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A History of my Photography

Light Meters


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    Light meters have been around a long time. The first electrical ones used a selenium photovoltaic cell. This cell generated a minute electrical current that varied with the amount of light striking it. This tiny current required a micro ammeter to measure it. This led to one of the two major drawbacks of this  type of meter - micro ammeters are extremely delicate. The other problem was that the combination of photo cell and micro ammeter had little sensitivity to low light. Of course the films back then were relatively slow any way. The advantage of this metering system is that it does not require batteries.

    The next generation of light meter used a cadmium sulfide (CdS) sensor. This is a device that changes resistance in proportion to the light striking it. This means a meter must contain a battery and can use a milli-ammeter instead of a micro ammeter. This is much more rugged and reliable. Since the resistance is extremely high in darkness some of the early meter just provided a light tight cover over the sensor to turn the meter off. I think public reaction to this method caused it to be replaced with a switch. Many hand held CdS meters were dual range devices. The base meter was a very sensitive device and a neutral density filter provided a high light level range. Both the Gossen Luna-Pro and the Minolta Autometer used this system. The neutral density filter was about 10 f-stops - this is the amount of filtering you need to safely view the sun in daylight. This provides a more accurate metering system than that used for full aperture metering. The sensor can operate over a narrower more linear range.

    The earliest camera metering systems used a simple system of a variable resistor, a meter, a battery and a sensor. A sometimes complicated mechanical system moved the variable resistor according to f-stop, shutter speed, and film speed. These meters used mercury batteries that delivered a precise 1.35 volts for nearly all of their life. Since the Whining Woo-Woos of the EPA have managed to have mercury batteries banned in the USA other sources of 1.35 Volt batteries must be found since the common 1.5 volt replacement won't work with these designs.

    Later designs used a Wheatstone Bridge circuit with a center zero meter. This design allowed multiple variable resistors to be used and the system is not sensitive to the voltage used. The most unique system was that provided by Canon with their booster meter. This was an amplifier unit that connected to the camera and provided a much greater low light level range. It's biggest drawback was that it was a clumsy system to use requiring that you remove the batteries to connect it and a separate metering unit that sat in the accessory shoe. The Canon F1 improved on this by using a special finder, but no one else copied the idea so it never sold well.

    The biggest drawback of the CdS meter is called the memory effect. When exposed to bright light the sensor can some time (up to a minute or more) to accurately read a low light level. The next generation of meters solved the by using silicon photodiodes or photo-transistors. These worked with the same circuitry but had no memory. Their response time was so fast that they could be used to measure electronic flash intensity. Some early flash meters used a mechanical shutter to restrict the exposure time to the shutter speed, because studio flash units can have fairly long flash durations. Others just looked for changes in the light due to the flash. Current flash meters measure both the flash and ambient light and indicate how much effect the ambient light will have on the picture.

    Olympus was the fist company to introduce TTL flash metering. Since the normal metering systems are inoperative during picture taking when the flash fires the flash intensity must be measured with a different system. Olympus did this by measuring the light reflected off the film during picture taking. When sufficient light has reached the film a signal is sent to the flash ordering it to shut off. This system was copied in one form or another by all the other Japanese manufactures (Patents?) and has worked great until the age of digital cameras. Digital sensors reflect so little light off axis that dependable reading were impossible This leaves two possible systems - you can use pre-flashes or if you lenses provides range information back to the camera you can use a guide number like system.

    When Nikon introduced the FA in 1983 it caused a major stir in the camera industry. The metering system in the camera divided the image into five zones and metered each one independently. It then compared these values to a table of stored values to determine the best exposure. They called it Multi-segment Metering, but later called it Matrix metering. This proved a popular system and was copied by the other manufactures. Since that time Nikon has added more zones, sensors for camera orientation, and used distance information also. The final version added RGB filters on over a thousand sensors to provided the most intense analysis or the required exposure.

    What does it all mean? The light meter built into a camera is supposed to give a setting that will align the center value of the scene you wish to photograph with the center value of your film. This is it's idea of the "correct" exposure. It's a robot designed with this purpose in mind. Yours can be different. There really is no such thing as a "correct" exposure.

 

Gossen Luna-Pro

Gossen Luna-Pro showing the meter scale and the exposure dial.

    I received this light meter from my folks about 1971. The Luna-Pro (know as the Luna-Six in Europe) was a CdS light meter. There was a rocker switch on the side that you pressed one side of to take a high range reading or the other to take a low range reading. The total range was 22 f-stops. Once you took the reading you transferred it by hand to the calculator dial which provided you with a scale of shutter speed and f-stops. There was a sliding white dome which you slid over the sensor to take incident readings. I found the incident reading were not as accurate as the reflected readings. I think the white dome was designed more as an add on than as a precision device.

Gossen Luna Pro

    There were several accessories made for this light meter and I purchase two of them: the narrow angle attachment and the enlarging meter attachment. I should have save my money. The meter by itself had a 30 degree field of view and the narrow angle attachment reduced it to either 15 degrees or 7.5 degrees. Unfortunately the view finder provided on the attachment was nearly useless. The enlarging attachment was also useless since the meter was impossible to read under a safelight.

