I recently stumbled upon this ‘Wellcome’ Photographic Exposure Calculator in the archives which is both a photography handbook and a diary for 1939 and belonged to Paul Graney. For someone using a camera without an inbuilt exposure meter (something we take for granted today) or a hand-held light meter (a gadget for reading light for use with cameras) this must have been an essential tool for calculating camera shutter speeds and aperture values in the 1930-40s. I was really intrigued to understand how it worked as it had obviously been very well used.
The handbook has various tables and information to guide the photographer, from taking a photograph to developing and printing it. The printed calculator wheel at the back of the handbook (see above) is a tool to help a photographer set up a camera. The wheel turns to align film information with subject and light information (‘light value’) to find the settings required to take a photograph that is ‘correctly exposed’ – that means that the photograph will not be too dark, nor too light. The two camera settings that combine to create a correctly exposed image are aperture and shutter speed. The aperture is a hole within the lens through which light passes to reach the film. The size of this aperture is adjustable by the photographer to let more or less light in. The shutter speed refers to the length of time that light passes through the aperture to the film. It works a bit like a curtain and the time is measured in fractions of seconds and seconds (and sometimes, minutes). On a manually operated camera a photographer can select the shutter speed and aperture before taking each photograph.
Here’s an example of how we can use this handbook and exposure calculator to work out what aperture and shutter speed we need for a particular subject:
Let’s say the month is July and we want to take a photograph of Manchester Town Hall at 12.30pm. In the handbook there are light tables for each month for locations that fall at latitude 52 degrees north (it’s close enough for us in Manchester at 53°28′N) and so if we use the light table for July, during British Summer Time, and after assessing the weather on the day we decide that the “sun is shining unobscured”, we can determine that the light value we need to take from monthly table is 1/8 (see below).
Next we need to consider the ‘speed’ of the film to be used. The film speed refers to how quickly light burns an image into the film. Today this is standardised and known as the ‘ISO’ but this guide predates that time and lists the different films that were available and the required speed factor for each. If we were making this photograph in the 1940s then we may have chosen to use Ilford’s ‘Fine-grain Com.Pan.’ film which could be a good choice for photographing a building due to its sharpness and high level of detail. The table indicates that for this particular film we should use an exposure factor of 1/16.
So we now have two values to apply to the ‘Wellcome’ Exposure Calculator. These will enable us to find the range of combined apertures and shutter speeds we could use to photograph the Town Hall (using the Ilford film we selected, on a sunny day in July). We need to move the wheel to line up the July monthly table light value of 1/8 (in black on the outside of the circle) to the Ilford film factor of 1/16 (in green).
Revealed on the other side of the wheel are the shutter speeds which are shown in red and represent fractions of a second. These are aligned to aperture values which are shown in black with f in front of them (which is the symbol for aperture). To make it easier to read here is a summary of the aperture and shutter speed combinations, shown above, that are suitable for taking the photograph of the Town Hall on a sunny July day:
So how come all of these combinations will give a correct exposure and how do we decide which to use?
Firstly all of these combinations of aperture and shutter speed equal the same amount of light exposure. That is because as the aperture is made smaller, reducing the amount of light that shines onto the film, the duration of the light exposure is lengthened to enable the film to still receive the same amount of light in total. So all combinations would produce a correctly exposed image.
To decide which to use we must first look at the camera lens as if the lens has an aperture range of say f 4.5 – f 22 then we can only select from those apertures. Next we must consider whether we have a tripod available to take this photograph or whether we will be hand holding the camera. This is important because at slow shutter speeds the hands are unable to hold the camera completely still for the duration of the exposure and this would result in a less than sharp image (the duration at which a tripod is needed to steady the camera depends upon a range of factors, including the size of the camera and lens). If the subject itself was moving (not likely with the Town Hall!) then we would need to choose a shutter speed fast enough to freeze the action, unless for creative purposes, we wanted to show a blur of movement.
Finally we need to consider the ‘depth of field’ we require in our photograph and this is directly connected to the aperture value that we choose. The smaller the aperture the greater the depth of field and this means that more of the image, from front to back, will be in sharp focus. It often confuses people that f 32 is a much smaller aperture than f 4 but this is because the aperture number is actually representing a fraction e.g. f 32 is actually 1/32 which is therefore smaller than f 4 which equals ¼.
So after following this guide we can now set up our camera and if we didn’t bring our tripod then we could choose f 8 at 1/150 of a second, which should give is a very good, correctly exposed image. If we were able to use a tripod then f 22 at 1/15 of a second would produce a slightly sharper and more detailed image as a result of the smaller aperture.
There are many more variables covered in this handbook (for example other subjects need further adjustments) but hopefully this basic explanation can help improve the understanding of photographic exposure and also show how Paul Graney, and other photographers of this time, were able to use cameras that had no automatic settings. To follow are a selection of photographs from Graney’s archive, some of which he would have consulted the handbook for guidance. He documented many themes including: sports & leisure, industrial scenes, parades and folk festivals, children, cities and towns and ‘places from the past’.
Find out more about Paul Graney and his unique sound archive here.
To follow are a selection of other pages from the handbook that you may find interesting or see our flickr site for even more: