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The "zone scale" of the Gossen Luna Pro SBC expsure meter

Doug Kerr

Well-known member
A message recently posted here (and on several other photographic forums) by (here) a "new member" (of yet-uncertain legitimacy) inquires about the "zone scale" on the Gossen Luna Pro SBC exposure meter.

Independent of the matter of its source, it is an interesting question.

The version of the manual for this meter I have shows no such thing, but a bit of poking around lets me guess what is being spoken about.

I will (as you might expect) first give some background for context.

The modus operandi of the Gossen Luna Pro SBC exposure meter

Simplistically, this meter is aimed at the scene (for reflected-light metering) and the so-called "power switch" is pressed momentarily.

The measured luminance is stored for a while in the instrument.

The actual "meter" has a zero mark at mid scale, on either side of which are marks at intervals of "one stop" (the whole scale typically labeled 3, 2, 1, 0, 1, 2, 3, with the left side marked "under" and the right side "over"). (Some versions may have a scale with a greater range.)

These are described as being at intervals of "1 Ev", not unreasonable in the context in which the meter is used​

After the luminance has been captured, the main dial is rotated until the meter reads "0". At that time, combinations of shutter speed and aperture that will produce the "recommended" exposure can be read from two concentric rings on the dial. (We assume that the appropriate "exposure index" has been set on the "ASA" sub-dial.)

If one wants to apply "exposure compensation", of, for example, -1 Ev, one can just turn the dial until the meter reads, for example "[under] 1".

There is, however, a way to set exposure compensation into the dial system so that it will be in effect when the dial is turned until the meter reads "0".​

The zone system of exposure planning

The zone system of exposure planning (devised by Ansel Adams and Fred Archer), simplistically, seeks to "plant" the photometric exposure for scene elements of different reflectance at levels such that the exposure result ("density", in the case of film) for each will be at an "appropriate" place on the tonal scale.

The metaphorical description is that this approach assures that a while cat on a snowpile will come out looking like a white cat on a snowpile, not a gay cat on an ash heap (as would happen with classical reflected light metering).

The system as usually taught identifies ten different zones of exposure, each "one stop" wide, identified with roman numerals. "Zone V" is considered to be the exposure whose exposure result would be "mid scale" (whatever that is).

Typically (very simplistically) the practice of this scheme involves taking some prominent area of the scene that can be recognized as deserving to end up in a certain zone, take a reflected light meter reading on that area, and by proper manipulation of the indication of the meter, determine what photographic exposure (combination of shutter speed and aperture) will result in that.

Typically, if the meter exposure "recommendation" is taken at par (with no manipulation), the result is that the area on which the reading is taken will end up exposed in "zone V".

Zone system work with the Gossen Luna Pro SBC exposure meter

The manual I have for the Gossen Luna Pro SBC exposure meter (thanks to Mike Butkus for reconstructing it) is for a version not having an actual "zone scale". It includes a clumsy, incomplete, and confusing discussion of the zone system and how to practice it with this meter.

To sort that out: basically, we if we capture a measurement on a scene object we consider as worthy of "zone III" exposure result, we turn the dial until the meter reads [under] 2 and shoot at the exposure recommendation that gives.

The Gossen meter "zone scale"

I have read discussions of a version of the Gossen Luna Pro SBC exposure meter in which there was a "zone scale" beneath the scale I described above on the actual meter. I have yet seen no picture of this.

My guess is that this is a scale of zone numbers, with "V" associated with "0" on the main meter scale, "III" associated with "[under] 2" , "VIII" associated with "[over] 3, and so forth.

Then, if we capture a measurement on a scene object we consider as worthy of "zone III" exposure result, we turn the dial until the meter reads "III" on the zone scale and shoot at the exposure recommendation that gives.

Just a guess.

Best regards,

Doug
 

Jerome Marot

Well-known member
This is an image of the meter (an advertisement of the times) showing the zone scales:

6678173543_ae96cf025e_o.jpg

I found it here and there are other advertisements of the time on that flickr account, most notably this one.

You will notice that there are two zone scales, one with 11 zones and one with 10 zones.

Coincidently, the wikipedia article about the zone system lists also 11 zones and I would believe that overdevelopment, so called "zone expansion" would lead to 10 zones. I would therefore believe that the top scale is simply the zone system and the bottom one, the "N+1" expansion case.
 

Doug Kerr

Well-known member
Hi, Jerome,

This is an image of the meter (an advertisement of the times) showing the zone scales:

6678173543_ae96cf025e_o.jpg

I found it here and there are other advertisements of the time on that flickr account, most notably this one.

Thank you so much. That is doubtless what is being spoken of.

I believe that this is the older (pre-SBC) version (CdS detector).

Then "main"scale of the meter here shows, rather than 3-2-1-0-1-2-3, 1-12, but it looks as if that scale can be changed. I note the apparent rocker switch on the right. There may be two "ranges" on that instrument.

