I wrote this article because I was looking for a good used hand-held light meter for large format, and Gossen in general (and the Lunasix meters in particular) have a reputation for sensitivity, accuracy and reliability. While looking at used Lunasix meters on eBay I was noticing variations between models and wanted to get a definitive picture of their differences, advantages and disadvantages.
When new, Gossen’s Lunasix light meters offered unprecedented sensitivity, accuracy and reliability. Their main disadvantage today is their selling price: people are asking too much. For the same money (or a little bit more) you can get a Gossen Profisix or Lunasix F, both of which could be 30 years younger, are based on faster SBC light sensitive cells, and use easier-to-find 9V batteries.
If you use old Synchro Compur shutters, the original Lunasix and the second version (confusingly also named Lunasix: more on that later) are good choices because they have markings for the “old” 1/5, 1/10, 1/25, 1/50 and 1/100 shutter speeds. Don’t underestimate that convenience.
Where the Lunasix 3 (Luna-Pro in USA) becomes a great value proposition is when it’s combined with the Tele attachment to make a narrow-angle (almost spot) reflected light meter. The Tele attachment’s waist level viewfinder makes it easy to see what you’re measuring and switching between 15˚ and 7.5˚ is simple. Note that the cheapest way to buy a Tele attachment is to get it with a meter: the attachments often sell for more by themselves.
Introduction
The Gossen Lunasix is a high quality light meter first introduced in 1961 featuring the then-new CdS cadmium sulphide photosensitive cell which offered unprecedented accuracy and low-light sensitivity. According to an early user manual, it is named LUNA because it yields reliable measurements all the way from moonlight to the very brightest sunlight.
The Lunasix was meter-of-choice for photographers around the world for decades. Thousands were made and they are easy to find on internet auction sites. Don’t let their age dissuade you from buying one: they are an accurate and reliable light meter for your daily photography.
Incident and Reflected Light
All Lunasix light meters measure both incident light (the light falling on the subject) and reflected light (the light coming from the subject). The meter’s normal reflected light angle of view is 30˚ which is equivalent to a normal lens (ie a 50mm lens on 35mm cameras or 80mm on 6×6).
The Lunasix 3 and later models accept clip-on attachments like the “Tele” which has a built-in waist-level finder and provides 7.5˚ and 15˚ angles of view (without the attachment the meter measures 30˚). If you want a cheap spot meter the Lunaxis 3 and later models with the Tele attachment are recommended, however if all you need is incident and 30˚ reflective (and generally it is all you need) then an older Lunasix model from 1961 or 1964 will be a good choice.
The CdS Photo Cell
The Lunasix was Gossen’s first light meter to use a CdS photo cell; until then, all Gossen’s light meters used selenium cells. Selenium cells work like solar panels to generate electricity when light shines on them, which powers the meter; selenium cell meters don’t need batteries. Light meters based on selenium cells (such as the Weston Master series) aren’t as sensitive so they don’t work in low light, and their accuracy varies between light and dark brightness levels. Note that selenium cells have not aged well, most are dead or dying and replacements are not being made, and there is no modern substitute to replace them with.
CdS cells don’t generate electricity when exposed to light, they change their resistance. They are very sensitive and the change in resistance is very even so they can be used over a large range of light levels from candlelight to brightest daylight. One down-side to CdS cells is that they can take a couple of seconds to respond to a change in brightness (say, from bright to dim light) so their response is not instantaneous. This delay is often referred to as “memory”. CdS cells were replaced with silicon cells (often called silicon blue cells or SBCs) which have the same or better light sensitivity as CdS cells and have an instantaneous response to light changes.
Operating the Lunasix
All versions of Lunasix light meters have “low” and “high” ranges for measuring dim and bright light, and can make both reflected and incident light readings. They all work the same way too. To prepare the meter for a reading:
- Perform a battery check to be sure it has sufficient charge.
- Set the film speed (ASA or DIN) on the exposure calculator dial.
- Slide the dome on the front to make a reflected or incident light reading as required.
To measure the light:
- Point the meter at the subject and press the meter activation button, either low or high range, to start the measurement.
- Hold the button a couple of seconds until the meter needle stops moving, then release the button to lock the needle in place.
- Read the light level from the scale.
- Turn the exposure calculation dial to the measured light level.
- Read the shutter speeds and f-numbers needed for correct exposure from the calculator dial.
If using neutral density (ND) or other filters on the camera lens, or if doing close-up or macro work, remember to make an adjustment for filter factor or bellows extension.
Note that most general purpose photographic film suffers from reciprocity failure at shutter speeds 1 second or longer. The readings from the light meter calculator dial do not include corrections for reciprocity, these must be applied separately. Correction factors are usually available from the manufacturer or can be determined by practical test, and for some films (notably Fomapan 100 and Shanghai GP3 100) the corrections are necessary and can be significant.
