What are the better light meters for very low level light?

Ted Harris said:

I checked and actually the Mavolux measures down to the same 0.001 fc that theProfisix mesures .... but as I noted it is a harder meter to use and also costs more.

One more question here .... how important is this actually? Considering that a light meter is a reference tool only and when we are talking extremely long exposures a light meter goes from being a reference point to a broad guideline. Extremem low light exposure may jsut be oneof those areas where experience counts for as much as all the technology we can muster.

Click to expand...

Yes, sorry Ted, my assumption was wrong. The Mavolux 5032 B is more sensitive than most other luxmeters in general use (later edit: wrong again! I was going from memory, always dangerous for me, and the number I thought of was in lux, not fc, dammit), and it costs about $1000 if anyone is interested. A calculator disc is available as an accessory for converting the measurements into photographic exposure. I totally agree about the usefulness of meter readings for photography at those levels.

Michael,

In practice, how important in photographic terms is the difference between the reading from a simple luxmeter like the Mavolux* and a photographic lightmeter? I wonder if, in many cases, the differences between the meter's spectral sensitivity and that of the particular film in use are greater than the difference between the spectral response of a luxmeter and that of photo lightmeters (which have a wide range of spectral responses).

Put another way: in many photographic applications, especially where we have calibrated our film/meter combination (which we will do in representative lighting conditions if we require accuracy), is the difference between the spectral response of a luxmeter and a photo lightmeter any more significant than the difference between different photo meters (which many people accept).

Re moonlight etc. Is it blue? It is 'less blue' than daylight - ie it has a lower colour temperature. Are our rods most sensitive to blue? Their peak sensitivity, according to the CIE standard spectral luminous efficiency function for scotopic vision, is at 507 nm. Wouldn't most people call that green?

Best,
Helen

*The Mavolux uses a colour corrected silicon photo diode, the colour correction being matched to the luminous efficiency function for photopic vision by the use of a filter, with different accuracies of match available. It does not apply photopic luminous efficiency functions to spectral data.

Lee L said:

As I stated in earlier post to this thread, the instructions for the LunaPro F (a variant of the SBC with flash metering added) specify -8 EV. That's a manufacturer's spec, verified by Helen on an equivalent model. Good enough for me. I'll be meeting John, this thread's originator, with my LunaPro F at the site he wishes to meter tomorrow.

Lee

P.S. Manuals for older Gossen meters (English sections) are at: http://www.gossen-photo.de/english/...icts/navi_foto_sammler_sub_01_anleitungen.gif
German language manuals are on another part of that site.

Click to expand...

I stand corrected, good to know that there's another low light meter available out there.

Helen,

You are right, I should have said blue-ish. And this is an example of how our vision differs greatly from film. As the light levels go down, our sensitivity shifts toward the use of rods, and the 507nm you mention. So while there is a clear shift in human sensitivity, there may (or may not) be a real large shift in film sensitivity.

Of course, there's also the issue of human color adaptation that film generally does not do as well, so nightime photos under electric light sources can produce very different results than out mind is seeing.

There's no harm in using a scientific meter, as long as the linearity of the meter matches film reasonably well. I've never checked to see if that is the case, though, so I don't know if that would be an issue or not. Beyond that, it is probably no better or worse than any other metering approach, except when you are discussing metering out in the field, you would have to remember that your metering results will probably never match those of a photo meter, even if both are calibrated properly.

Speaking of calibration, Minolta makes one of the most used lighting meters in the lighting specification field, and it is somewhat considered the indstry standard.

http://www.tequipment.net/MinoltaT10.asp

They recommend that they be calibrated once a year, and Minolta has a service to do that for their meters. I've never asked if they will also do their photo meters with the same service, but I would absolutely trust them implicitly to do a proper calibration on my meters (at least the Minoltas I have).

Also not that the specifications for these scientific meters are + or - about 3% or so. Thats a much more accurate meter than a photo meter (+ or - 1/3 stop), but that accuracy is of almost no value to photographers because the entire photography process can't be maintained to that level of accuracy.

