheiner	ASA	DIN	Weston	BS log	GE	American	Ilford	European	Compass	Smethurst
				Old			Scheiner		H & D		High-Light
15							4
24							6
40							8	A		+6	1
64		4	7				10
170	19	5	8				14
220	20	6	9		19	8	15	B		+4	2
270	21	8	10		20		16		750
350	22	10	11		21	12	17
450	23	12	12	8	22		18	C		+2
570	24	16	13	12	23		19		1500		5
720	25	20	14	16	24		20
	26	25	15	20	25	32	21	D		0
1000	27	32	16	24	26				3000		10
	28	40	17	32	27		22
	29	50	18	40	28	64	24	E		-2
2000	30	64	19	50	29	80	25		6000		20
	31	80	20	64	30		26
	32	100	21	80	31	125	27	F		-4
4000	33	125	22	100	32	160	28		12000		40
	34	160	23	125	33	200	29
	35	200	24	160	34		30	G		-6
8000	36	250	25	200	35	320	31		24000
	37	320	26	250	36		32
13000	38	400	27	320	37	500	33	H	40000
		500	28	400	38
		650	29	500	39
		800	30	650	40	1000

In the above table every 3rd line is a change in relative speed equivalent to 1 stop.

The relationship between H & D, Watkins and Wynne is fairly well fixed, conversion is:^{47 48}

• Watkins = 50/i where i is inertia so Watkins = H & D × 50 / 34.
• Wynne = √Watkins × 6.4. Prior to 1901 the conversion was √Watkins × 8, at that time the sensitive paper of the Wynne was changed.

European H & D was 3 times H & D.

References & Notes