The Exposure Triangle and its history
explained as simply as I can
See also F8 and be there
The Exposure triangle and Sunny 16
The exposure triangle is a simple way to calculate photographic exposures and we can see how adjusting one element affects the other two. But it wasn't always like that. Well over a century of photography had passed before it finally became a reality. What is sunny 16? It's a simple, manual way to calculate exposures without using a meter. Back in the day, film boxes usually came with exposure recommendations based on the sunny 16 'rule'. So how does it work? The shutter speed value is linked to the film speed value (or as near to it as possible) so a 100 film speed requires a 1/125th shutter speed, 200 needs 1/250, 50 needs 1/60 and so on. Then the apertures work like this: Talking of film speeds, when the exposure triangle was formed, film speeds were in ASA (American Standards Association) this was adopted formally as an international standard (ISO) some time in the late 90s. The values ASA and ISO are interchangeable. Due to the nature of digital sensors and how they work, ISO in a digital camera is a close but only approximate equivalent of film ISO. The latitude of modern films compensates for any discrepancies or 'in-between' values. Naturally, as the light fades as we get towards the evening and in the early morning so an adjustment of one stop helps. Side note. The sunny 16 doesn't work too well where I live (53 north) so I use Sunny 11 replacing the values of 16, 11, 8, 5.6, 4 with 11, 8, 5.6, 4, 2.8. So who invented the exposure triangle? Well it wasn't really invented as such, as each element; Film speeds, lens apertures and shutter speeds all evolved over time until they were tweaked to become what we know now. How does the exposure triangle work? Each corner of the triangle has a value that halves or doubles. Halving one element doubles another, see the example below. So in one corner we have film speed F, this is the 'driver' of the exposure triangle and the fixed value, then S for shutter and A for aperture which are variables. The sunny 16 rule is based on this, the F and S are linked and the A is changed according to the conditions. |
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But what happens if you want a faster shutter speed? Well that's where the exposure triangle comes in. in this example the film speed is 100:
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But you want a faster shutter speed, say 1/500, that is two stops up (125, 250, 500) so the aperture must be adjusted by two stops down from f16 to f8. In this case the triangle would look like this:
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Remember in film, the film speed remains constant, digital allows us to change the ISO, if this is changed by a whole stop the others may be changed too, so in this instance you wanted a fast shutter speed but retain the aperture of f16, that's a four stop difference, two on the shutter speed, two on the aperture, then change the ISO from 100 to 800
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Right, back to film where the film speed remains constant.
EVs or Exposure values. In the late 1940's, exposure values, EVs, were created and many cameras with light meters that read out in EVs. The user transferred the readings to the camera which had the shutter and apertures locked together so changing one automatically changed the other removing the need for recalculation. Some people find it annoying but for me it works. EVs also had numerical values, sunny 16 in EV values would look like this 1/125th (based on 100 ISO/ASA film speed) |
whereas with 400 film it would look like this
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Each increase in EV value is equal to an aperture or shutter speed stop. In my mind I use the base of EV10 to calculate it (EV10 is 100ASA - 1/60th – f4).
Let's go back to each corner of the exposure triangle: Film Speeds Film speeds weren't always nice and regular like they are now. Flexible film was introduced in the late 1880s before that photographers used metals, glass and paper as a base for their emulsion, which is the sensitive part, so I'll refer to it as emulsions here. In the early days, photographers had to make their own emulsions and exposures were calculated by guesswork and experience. Emulsions had to be used wet and as it was drying, it resulted in longer and longer exposure times. Then dry plates were born, these could be prepared in advance using the same batch of emulsion, this allowed for a certain consistency. Soon, chemists and enterprising photographers were making plates to sell to other photographers, eliminating the need to prepare their own. The plates were consistent in speed and photographers would buy several packets with the same batch number. Processing was done visually under red light or 'safelight' as it was called, as the emulsions were only blue sensitive. It was discovered that by adjusting the chemistry of the emulsion, that faster or slower plates could be made, the faster emulsions allowing the photographer to take 'snap-shots' at fraction of a second speeds. Emulsions were applied to paper which could be rolled up and used in special cameras that didn't need a plate changing after each photograph. This revolutionised photography and soon the 'man in the street' could take snapshots of his daily life. Paper gave way to film and photography as we know it now was born. And for those who liked their big cameras, glass plates gave way to sheet films which was much lighter and portable and is still popular today. Film (for that's what we will call, it now) was sold in a variety of emulsions which were often described as fast, regular or slow. Development was still performed under safelight and films were not only blue sensitive but green sensitive too which increased speed. It was only a matter of time when panchromatic film (sensitive to all visible wavelengths) was created requiring processing in the dark. The photographer now needed absolute consistency as he couldn't develop films by eye. (He because it was usually men.) So, manufacturers created a speed rating for their films and introduced complicated charts for exposure and processing. These worked well but if the photographer wanted to try a different brand of film, he would have to learn a new set of rules. Speed conversion tables were available but not all film speeds were calculated the same so they were only approximate. I'm not going into scientific detail here, just the basics, if you like numbers, go to https://en.wikipedia.org/wiki/Film_speed In 1890 scientists Ferdinand Hurter and Vero C Driffield devised an exposure estimation device which they called and actinograph. This was a wooden slide rule that worked like sunny 16 but took into account the month and time of day. But it only worked with Marion & Co plates. This was a start, though, and soon Hurter and Driffield's speed rating was taken up by other manufacturers. Other manufacturers used a table like H&D's made specifically for their own brand of films. |
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Meanwhile Julius Scheiner created his own rating system and some film manufacturers took that up. Both Scheiner and H&D changed their systems as newer, faster films were introduced. In Germany the DIN system was created, inspired by Scheiner's more logical system it was easy to use.