Ah, yes, I see that referred to in the advert text!​

I'll look for a manual for it!

You will notice that there are two zone scales, one with 11 zones and one with 10 zones.

Coincidently, the wikipedia article about the zone system lists also 11 zones and I would believe that overdevelopment, so called "zone expansion" would lead to 10 zones. I would therefore believe that the top scale is simply the zone system and the bottom one, the "N+1" expansion case.

That might very well be. I'll try to follow up from that point.

Thanks so much.

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jerome,

I have the manual for that version of the meter (Gossen Luna Pro).


This figure from the Luna Pro manual is helpful:

Gossen_luna_pro_face-01.gif
Those two sets of black-white "stripes" are just the "scale divisions" for the two scales (each "stripe" runs between two consecutive "numbers").

The upper one matches the scale now in place ("low range") (1-12) (as we see in the picture).

The lower one matches the other scale (12-22), which is presumably exposed (below the bottom row of stripes - and the upper one hidden) when the rocker switch is pressed to the "ON - bright range" position (rocked "backward").

That scale, as we would expect from its range, is evidently shorter. I have no picture of the instrument in that situation.​

The lower "stripe scale" has black stripes for two adjacent "subspans" (16-17 and 17-18), to accommodate a little white one between them which is the "battery OK" range. Ugh!

There is no mention of zone technique in the manual.

We may be dealing with various people in the "chain of information" getting various things tangled together!

And of course the question itself comes from a so-far questionable source.

Thanks so much for your help here.

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jerome,

Here is the face of the Gossen Luna Pro S (very similar to the Luna Pro), with the "bright range" numerical scale in place:

Gossen_luna_pro_s_face-01.gif

Note that it has a more sensible bright range strip pattern, with the battery OK zone separate. Duh!

But:

Notice the obvious Zone numbers at the bottom of the dial!

I have to figger just how the algebra/logic of that would work.

Well, I'll see if I can get the manual for that!

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jerome,

Well, I got the manual for the Luna Pro S.

It has a section on zone system operation. It is almost useless:

Gossen_luna_pro_s_zone_text-01.gif


I will reverse engineer it from the picture.

And why do I even care?

Gotta go shower and get dressed now (it is 1253 local time) so we can go out to lunch.

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Jerome,

I think I've got it.

Here again we see the face of the Luna Pro S:

Gossen_luna_pro_s_face-01.gif

On the Luna Pro (and Luna Pro S), we read the luminance from the meter and then set it into the "computer" by moving the white arrow to the corresponding meter reading on the adjacent "white" scale.

But on the Luna Pro S, the white arrow has the "V" of the zone scale in it.

In the picture above, we see the "computer" set for a meter reading "10" (for ordinary metering), or for zone metering if we want our metering target rendered as Zone V.

I think that if we want the object metered to be rendered as other than Zone V, we use the zone number we want it to be as our "pointer" (not the white triangle with the "V"") and align it with the meter reading on the adjacent white scale.

I think.

Best regards,

Doug
 

Jerome Marot

Well-known member
Those two sets of black-white "stripes" are just the "scale divisions" for the two scales (each "stripe" runs between two consecutive "numbers").

The upper one matches the scale now in place ("low range") (1-12) (as we see in the picture).

The lower one matches the other scale (12-22), which is presumably exposed (below the bottom row of stripes - and the upper one hidden) when the rocker switch is pressed to the "ON - bright range" position (rocked "backward").

Indeed. Thank you.

I have a similar instrument, of Russian origin. It has the rocker switch and top/bottom scales.
 

Doug Kerr

Well-known member
Hi, Jerome,

Then Gossen Luna Pro F has the general design of the Luna Pro SBC but incorporates a zone scale much like that of the Luna Pro S (although its working is a bit different owing to the different paradigms of the two instruments with regard to picking up the meter reading itself).

We see its face here:

Gossen_luna_pro_F_face.gif

The manual has an elaborate section on zone work:

II. o. Zone Systems

There are times when the lighting range cannot be brought within the acceptable limits of the film because of an inability to fill or use "gobos" such as when photographing landscapes. When these situations arise, the exposure levels can be adjusted so that the most important parts of the scene receive proper exposure. This can be done with either black and white or color film. However, some detail, either in the highlight or shadow areas, will be lost.

When using black and white film, another technique is available to extend the capabilities of the film to record extreme lighting ranges. It is called the zone system. Instead of relying on standard measurement and exposure techniques, the zone system combines special measurement techniques, modified exposures, and altered film processing to expand the range of light values that the film can accept. In doing so, detail can be recorded which would otherwise be lost.

A complete discussion of this technique is far beyond this manual. References are given in the appendix which should help you become familiar with this very useful photographic tool.

The Luna-Pro F is well suited for use with most zone systems because of its null meter principle of operation and the zone system scales (20). Most systems are based on a central zone, representing a certain lighting level called Zone 5. From this level, the range of light from highlight to shadow is measured and referred to this central zone. The level of variance from the central zone determines the amount of exposure correction and processing modification, if any, that is needed.

