Help understanding gray card use

336v · Jun 5, 2025

xkaes said:
Are there meters that go down to f500? I've never seen one. If you are shooting pinholes, any meter will work fine, but I'd use an incident meter -- or a L'EGGS pany hose holder on a D200. And you'll need a sheet of paper to convert f4 at 1/125 into f378 at ?????

Would be nice, but no need. Luna Pro goes down to f/90, but I always measure for F/22 and then it's just one multiplier, which is square of the ration of my f/319 pinhole over f/22, which happens to be a fixed number 210. So I made a small table in excel for all required exposures corresponding to what Luna Pro shows me for f/22, and taped printed table to the back of the camera with clear Scotch. Attached is what it looks like. Easy-peasy, no math in the field. My final multiplier was tweaked based on actual tests after a couple of iterations.

Chan Tran · Jun 5, 2025

For a meter I only need the LV it gives me. I can figure out the aperture and shutter speed from there.

336v · Jun 6, 2025

Chan Tran said:
For a meter I only need the LV it gives me. I can figure out the aperture and shutter speed from there.

Sure, true statement for conventional cameras with lenses. For pinhole photography application it's actually twice as easy: - since the aperture is not variable and always the same for a given camera, you only need the light meter to show you EV number to determine a single parameter - the exposure time (for your fixed ISO). Never need to worry about aperture/speed combinations.

Chan Tran · Jun 6, 2025

336v said:
Sure, true statement for conventional cameras with lenses. For pinhole photography application it's actually twice as easy: - since the aperture is not variable and always the same for a given camera, you only need the light meter to show you EV number to determine a single parameter - the exposure time (for your fixed ISO). Never need to worry about aperture/speed combinations.

Yes and that what I refer to meter for general camera with variable shutter speed, aperture and even ISO.

Craig · Jun 6, 2025

gary mulder said:
Yes you can always overexpose your film to be at the save side.

Not with slide film, underexpose to safe with slides.

336v · Jun 8, 2025

Still planning to calibrate both of my Luna Pro meters with as simple (to me) means as possible. I think I know what to do, just need confirmation I don't miss anything obvious... A search for DIY light box yielded few treads from a few years ago, but people talk about importance of the right color temperature for proper calibration. I absolutely get that - if you don't already have a calibrated meter to compare against, then you need a light source with close to daylight color temp.

HOWEVER, I have a modern DSLR, factory calibrated. So, say, I build a miniature light box - the size of a plastic can for 35mm roll of film, with bright white LED shining straight into either the lens of the DSLR (through a plastic diffuser of some sort) or into Luna Pro sensor. The LED's light output is very easy to control by feeding it from the variable current power supply. LED's color temp does not really change with brightness, but for the sake of discussion let's assume I use bright an incandescent light bulb (like for a car headlight), which will be noticeably redder in color as brightness diminishes.

Tell me why should I care about color temp change if I'm adjusting Luna Pro circuit for a particular EV reading which will sure be wrong when compared to a day light's color temperature, but so will be wrong the perfectly calibrated DSLR's readout, error'ing *by the same amount*. All I care about is for both meters to show the same EV number, regardless if that EV number itself is really overexposing or underexposing because of "wrong" color temp. Basically, all I want is my Lina Pro to show identical readings to the DSLR, so it becomes as calibrated as the DSLR is.

Anything wrong with this logic? Basically, to get good prints on film/paper you DO need to know correct EV number for this particular color temp environment. But just for comparing two pieces of hardware you don't.
If both are off by 5EV because of wrong color temp, but track each other and one is factory calibrated correctly, I'm still happy and will assume the second piece of hardware will get calibrated just as correctly as the first "reference" one. Is that right?

Chan Tran · Jun 8, 2025

336v said:
Still planning to calibrate both of my Luna Pro meters with as simple (to me) means as possible. I think I know what to do, just need confirmation I don't miss anything obvious... A search for DIY light box yielded few treads from a few years ago, but people talk about importance of the right color temperature for proper calibration. I absolutely get that - if you don't already have a calibrated meter to compare against, then you need a light source with close to daylight color temp.