Other systems of measurement were created too and exposure tables became complicated. William Goodwin, working for Weston, in the US created a system which they used on the Weston photoelectric exposure meters. Film companies and other meter makers adopted this system which was very similar to the modern ISO with each number doubling the previous. This was used right into the 1950s. In the USSR this was adopted as the standard GOST. In the USA, Kodak's scientists were experimenting with panchromatic film speeds too. Already the largest film manufacturer in the world and inventor of many different size 'cartridge' (roll) films, they wanted to create a world standard film speed system. The resulting system was adopted by the American Standards Association in 1943 and tweaked until when in 1954 it was finalised. This linear system, similar to Weston's, where each doubling of numbers was one stop of speed became the first corner of the exposure triangle. The recalculation of ASA and doubling of speed in 1960 didn't affect the triangle. The German DIN system was used throughout the 50s to the 90s, particularly by makers of film in Europe and was usually displayed alongside the ASA. The DIN system didn't readily work with the exposure triangle and it was quietly dropped when ASA became ISO. 
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Now, the Apertures.
The word 'aperture' means opening, in modern cameras it is controlled by a mechanism called the iris diaphragm, a complicated mechanism comprising thin metal (or plastic) leaves that close down creating smaller, roughly circular, apertures by means of a ring round the lens or a mechanism built into the camera. The effect of this controls the amount of light entering the camera through the lens. The iris diaphragm is also referred to, by photographers, as the aperture, which is the term we shall use here. Early camera lenses didn't have apertures as the photographer needed as much light as possible, but with faster emulsions being made, he needed some control so small circles of black painted metal were inserted into the lens, this required unscrewing the lens, inserting the disc and screwing the lens back together. John Waterhouse devised a system whereby little slots were cut into the lens barrel and black squares of wood or metal with a hole inserted. This effectively reduced the light and a number of them were sold in sets of, typically, 3 or 4. The stops were known as Waterhouse stops. There were no specific values put on these, rather just arbitrary numbers, as these stops may be used with different focal lengths any specific values would be useless when used on a lens of a different focal length. Some lens makers created a disc that fitted in front of the lens with a selection of different size holes that could be selected. Eventually the iris diaphragm, which had been invented years before for microscopy, started being built into lenses making the whole process easier and infinitely variable. The aperture value could be calculated by the following formula: 
Where f is the focal length, and D is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by "f/", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N.
The focal length of the lens divided by the diameter of the aperture equals the f number. So 300mm lens divided by a 40mm aperture equals f7.5. (300/40=7.5) For an exact calculation, the aperture should be placed in the centre of the lens but measuring the rear of the lens would give a near approximation. These were known as f-numbers. With regular film speeds these diaphragms needed some sort of numbering system so many manufacturers used their own. These were the prototypes of the standard f-stop. Kodak used a simple system whereby each number doubled as the aperture doubled: 1, 2, 4, 8, 16 and up. There were at least half a dozen standards using different numbers. But as they were all just different expressions of f-numbers, tables to convert them were accurate. As time went on the different manufacturers started using actual f-numbers. But even then, they used different sets. |
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An f-number doubled became the next whole stop down. Halved, it became the next whole stop up. With graduations of a third or a half stop in between. With each graduation allowing more or less light than the previous one. You can see that each graduation is only half, doubling only every second stop. But doubling or halving lets four times as much/less light through according to physics.