    One other disadvantage of this light meter is in the needle it used. It was too sensitive to shock. I remember dropping the meter on a hard steel deck. When I picked it up the meter would strike the scale. I said what the hell, turned it over and dropped it again. The meter functioned fine after that. The light meter disappeared some time in the late 1970's, but I already had something better.

 

Gossen Pilot

Gossen Pilot showing the hard shell case it came with.

    I bought this light meter for a movie camera I had because it was one of the least expensive ones available with a cine scale. The pilot was a selenium meter. This meant that the meter movement was even move delicate than the CdS meters, and it also had very poor low light capability. It did work fine for my movie camera though.

Front view of the Pilot showing the diffuser for incident readings

 

 

Minolta Autometer

Front view of the Autometer showing the reading dial, the reading button on the side and the high and low indicators.

    This was purchased in 1975 and I still have it today. The Minolta Autometer was different from other light meters of it's time because it used a servo to move the scale. You just pressed the button and took the reading. It had two ranges - if you were on the wrong range for an accurate reading a red light would point you to the right range. You turned the outer ring of the dial to set the range. The high range moved an extremely dense filter in front of the sensor.

Rear view of the Autometer showing the battery check button and the incident/reflected selector.

     The basic meter was an incident meter with a hemispheric diffuser. I bought a flat diffuser and the reflected light attachment. To use the reflected light attachment you had to unscrew and stow the diffuser and screw on the reflected light attachment. Then you had to turn a control on the back of the meter from "inc" to "ref" with a coin. I tended to just use the hemispheric diffuser for incident reading. This meter is still accurate and reliable and if all Minolta's products were this reliable they would still be in the photo business.

 

 

Minolta Flashmeter

    I bought this meter in 1975 also. This was a cord only incident flash meter. There was a dial on the front to set the shutter speed, and a button on the side to fire the unit. My unit died in 1983. I was unable to find any picture or any units for sale on eBay so they all must have died out.

 

 

Minolta Spotmeter

Side view of the Spotmeter showing all the controls.

    I purchased this meter in 1983 to be able to balance the light on the color CRT monitors with the ambient light to provide a more natural looking picture. This was a digital meter and I found it infuriating to use. You set the shutter speed via buttons and read out the f-stop for use. While it worked fine for the previous use where because of the CRT refresh you used slow shutter speeds, for other uses I usually prefer to pick the f-stop and have the meter tell me the shutter speed  Here you have to take the reading then raise or lover the shutter speed to get the f-stop you want.

    I still have this meter and it still works well. I just don't use it. It needs a new battery every year.

 

Quantum Calcu-Flash II

Front view of the Calcuflash 2 the red window in the center on the bottom is the LED readout and the little know to its left is the shutter speed setting dial.

    I bought this meter in 1983 also. It was meant as a low cost replacement for my dead Minolta Flashmeter. This meter does its job, though it isn't easy or convenient to use. It can be used with or without a cord and provides the reading on a two digit seven segment LED display at the bottom. There is a button on each side, one activates the unit, the other clears the reading. This lets you accumulate multiple readings. There is a small very hard to read knob on the lower left to set the shutter speed to use. You read the f-stop to use opposite the LED reading on the dial scale. The unit came with two front units, a hi/lo diffuser dome attachment and a diffuser dome/reflected light attachment. The worst problem is using the dial, and having to replace the four batteries. This meter falls under the it works but there are better ways to do things catagory.

Sekonic L-558

Front view of the Sekonic L-558

    This is the Swiss army knife of light meters. It does it all, incident with dome or flat receptor, one degree spot readings, to read ambient light or flash, in cord, cordless or wireless mode. In ambient mode you can have shutter speed or f-stop priority. In flash mode it can measure multiple flashes and it displays the ratio of flash to ambient light. About the only thing this meter doesn't do is measure color temperature.

 

Gossen Sixticolor Color Temperature Meter

Sensor side of the Sixticolor. Because the meter does not require batteries I assume that it uses a selenium photovoltaic cell.

    One of the most expensive type of light meter is the color temperature meter. Color temperature represents the light radiated by a black body when heated to a certain temperature in degrees Kelvin. Sunlight is about 5800 K degrees and anything cooler is more red while anything hotter is more blue. The Sixticolor measure red and blue light to determine the equivalent color temperature. Pressing the button on top will free the needle to move to a position on the scale representing the color temperature. Release the button and the needle locks in position. The color temperature is read directly off the scale. Another scale provides the amount of necessary filtration using a filter system that is long obsolete.

Business end of the Sixticolor showing the readout and the filter scale. The operating button is at the top.

Color temperature meters are most important in color film photography were film has a fixed color temperature. Digital cameras allow adjustment of the color temperature and some have the ability to sample the light and set themselves to match the light. Still a meter like the Sixticolor is helpful when dealing with indoor lighting or mixed lighting situations.

The Sixticolor with its manual and case.

    Gossen has replaced the Sixticolor with a newer three color meter (it also measures green light) that is much more sensitive and can also read electronic flash color temperature.

The Sixticolor box

 

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