Light (or exposure) meters

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Light meters (which are also called exposure meters) are a modern invention and early photographers had to guess their exposures and rely on experience to get it right.  An early system used to make guessing easier was to use the Sunny 16 rule which says:
  • “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.” 

 (from Bernhard J. Suess (2003). Mastering Black-and-White Photography, Allworth Communications. ISBN 1-58115-306-6)  So, if you are using Ilford FP4 film, which is ISO 125, on a sunny day you set the aperture to f16 and the shutter speed to 1/125 seconds.  This will generally give you a usable picture even if not a perfectly exposed one.  You do need to be aware that the amount of light on a sunny beach will be more than the amount of light in a sunny field – entirely due to the amount of reflected light.  You can easily adjust either the aperture or the shutter speed to compensate.  You also need to be aware that the amount of light present on a sunny day depends on how close to the equator you are.  A photographer in Norway using the Sunny 16 rule will get a very different result than a photographer in Nigeria would.  I have been trying this Sunny 16 rule out in Lincoln this week and in the middle of the day it produced the same exposure as my Ikophot exposure meter suggested.  By four in the afternoon, the Sunny 16 rule was a whole stop out.

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
 
These, although simple and not able to go wrong, had their drawbacks.  They depended on the quality of the eye sight of the user and that is very variable.


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:
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Table 1. Exposure times, in seconds or minutes (m), for various exposure values and f-numbers
EV f-number
1 1.4 2 2.8 4 5.6 8 11 16 22
−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
2 1/4 1/2 1 2 4 8 15 30 60 2 m
3 1/8 1/4 1/2 1 2 4 8 15 30 60
4 1/15 1/8 1/4 1/2 1 2 4 8 15 30
5 1/30 1/15 1/8 1/4 1/2 1 2 4 8 15
6 1/60 1/30 1/15 1/8 1/4 1/2 1 2 4 8
7 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 2 4
8 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 2
9 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1
10 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2
11 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4
12 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8
13 1/8000 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15
14   1/8000 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30
15     1/8000 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60
16       1/8000 1/4000 1/2000 1/1000 1/500 1/250 1/125
17         1/8000 1/4000 1/2000 1/1000 1/500 1/250
18           1/8000 1/4000 1/2000 1/1000 1/500
19             1/8000 1/4000 1/2000 1/1000
20               1/8000 1/4000 1/2000
21                 1/8000 1/4000
EV 1 1.4 2 2.8 4 5.6 8 11 16 22
f-number
To my light meters:
I have four old meters, each of the selenium type.  They are a Weston Master III, a Leningrad 4, a Ikophot and a Bewi Automat.  The Weston does not work – I tried to adjust the zero setting and managed to wreck the meter.  The blame for this lies entirely with me, not Weston’s design or manufacturing standards.

First, the Weston.

This is a Weston Master III
 
Weston Master III
It is the most complicated meter of the three I have – presumably it will do more than the other two.  The draw backs for me is firstly there is no ASA or DIN setting, it uses instead Weston values which are their own proprietary system. As films do not come with a Weston speed marked on them, it makes it unnecessarily difficult to use.  Secondly, I find the plethora of black and white marking difficult to distinguish.  As I mentioned earlier, I managed to break this meter, so my complaints about it are moot.

Secondly, Leningrad 4:
Leningrad 4  
This is much simpler in layout than the Weston and much easier to use.  The needle points at red numbers, these red numbers are then set against a large pointer on the other end of the meter and the aperture/shutter speed combination are read off a black and white scale.  The film speed can be set in either ASA or DIN and will work with modern films with ISO speeds as they are the same as ASA.  A note of caution:  these red numbers are NOT Exposure Values – they seem to be arbitrary numbers and are different to the equivalent numbers on the Ikophot meter.

Thirdly, Zeiss Ikon Ikophot:
Zeiss Ikon Ikophot  
This mirrors the Leningrad in as much as the meter needle points to red numbers and the red numbers are then aligned against a red pointer.  The aperture/shutter speed combination is then read off the scale – aperture in black and shutter speed in white on red.  Again, film speed can be set in either ASA or DIN.  A note of caution:  these red numbers are Exposure Values but only for 50 ASA/18 DIN film (I expect 50 ASA/18 DIN was seen as standard by Zeiss Ikon).  It is possible to use them directly on cameras with a EV scale on the shutter, but you will need to adjust for the speed of the film you are using.  This is simple enough – for 100 ASA, 21 DIN film, subtract 1 from the EV; for 200 ASA, 24 DIN film subtract 2 from the EV and for 400 ASA, 27 DIN film, subtract 3 from the EV.
The DIN/ASA scales on this meter are a bit unique.  DIN 21 SHOULD be ASA 100, but it is ASA 80.   I use the DIN exclusively and that works just fine.  I suspect using the ASA scale would also be fine as the difference between 80 and 100 is fairly small (1/3 of a stop).  As DIN is a German standard and Zeiss Ikon are a German firm, I would expect them to get DIN right.

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

This is a German meter (made in West Germany).  Bertram have been making light meters since 1928 . This meter works differently from the meters above.  For one thing, it has a digital read-out rather than an analogue needle pointing to a scale. 

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.
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