The stops we are used to now are half stops: f1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. But some lenses lenses used in-between stops instead so it’s possible to see on old lenses the numbers in third stops: f1.2, 1.8, 2.5, 3.5, 5, 4.1, 10, 14, 20, 29 or in quarter stops: 1.1, 1.5, 2.2, 3.1, 4.4, 6.2, 8.7, 12, 17, 25. Confusing, but importantly each number was double the preceding number. Light meters became complicated and used dozens of these values arranged in order on the calculator. After WW2, makers started bringing aperture numbers in line with the ASA film standard, finally settling on the sequence f1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 When using lenses with the older style markings, use the nearest modern number. For example f12 use f11 f9 use f8 and so on. The wide latitude of today's films compared to those a century ago is extremely wide and forgiving. The numerical value of an aperture is calculated by the formula I gave above, but in reality different things can affect the amount of light being transmitted through the lens: the number of lens elements, lens coating. In 1938, C.J. Bentley reported that when photographing a circus with his camera lenses at full aperture his 35mm f2 coated lens was 'brighter' than his uncoated 50mm f1.5, clearly indicating the benefit of lens coating. However there is another method of calculating a lens's light transmission: t-stops. They are similar to f-stops in that the numbering system is the same but each lens has been tested and the true light transmission calculated. t-stops are mainly used in the movie or video industry where manual exposures are made and need to be consistent from shot to shot if the lenses are changed. With t-stops each lens transmits exactly the same amount of light as another at the same aperture. Different as they are, t-stops work with the exposure triangle and are more accurate than conventional f-stops. Side two of the exposure triangle was complete. Shutter speeds. The first photographs had exposures of hours while the photographer waited for his silver coated plate to darken, but when it was discovered that a latent image could be developed, exposure times dropped dramatically to minutes. Exposures were controlled by taking off the lens cap, counting, then replacing it. As emulsion speeds grew faster so too was the need for an accurate method of exposing the plate. Thus the shutter was introduced. There were dozens of shutters made for cameras, most of them fitted on the front of the lens and used a pneumatic bulb to open and close them. Some were simple 'drop plates' a perforated sheet of wood or metal above the lens, that when dropped exposed the plate briefly on its way down. Mechanical, clockwork or pneumatic shutters were faster and more accurate. The speed of these were arbitrary often just being marked slow, medium, fast and rapid. There was no connection between shutter speed, aperture and emulsion speed then so the photographer relied on charts printed in the Amateur Photographer or British Photographic Society magazines or the American Amateur Photographer in the states. Developing was done by eye as I mentioned. German manufacturer Voigtlander created the leaf shutter which was quiet, compact and accurate. Other makers improved on the design and the leaf shutter that dominated photography for over 50 years was born. The range of speeds was low and often there was no connection between them, no convenient one-stop differences. Manufacturers like Kodak made simple spring 'guillotine' shutters, often marked I for instantaneous (between approximately 1/15th to 1/25th of a second) with more ‘advanced’ shutters being capable of 1/25th, 1/50th and 1/100th with some degree of accuracy. B and T on shutters refer to Bulb and Time. On B the shutter stays open as long as the button is pressed, closing when it was let go. Typically it was operated by a pneumatic rubber bulb - hence the name B, that we use today. Or BulB as it says on digital cameras. T opened on the first press and closed on the second press. Early shutters tended to have speeds in seemingly random values. With the speeds being close together: 1, 1/2, 1/5, 1/10, 1/16, 1/20, 1/25, 1/30, 1/40, 1/75, 1/100, 1/200, 1/300 These were accurate but complicated exposure tables had to be made with all the different speeds. Ernst Leitz, creator of the Leica camera made his first prototype to calculate the speed of each batch of 35mm movie film. He used lenses with a system of f-numbers and the shutter speeds on the camera matched those of the movie cameras being used namely 1/20, 1/40 and 1/60. These speeds (and more) made their way into production Leicas in the 20s. The reason for shutter speeds to be close together was because of the lack of tolerance film had in those days, half a stop could make a noticeable difference to the brightness of the image. As film stocks became more sensitive, reliable and with a wider latitude, so shutter speeds became more separated, with a doubling-up being normal in the 30s onwards, resulting in this standard sequence: 1, 1/2, 1/5, 1/10, 1/25, 1/50, 1/100, 1/200, 1/300 This carried on into the 1950s, but it didn't quite fit in with the standardised film speeds and apertures and so the manufacturers changed them to the more familiar system of: 1, 1/2, 1/4, 1/8, 1/16, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000th of a second By 1960 shutters on almost all types of cameras had a selection of speeds within this range. Thus the exposure triangle was born. Some say technically its an exposure quadrangle as the light brightness has an effect but that is a constant and can't be changed at all. Of course it affects everything else. But light is infinitely variable, not in nice even steps like the exposure triangle. |
Aperture and film speed converter and Actinograph images from Wikimedia commons