With most meters, the zone equivalents must be marked on the face or transferred to another sheet to correlate with changes in light level. However, with the LunaPro F. the computer dial can be used to adjust the meter needle at any light level. Any subject can be nulled for and light variations from that level read in zone system values I thru IX directly off the meter face in EV. This means that you can always set null for Zone 5. Or, you may wish to bias the reading in one direction.

Simply turning the computer dial of the Luna-Pro F allows you to put the meter needle anywhere from -3 to + 3 EV from a normal value.

To use the zone system scales of the Luna-Pro F. take a measurement of a central zone (Zone V), noting the corresponding EV values for each zone value. Using the EV scale as a reference point, measurements of highlight and shadow areas can then be translated into zone values.

Example:

Take a reading of a central zone or mid-tone (Zone V) and null the meter. Assume the EV indicated for Zone V is '8'. Note the EV values indicated for the other zone values. In this example:

Zone I = EV 4 Zone VI = EV 9
Zone II = EV 5 Zone VII = EV 10
Zone III = EV 6 Zone VIII = EV 11
Zone IV = EV 7 Zone IX = EV 12
Zone V = EV 8

Now you can take a reading of a highlight area, null the meter and note the EV indicated. Assuming the highlight reading corresponds to an EV 12, the zone value would then be Zone IX (see chart above). Now take a shadow reading, null the meter and note the EV indicated.

Assuming the EV value is '5', the corresponding zone value would be a Zone II.

The zone system scale of the Luna-Pro F enables you to easily measure zone system values and establish information regarding tonal range in a scene.

In the example above, Zone V (a mid-tone) is used as a reference point; however, with the Luna-Pro F you can select any zone or light level as a reference. A reading is taken of a particular area, the meter is nulled and the EV value noted (i.e., EV = 7). If you would like that area to be represented as a Zone III rotate the computer dial until EV 7 is directly above Zone value III.

At this point, a reading of the appropriate apertures and shutter speeds will render the particular area measured as a Zone III. With a little practice, you will be amazed at the ease of obtaining this specialized information.​

It will take me a little while to follow all that, but it looks credible so far..

This may be what our mystery correspondent was asking about.

Best regards,

Doug
 

Doug Kerr

Well-known member
The principle behind the Gossen zone system scales is the presumption that the metering equation used by the meter will place an object whose individual luminance is directly measured (on what amounts to a spot basis) at a photometric exposure that would give an exposure result that we would think is appropriate for "Zone V".

I do not know the reflected light meter calibration constant supposedly used by the Gossen meter.

I can reverse engineer if from the tables on the rear of the instrument, but not without taking off my shoes.​

However, if:

• The reflected light calibration constant used by the meter is essentially the one implied by the ISO standard for automatic exposure systems (the standard for free-standing exposure meters allows a generous range of calibration constants), and

• The exposure index set into the meter (via the "ASA" dial is in fact the ISO speed of the sensor system (saturation basis)

Note that today the sensitivity of digital cameras are usually stated in terms of the ISO SOS rating, which is lower than the the ISO speed by about half a stop.​
then if we meter on a particular scene item, we can expect that scene item to have a photometric exposure of about 12.8% of the saturation value.

Now if that's your idea of a good "Zone V rendering", that's nice.

If in fact the exposure index is set to the ISO SOS value of the camera (which should be what we have set the camera to, for modern digital cameras), then if we meter on a particular scene item, we can expect that scene item to have a photometric exposure of about 18.1% of the saturation value.

Now if that's your idea of a good "Zone V rendering", that's nice.

Best regards,

Doug
 

Doug Kerr

Well-known member
While we are looking into every orifice of the Gossen Luna Pro exposure meter family, I note for the record:

For the instruments that show a numerical value on the meter proper (the Luna Pro and Luna Pro S), in the reflected light mode, that value is Bv + 10 (Bv being the APEX logarithmic measure of scene luminance ("brightness").

This 10-unit offset is no doubt so that for very low values of Bv the meter proper does not give a negative value (it seems that it can give readings down to Bv -9, which would be "1" on the meter scale).

In the instruments of the family that use a "null the meter" reading approach (Luna Pro SBC and Luna Pro F), Bv is not visible that I know of. It can of course be reckoned by:

Bv = Ev - Sv​
where Sv is the APEX value of the exposure index set into the meter's computer, where ASA/ISO 100 is Sv 5.

Best regards,

Doug
 

Doug Kerr

Well-known member
Here we see a picture of a Gossen meter clearly labeled Luna-Pro SBC that has an EV scale on the computer ring:

gossen_lunaprosbc.gif

This is very much like what I have seen as the Gossen Luna-Pro F.

This may be what the phantom was describing.

Very interesting.

Best regards,

Doug
 
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