I'm not one to trust a meter over my experience, so I generally consider a meter reading as a suggestion rather than the 'correct' exposure (as if there were such a thing, anyway).


---Michael

Michael Mutmansky said:

...
There's no harm in using a scientific meter, as long as the linearity of the meter matches film reasonably well. I've never checked to see if that is the case, though, so I don't know if that would be an issue or not. Beyond that, it is probably no better or worse than any other metering approach, except when you are discussing metering out in the field, you would have to remember that your metering results will probably never match those of a photo meter, even if both are calibrated properly.
...

Click to expand...

Michael,

There’s something I don’t understand here. Are you suggesting that photographic light meters are made deliberately non-linear? There's no sound technical reason for that, surely? What reason is there for a luxmeter and a photo lightmeter to disagree by design, other than any differences in spectral sensitivity?

Thanks,
Helen

Helen,

No, I'm suggesting that scientific meters that are designed around human visibility may be non-linear. I've not looked into it to see, but it's not outside to realm of possibility based on human response at different illuminance levels (actually at different luminance leves, since our visual system only works on luminance).

The issue comes into play when considering photopic vision vs. mesopic vs. scotopic. They have different sensitivities and it's not an abrupt transition, but rather a gradual change. So really, if a meter reads to 0.01 fc, and if the meter is built around photopic vision, then the meter is reading well beyond the range of photopic vision, and the values are somewhat meaningless in terms of human vision.

Frankly, I don't think there would be any problems, but I don't want to say that there would be NO problems without having seen the response curves from both meters. As I said, it's probably a more precise meter than any 'regular' light meter, but given the slop in a photo sequence.

---Michael

Michael,

So you're suggesting that scientific light meters are designed with something analogous to a built-in set of Fletcher-Munson audio response compensation curves? Interesting idea, but I'd doubt that is the case, and would expect that the instruments would measure actual light levels, with adjustments for human response applied to the results as needed.

Lee

Lee L said:

Michael,

So you're suggesting that scientific light meters are designed with something analogous to a built-in set of Fletcher-Munson audio response compensation curves? Interesting idea, but I'd doubt that is the case, and would expect that the instruments would measure actual light levels, with adjustments for human response applied to the results as needed.

Lee

Click to expand...

Lee,

That is exactly how scientific illuminance meters are designed. There is no 'actual' illuminance (footcandles or lux) other than the amount based on the CIE standard observer for normal applications.

You could measure the light in other units that are not based on human visibility. Spectrophotometers can do that and some can integrate over spectral sensitivity curves as well. That's outside my sphere of knowledge, however, as I deal with human visibility issues, not plants or other animals, so I work in footcandles or lux.

Basically, when you say 'footcandles' or 'lux', you implicitly include the CIE standard observer spectral sensitivity.


---Michael

MIchael,

Thanks for the response. I suspect that I should have said illuminance meters rather than light meters. I'd not run across that information before.

Lee

Michael,

"No, I'm suggesting that scientific meters that are designed around human visibility may be non-linear."

Why?

The CIE definition of luminous flux (the fundamental property, from which the others can be defined) takes the spectral luminous efficiency of either photopic or scotopic vision into account. These functions are not intensity-dependent (ie they are not a source of non-linearity). There is no other variable observer characteristic (human response) in the definition. The other factors are radiant flux (power, in watts) and luminous efficacy (in lumens per watt - this is not intensity-dependent either*). There is no non-linearity in the equation for luminous flux, there is only the choice of photopic or scotopic spectral response function. So how can there be any scientific reason for a luxmeter (and a photographic lightmeter) to deviate from linearity by design? Error, yes.

Though a human observer is the basis for brightness matching (ie the decision about whether or not two different patches are the same brightness) other, non-human, methods are used for determining relative brightness - like the inverse square law. So a meter that reads 100 lux when it is one metre away from a small source must read 25 lux when it is two meters away. Otherwise it is wrong.

A reading from a luxmeter that consists of a single receptor that is colour corrected by a filter to match the photopic response cannot be corrected to a scotopic reading without spectral data, of course. This is a similar case to converting from Status A RGB densities to visual density: it can't be done correctly. So a simple photopic meter cannot give a scotopic reading - it cannot slide between the two at some particular value, either.

