- "On a sunny day set aperture to f16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight."
A better system was the extinction meter. This used a piece of translucent celluloid that was painted black apart from a series of grey numbers. Each number was printed in a darker shade of grey than the last number. So, in my Braun Paxette, the number 1 is nearly white and the number 16 is nearly black. In use, the user looks through the extinction meter and notes the darkest number that is visible. This number is then looked up in an exposure table to get suitable aperture and shutter speeds. This picture of my Braun Paxette show one of these tables. It is set up for 50 ASA (ISO) film and tells you to halve the exposure for 100 ASA film and double it for 12 ASA film (films were much slower then than we are used to now). This chart refers to 'diaphragm' rather than 'aperture' but it is the same thing.
|Extinction meter table on my Braun Paxette|
The next development was the electronic light meter. Initially, these had a cell made from selenium that produced a small electrical voltage on exposure to light. This voltage was then used to move a needle across a scale. This needle would then point to an arbitrary number that has to be set on a scale. The scale then indicates a range of appropriate aperture and shutter speeds. It is tempting to think of these as EV numbers but they are not - at least are not on the three light meters I own. These worked very well in reasonable light but were poor performers in poor light. These fell out of fashion and were replaced by CdS meters. The advantage of selenium meters is that they do not need a battery to work. A big disadvantage is that they lose sensitivity with time. Towards the end of their useful life they give a low reading which will eventually cause over-exposed photographs. For this reason, some people will not use old selenium meters but my old (fifty years old, plus) meters all agree with my modern light meters.
I suspect that the rate of deterioration depends on how the meter has been stored over the years. If the meter is in the dark inside a case apart from when actually taking a reading, the deterioration doesn't seem to matter over a period of sixty or so years.
The next development was the CdS meter (Cadmium Sulphide). These do not produce a voltage on exposure to light but act as a resistor that changes its resistance to electricity on exposure to light. These always need a battery to work to provide the voltage. Most modern light meters work this way. These work in much lower light levels and do not significantly deteriorate with time. The draw back with these is that battery technology changes and it can be hard (or impossible) to find batteries for older CdS meters. In particular, mercury batteries are no longer made and the modern equivalents produce a different voltage which alters the accuracy of the meters.
A lot of meters from the 1950s used what are called Exposure Values (EV). The idea is that you set your shutter to the indicated EV and this sets a combination of shutter speed and aperture. As you then alter the aperture, the shutter speed will alter in unison - and vice versa. I like the system but many people do not.
Here is a table of EVs and their associated aperture/shutter speeds:
|Table 1. Exposure times, in seconds or minutes (m), for various exposure values and f-numbers|
|−6||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m|
|−5||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m|
|−4||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m|
|−3||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m|
|−2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m|
|−1||2||4||8||15||30||60||2 m||4 m||8 m||16 m|
|0||1||2||4||8||15||30||60||2 m||4 m||8 m|
|1||1/2||1||2||4||8||15||30||60||2 m||4 m|
First, the Weston.
This is a Weston Master III
|Weston Master III|
Secondly, Leningrad 4:
Thirdly, Zeiss Ikon Ikophot:
|Zeiss Ikon Ikophot|
Not cheap - it cost £10/13/5 in 1957 - which was just over an average man's weekly wage (so about £500 in 2013 values).
This is the meter I most often use as it is nicely made, feels good in the hand and produces satisfactory results. A scanned copy of the Ikophot manual is available for download as is a scanned copy of Zeiss Ikon's 1930s exposure guide.
Lastly, my Bertram Bewi Automat.
|Bertram Bewi Automat|
The meter is rather larger than is usual for analogue meter - it measures 90 x 65 x 25 mm not including the activating button. It is encased in ivory plastic.
The only control is a ring to set the film speed. This is calibrated in ranges. For instance, the DIN setting is a range of three numbers - 11-13, 14-16, 17-19, 20-22, 23-25. That is one stop difference between each range. Given the exposure latitude of film that is plenty accurate enough. There are also ranges for ASA and Weston (ASA is the same as ISO and Weston is a defunct film speed system devised by the makers of the Weston series of exposure meters.
Using the meter is also different to analogue meters. If you point this meter at the scene you intend to photograph, nothing happens. You need to point the meter, depress the activating button for about three seconds and release. Once you have done this, the shutter speed scale lines itself up with the aperture scale and you can read off suitable combinations of aperture and shutter speed.
The read out also has exposure values indicated - under L on the aperture scale. These are adjusted for film speed and can be set directly on an EV enabled shutter.