HOWEVER, I have a modern DSLR, factory calibrated. So, say, I build a miniature light box - the size of a plastic can for 35mm roll of film, with bright white LED shining straight into either the lens of the DSLR (through a plastic diffuser of some sort) or into Luna Pro sensor. The LED's light output is very easy to control by feeding it from the variable current power supply. LED's color temp does not really change with brightness, but for the sake of discussion let's assume I use bright an incandescent light bulb (like for a car headlight), which will be noticeably redder in color as brightness diminishes.

Tell me why should I care about color temp change if I'm adjusting Luna Pro circuit for a particular EV reading which will sure be wrong when compared to a day light's color temperature, but so will be wrong the perfectly calibrated DSLR's readout, error'ing *by the same amount*. All I care about is for both meters to show the same EV number, regardless if that EV number itself is really overexposing or underexposing because of "wrong" color temp. Basically, all I want is my Lina Pro to show identical readings to the DSLR, so it becomes as calibrated as the DSLR is.

Anything wrong with this logic? Basically, to get good prints on film/paper you DO need to know correct EV number for this particular color temp environment. But just for comparing two pieces of hardware you don't.
If both are off by 5EV because of wrong color temp, but track each other and one is factory calibrated correctly, I'm still happy and will assume the second piece of hardware will get calibrated just as correctly as the first "reference" one. Is that right?

I am not sure but I think the DSLR and the CdS sensor may shift their response to light vastly different from daylight differently. By the way I think they use 4700K light for calibration. But I would try to keep the light fairly close to that color temp. I have some success dimming an LED light but at low level the LED light brightness changes so abruptly. I had much better luck by dialing the same amount of filtration on a dichroic color head.

Bill Burk · Jun 8, 2025

With a bright light illuminating a frosted or opal glass screen, you can decrease illumination without changing color temperature by moving the glass further from the light source.

You can also attenuate the light with true neutral density by making a physical "screen". I created a 2-stop screen by cutting a couple circles from a Sony radio speaker grill and mounting them in a filter ring. I rotated them against each other until the rosette moiré pattern emerged then soldered the edge in a couple places to hold them together like that. Since we are not forming an image with this light, this is an effective way to reduce test light without changing color temperature.

I use a device that has a small enlarger bulb (I think it's same bulb as in a Beseler 23CII). I found color temperature varied immensely with changes in voltage. But when driven at near line voltage, and using a blue color correcting filter like 80B it's enough light to work with.

I use a microscope stand to move the opal glass screen up and down to further adjust.

I also created a holder for the "slides" that come with Sekonic incident light meter. These are calibrated to third-stop increments, so changing between the 80 ASA slide and the 100 ASA slide changes the light by 1/3 stop.

Other people have done the LED light source route. Serhiy Rozum has a github repository for a film camera tester, that you could simply build the light source from.

336v · Jun 9, 2025

Good ideas Bill, thanks. I was thinking of LED current regulation for just final fine adjustments to dial desired illumination to get particular EV exposures. Rough illumination without changing color temp would be pure mechanical - I would make crude miniature version of window blinds - you can adjust them from practically not interfering to totally blocking the light. I thought about printing black stripes on two pieces of transparency film and overlapping them you can easily set any light attenuation moving one relative to the other. It could be elongated triangles printed, which you can insert or remove as black "wedges" on the path of light. The most simple way would be to just use white paper - stacking as many pieces as needed for desired light blockage. As I mentioned, the fine adjustment between [within] coarse steps per sheet of paper is done by adjusting the LED current.

Many ways to do it, and your post has added food for thought, very much appreciated! Just need to pick the most effortless way to accomplish this. If people are interested, I'll report the outcome here when I'm done done making something.

koraks · Jun 9, 2025

336v said:
The LED's light output is very easy to control by feeding it from the variable current power supply.

You can easily get more dynamic range by just PWM-controlling the LED. Pick a sufficiently high PWM frequency and the inertia of the meter will not be confused with it; something like 10kHz will likely work. 10kHz PWM with 16 bit resolution can be made with e.g. an ESP32 microcontroller that costs a few bucks.