Four Versions of Lunasix
Gossen released four versions of the Lunasix light meter. The overall shape, size and user interface of the Lunasix remained fundamentally the same for all versions over the 30-odd years it was in production. In the USA, some versions were sold with different names to the rest of the world.
Gossen Lunasix (Version 1)
The first version was sold from 1961 to 1964 as the “Gossen Lunasix” in all markets. It boasts a 20-stop exposure range and takes incident and reflected light readings. The built-in sliding dome for incident light readings was a big feature for the Lunasix, with many other contemporary light meters (for example, the popular Weston Master V) the dome was a separate attachment.
The first Gossen Lunasix model can be identified by:
- two separate meter activation buttons, one dark and one light, for low and high range readings
- a meter scale marked with both low range (1 to 11) and high range (11 to 21) numbers printed in black on a white background
- a small pointer on the front, visible from the top of the meter, that aligns with embossed markings (a “pine tree”) on the case to indicate the position of the dome and whether the meter is making incident or reflected light readings
- the exposure calculator dial turns from right to left: that is, rotating the dial clockwise decreases the EV number
- the meter uses only one 1.35V mercury battery
This is the only version that has two separate meter buttons and uses one battery. While not rare, these first Lunasix meters appear less frequently on internet auction sites than later models. This meter does not accept the clip-on attachments (Tele, Lab, Micro, Repro etc) designed for the Lunasix 3.
Gossen Lunasix (Version 2)
The second version, released in 1964 and produced until 1966, was also sold as the “Gossen Lunasix” in all markets. It could have been named the Lunasix 2 because it has several significant changes both inside and out. The main improvements are:
- a single “see-saw” rocker switch replaces the two separate meter activation buttons: press up for low readings, press down for high readings
- the meter displays only one set of numbers (low or high) that apply to the last measurement taken
- the highest reading is increased from 21 to 22 which is a doubling of the light measuring level
- the direction of the exposure calculator dial is reversed and now reads from left to right: rotating the dial clockwise increases the EV number
- the meter uses two 1.35V mercury batteries
Like the original, this second version of the Lunasix has a small pointer on the front, visible from the top, that aligns with embossed markings (the “pine tree”) on the case to indicate whether the meter is set for incident or reflective measurement. The attachments (Tele, Lab, Micro, Repro etc) designed for the Lunasix 3 do not fit this meter.
The see-saw switch and the round exposure dial became the Lunasix meter’s most identifiable features and became the template for Gossen’s subsequent analog meters, the Profisix and the Lunasix F.
Some examples of this second version of the Lunasix have the light value numbers in black printed on a white background, while others have black numbers printed on a yellow background.
Gossen Lunasix 3/Luna-Pro
Gossen updated the Lunasix again in 1966 and sold it as it the Lunasix 3 in all markets except the USA where it was sold as the Luna-Pro. The Lunasix 3 retains the increased sensitivity, see-saw switch and articulated meter scale of the second model, and incorporates three major changes:
- the front of the case is re-designed to take clip-on attachments (Tele, Lab, Micro and Repro)
- the exposure calculation wheel has additional Red and Green setting marks for use with the Tele attachment
- the case no longer has the embossed “pine tree” or the incident dome position pointer
The Lunasix 3 and the USA-market Luna-Pro appear to be identical except for film speed indicators on the exposure calculation dial: Lunasix 3 meters have both ASA and DIN markings, while some Luna-Pro meters have both ASA and DIN and some only have ASA.
The Lunasix 3 remained in production for more than 20 years. The only major external change appears some time in the late 1980s when the body colour went from two-tone grey gloss to satin black plastic.
Gossen Lunasix 3S/Luna-Pro S
The last version is the Gossen Lunasix 3S – called Luna-Pro S in USA. Gossen changed the body material to satin-black plastic and changed the calculation dial to include an additional EF/EV exposure compensation ring (omitting the CINE shutter speed scale found on earlier models) making it similar to that used on Gossen’s (then new) Profisix and Lunasix F.
This version is the least common probably because it didn’t sell as well: meters with SBC cells had already been released (like Gossen’s Profisix and Lunasix F) and the Lunasix 3S with its CdS cell was considered old technology. At this stage of its life the Lunasix had become a “budget” model in the Gossen range.
Both “S” models appear to be largely similar, though there are a few differences in detail: the Lunasix 3S meter scale has white printing on a black background and black meter switch, while the Luna-Pro S has black printing on a white background and a grey meter switch like the earlier version.
Late examples of the Lunasix 3S and Luna-Pro S were sold with the GOSSEN battery adapter V206A (more on this below) and used two 1.5V silver oxide batteries instead of two 1.35V mercury batteries.
The Mercury Battery Problem…
Most precision portable electronics designed in the middle of the 20th Century – like the Gossen Lunasix, the Olympus OM1, Leica M4, and Nikon Nikkormat FT to name a few – used 1.35V mercury batteries as their power source because they provided an accurate and reliable voltage that remained constant for the life of the battery.