I suspect that it won't be long before USB spectrophotometers hooked up to a PDA or whatever take over from simple luxmeters, just as they have taken over from densitometers.

Best,
Helen

*though it is different for photopic and scotopic cases: 683 for photopic and 1700 for scotopic.

Helen B said:

"... The CIE definition of luminous flux (the fundamental property, from which the others can be defined) takes the spectral luminous efficiency of either photopic or scotopic vision into account..."

Click to expand...

Huh? Oh, yeah! I used to have one of those, but the wheel fell off. ;-)

It was a very badly made wheel to begin with, and should have been recalled a long time ago.

Best,
Helen

Helen B said:

"... It was a very badly made wheel to begin with, and should have been recalled a long time ago..."

Click to expand...

That's good to hear. All these years I thought it was something I had done to the photopic optic -- probably something having to do with its non-linearity or its spectral intensity. But now that you mention it, the rubber was kinda hard. And the lux flux was all rusted.

Thanks for clearing that up. ;-)

I hope that the graph is legible. It compares the CIE photopic luminous efficacy (in lumens/watt) with the spectral sensitivity of 400TMY and P3200TMZ (reciprocal of exposure in ergs/cm² to reach 0.3 above D-min, not a tungsten wedge spectrogram as used by Ilford for example). You can see the big difference at the blue end of the spectrum. (edit: I attached the wrong graph - this one is part log, part linear on the y-axes. I'll fix it later)

If incident (illuminance) readings were being used you could calibrate an industrial light meter for use with film if the light source was fairly consistent in terms of spectral distribution. You probably couldn't use the calibration in other lighting conditions.

Best,
Helen

Too late to edit and replace the graph. Here is a version with both vertical scales linear.

Out of interest I decided to have a look at what the ISO have to say about the spectral response of photographic lightmeters because I knew that they specify the calibration factor in terms of lux or cd/m², whichever is appropriate, not in terms of radiant power.

The ISO suggest that the calibration should be made at a correlated colour temperature of about 4700 kelvins – a ‘reasonable compromise’ between daylight and tungsten. The spectral behaviour of the meter is determined by comparing the meter with a meter conforming to the CIE standard (photopic) observer under two CCT conditions: 2855.6 kelvins and 4700 kelvins.

These are just ISO suggestions and not requirements and there is no need for manufacturers to follow them.

Best,
Helen

Thanks Helen.

Lee

I did not read what other have said, but I find my Pocket Spot meter by METERED LIGHT CO, can read better in lower light conditions then any other meter. It can read down to a -2 actually.

Ryan McIntosh said:

I did not read what other have said, but I find my Pocket Spot meter by METERED LIGHT CO, can read better in lower light conditions then any other meter. It can read down to a -2 actually.

Click to expand...

Thank you Ryan. If you are linterested in better capabilities you might find reading the rest of the thread interesting.

John Powers

Hi !
I'm late on this discution, but :
1) the Luna Pro and Lunasix 3 are the same emters. (different marketing strategies/pricing at the time)
2) all Lunasix 3 /lunapro using 1.35V mercury batteries can be converted to use 2x1.55 V silver oxyde (gossen sells an adapter which is sold new in the actual Lunasix S which is IIRC still made) B&H has it.
3) I've made a lot of "availlable light" photography in the past and relied on my "nose" and guessing to measure the light. I used always the same film (Fuji RDP ) and exposed for 20 to 45 seconds. And make bracketing up to 1min, 2 min and 3 min. I've always got one picture which was technically correct. Bear in mind that YOU SHOULD stop down from wide open to get rid of coma and chromatic aberations seen at these openings. If you calibrate the process, once your eyes are accustomed to the low light level, you can quite predict how much exposure you need.
4) IMHO the tripod problem is more important than the meter problem... finding a stable and sturdy tripod able to hold still a camera during these long exposures and light enough to be carried on location is a real challenge...
Hope this helps !