336v · Jun 10, 2025

Sure. This is actually wrong forum to discuss embedded electronics but briefly - any microcontroller based project will require some level of programming and I'm not a programmer (and no plans to become one) even if PWM is primitive. Then I'll have to build a driver for the power LED a dev. I Can get 0-100% PWM signal from my arbitrary form function gen without bothering with EPS32 or any other uC. But my main argument would be - controlling current limit of a power supply with simply turning a knob already gives me 0-100% dynamic range of a LED - from completely off to full on. No PWM or any other technique will exceed 0-100% average current control (e.g. brightness) range anyway.

Bill Burk · Jun 10, 2025

336v said:
Good ideas Bill, thanks. I was thinking of LED current regulation for just final fine adjustments to dial desired illumination to get particular EV exposures. Rough illumination without changing color temp would be pure mechanical - I would make crude miniature version of window blinds - you can adjust them from practically not interfering to totally blocking the light. I thought about printing black stripes on two pieces of transparency film and overlapping them you can easily set any light attenuation moving one relative to the other. It could be elongated triangles printed, which you can insert or remove as black "wedges" on the path of light. The most simple way would be to just use white paper - stacking as many pieces as needed for desired light blockage. As I mentioned, the fine adjustment between [within] coarse steps per sheet of paper is done by adjusting the LED current.

Many ways to do it, and your post has added food for thought, very much appreciated! Just need to pick the most effortless way to accomplish this. If people are interested, I'll report the outcome here when I'm done done making something.

It doesn't have to be film, it could be brass sheets - which may be better because you could cut them accurately and they wouldn't deform/melt/fade over time.

koraks · Jun 11, 2025

336v said:
controlling current limit of a power supply with simply turning a knob already gives me 0-100% dynamic range of a LED

But dynamic range in practice will be very limited. That's why suggested PWM. If you combine both approaches, you have a useful dynamic range for testing a light meter. But if you don't want to go there, that's fine.

Chan Tran · Jun 11, 2025

336v said:
Sure. This is actually wrong forum to discuss embedded electronics but briefly - any microcontroller based project will require some level of programming and I'm not a programmer (and no plans to become one) even if PWM is primitive. Then I'll have to build a driver for the power LED a dev. I Can get 0-100% PWM signal from my arbitrary form function gen without bothering with EPS32 or any other uC. But my main argument would be - controlling current limit of a power supply with simply turning a knob already gives me 0-100% dynamic range of a LED - from completely off to full on. No PWM or any other technique will exceed 0-100% average current control (e.g. brightness) range anyway.

I think since PWM power the LED at full power there is no color shift. Reducing current can possiblly cause color shift?

koraks · Jun 11, 2025

Color shift is not an issue on LEDs for this application are long as they're driven within their specifications. It doesn't matter whether they're PWM-ed or linearly driven.

336v · Jun 29, 2025

I was working on my second Luna Pro meter and discovered that evidently not only appearance had changed over years of production, but actual internal design. From the outside you can see that Some Luna Pro's have ISO speed scale implemented only as ASA numbers, while other models also have corresponding DIN scale. This is rather "cosmetic" meaning no guts are different, just added new DIN tick-marks on the same dial. However, after taking apart both units I discovered that the PCBs are a bit different, which is result of the circuit also being slightly different - see attached.

That might explain discrepancies of schematics posted by various people - they just describe the unit they happen to have. The ASA-only Luna does not have a fixed resistor in series with trim pot for battery check calm and the calibration circuit is connected to the galvanometer through the independent fixed resistors divider rather than tap to the trim pots for the slope calibration as is implemented in Luna with dual ASA and DIN scale. I've traced schematics of both now and I can see what they were trying to accomplish.

I don't know which variant came first, but with independent divider your battery check calibration is not affected by changing slope or offset trim pots. It does (somewhat) for ASA/DIN version, thus it is always suggested to calibrate batt check function last, after slope and offset are settled. For the ASA-only version the order of adjustments makes no difference.

Just sharing something new I've learned about these meters. I only have two and it's very possible/probable Gossen had implemented more than just two actual Luna Pro's circuit variations during their long production run, not to mention Lunasix-3 European version(s).

I bought a third "untested" Luna Pro one on ebay for $20 - it was worth just to have spare parts, but I wonder if I'm going to discover a third version of this meter's guts. If so, I'll trace and publish the schematic of what I'll discover.

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