Mercury batteries were were banned from production in 2000 for health and environmental safety reasons, and most modern-day battery alternatives cannot be used to directly replace them because either their voltage is too high or their voltage drops as they discharge (and sometimes both).
Gossen’s solution was their GOSSEN battery adapter V206A with silver oxide batteries. The adaptor is a short plastic tube to hold the smaller silver oxide batteries in place, with a diode to reduce the total voltage to 2.7V. The adaptor was included with late model Lunasix 3S and Luna-Pro S meters and was also available separately as a conversion kit for the older meters that used two batteries. The adaptor is no longer available but it is mechanically and electrically simple, and making something equivalent is relatively easy.
Alkaline batteries are available but DO NOT USE ALKALINE BATTERIES. Their voltage does not remain constant: it starts off too high and drops as they discharge and becomes too low. Circuits designed for mercury batteries assume (require) that the voltage remains constant.
The best mercury replacements are silver oxide batteries and zinc-air batteries, but they are not perfect:
- silver oxide batteries are 1.5 volts which is higher than the 1.35V from mercury batteries, so either the higher voltage needs to be reduced to 1.35V or the meter needs to be re-calibrated to use 1.5 volts
- zinc-air batteries are the correct voltage (or close enough to not matter) but they have a high self-discharge rate and go flat quickly even if not being used; however they are cheap and easy to get
- neither silver oxide nor zinc air batteries are available in exactly the same shape and size as the original mercury battery, but they are available in a size that can be easily adapted to fit
- alkaline batteries are available in the same shape and size as the original mercury batteries but their voltage starts too high and drops as the battery discharges, causing the meter to fall out of calibration. Do NOT use alkaline batteries. Use only use silver oxide or zinc-air batteries.
Here are the options for replacing the mercury batteries:
| Option | Advantage | Disadvantage |
|---|---|---|
| use 1.35V zinc-air cells | the meter probably won’t need recalibrating | the batteries won’t last long even if not used often, but the batteries are cheap and easy to get |
| use silver oxide cells with voltage adaptor | the meter probably won’t need recalibrating | the voltage adaptors are expensive (as much as a used meter) but they can be home-made easily |
| use silver oxide cells and add a diode to the meter circuit | the meter probably won’t need recalibrating, and there is no need for battery voltage adaptors | the diode is cheap but needs skill with a soldering iron and experience with electronics to fit |
| re-calibrate the meter for 1.5V silver oxide batteries | no need for battery voltage adaptors | calibration requires high-level skills, and not all meters can be recalibrated |
A reminder: do NOT use alkaline batteries. They don’t have a constant voltage and the meter will not remain accurate after re-calibration because the voltage changes as the battery drains. Use only silver oxide or zinc-air batteries which have stable voltage.
Reports are that some Lunasix meters can be re-calibrated to 1.5V and some can not, depending on the example at hand. This is because there was variation in CdS cells when new, and the meter circuit was designed to have enough adjustment to account for the variation, but there isn’t enough adjustment to compensate both for cell variation and higher battery voltage.
I attempted to recalibrate a Lunasix meter (two, actually) and despite being experienced with electronics I found the job beyond my abilities and ended up junking them. (It was a learning experience and the meters were sold cheaply as “parts/untested” but I hate breaking things.)
My recommendation is to use either zinc-air batteries with a simple insulating sleeve to fit them into the meter, or use silver oxide batteries with a voltage-reducing adaptor (either buy the adaptor or make it yourself with a diode). You will need to test the meter and see if it is accurate with the new batteries (remember to first zero the meter needle with the adjustment on the back). If the meter works with silver oxide batteries and voltage adaptor and your skills are good, you could go the the next level and open the case to hard-wire a diode into the meter circuit. If you do this you can replace the voltage-reducing battery adaptor with a simple sleeve.
I have a couple of Lunasix light meters and converted one to use zinc-air batteries and sleeve, and another to use silver oxide batteries in a home-made voltage-reducing adaptor. In both cases the accuracy of the meter is excellent and after adjusting the pointer zero it has matched the readings of newer Profisix and Lunasix F meters to within 1/6 of a stop (ie, a needle width).
Tips for Buying on eBay
- Avoid meters that are listed as “for parts or repair” or have descriptions that describe a fault eg, “works on high mode but not on low mode” because spare parts have not been available for a long time and despite being relatively simple the meters are difficult to repair (personal experience). Calibrating a meter is difficult, too (more personal experience).
- There are a lot of good meters around so wait for one that works and is well priced.
- If the eBay listing is buy-it-now, don’t buy it right now: save the item to your watch list and wait a day or two. Chances are you’ll be sent a special offer with a discount.
- You will often pay more for a clip-on Tele (or other) attachment by itself than the same attachment together with a meter.
Hopefully you’ll buy an an old Gossen Lunasix, install zinc-air batteries or convert it to use silver oxide (not alkaline!) batteries and give it a new life for your photography.