Light meter spectral sensitivity and sensitometry

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tanner

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What is the spectral sensitivity of light meters used in photography? Does is vary with brand, model, built-in, standalone? Does it approximate the luminosity function of the eye? I know that at least in my meter employing a Si diode (Sekonic L-308) there is a magenta looking filter over the sensor that modifies the bare Si spectral response in some way.

Film and sensor spectral sensitivities vary and are very different than that of the eye, at least in the film case, AFAIK. Density curves also use units of lux-seconds, which is puzzling. Shouldn't sensitometry and densitometry units be observer agnostic, i.e. use radiometric units instead of photometric?
 

Mark J

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It's a good subject. I haven't looked into it myself but can comment that Fred Picker ( Zone VI ) used to supply or modify typical spot meters ( Soligor, Pentax etc ) for large-format users, a few years ago. I think he was aiming to make the sensitivity of them closer to typical B&W film.
 

Sirius Glass

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The spectral ranges of films, color slide, color print and black & white are available on the manufacturers' data sheets.
The spectral ranges of light meters is available also from the manufacturers' and can also be found on line by type of sensor.
The spectral ranges of digital cameras are available from the manufacturers.
 

koraks

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The spectral ranges of light meters is available also from the manufacturers' and can also be found on line by type of sensor.

Kindly point me towards the spectral sensitivity of the Sekonic meters, please. Thinking along the same lines, I've searched for a minute or so, but drew a blank.

The spectral ranges of digital cameras are available from the manufacturers.

Not always, I think. Besides, relating such information to light meters is a whole different ballgame; I wouldn't know where to begin and I'm no dummy.
 

ic-racer

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Recent thread:... 14 meters tested in 1961 (diagram below). Would be nice to have updated info as koraks mentions. Sekonic does not provide this information in users manuals I have seen.

Either way ISO 2027-1974 recommends 4700k with this wording but suspicious for it relying on the 1961 data in the diagram.

The light source used for calibrating meters shall operate at such a correlated colour temperature that it represents a reasonable compromise between the requirements of photography under tungsten and daylight lighting conditions, and shall closely match the relative spectral energy distribution of a black body at this temperature. A possible correlated color temperature for this purpose is 4700K.


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BrianShaw

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Hi @Stephen Benskin ! May I please ask for a professional opinion/judgement/assessment? Do any of these different curves really affect photography practices in any significant (statistically or practically) way? To me they are interesting characteristics but not much more than that. Maybe I’m mistaken, though.

And why is the Silicon curve so wonky? I may never have experienced a meter with that kind of cell.
 

ic-racer

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From what Steve posted, one could infer the spectral sensitivity of Sekonic meters like 558 which have Silicone Blue cell.
 

ic-racer

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Good point. I have not taken my 558 apart to see and there is no mention in the literature. My Sekonic L206 has a bluish filter, but it is CDS.
 

koraks

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My Sekonic L358 also has a bluish filter over its sensor; the sensor itself is a fairly large photodiode, and the filter is a dichroic filter that is very strongly magenta when held against the light, but seems pale blue at a perpendicular angle. It's also noticeably larger than the actual sensor, so it's not part of the sensor die itself.
 

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Silicon is very red sensitive and you see a lot of IR silicon photodiode and transistor. They have to filter it to make it a good sensor for visible light or exposure meter.
 

ic-racer

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My Sekonic L358 also has a bluish filter over its sensor; the sensor itself is a fairly large photodiode, and the filter is a dichroic filter that is very strongly magenta when held against the light, but seems pale blue at a perpendicular angle. It's also noticeably larger than the actual sensor, so it's not part of the sensor die itself.

Makes sense.
 

Chan Tran

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Kindly point me towards the spectral sensitivity of the Sekonic meters, please. Thinking along the same lines, I've searched for a minute or so, but drew a blank.



Not always, I think. Besides, relating such information to light meters is a whole different ballgame; I wouldn't know where to begin and I'm no dummy.

Agree. I have never seen any meter manufacturer publish their meters spectral sensitivity. Also digital imaging sensor spectral sensitivity is adjusted during the raw conversion but no manufacturers publish base response of their sensor.
 

Sirius Glass

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The spectral ranges of films, color slide, color print and black & white are available on the manufacturers' data sheets.
The spectral ranges of light meters is available also from the manufacturers' and can also be found on line by type of sensor.
The spectral ranges of digital cameras are available from the manufacturers.

Kindly point me towards the spectral sensitivity of the Sekonic meters, please. Thinking along the same lines, I've searched for a minute or so, but drew a blank.



Not always, I think. Besides, relating such information to light meters is a whole different ballgame; I wouldn't know where to begin and I'm no dummy.


With film one can only use filters to change the response: blue filter with white flash bulbs for daylight film and orange filters for tungsten film in daylight.
Now with digital cameras have the capability to set to White Balance to: Sunny Bright, Daylight, Cloudy, ... and alter the response in post processing well beyond the adjustments with film in the past. Therefore the manufacturers probably do not feel a need to supply the sensors' response curves.
 

DREW WILEY

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Somewhere I still have spectral curves on file for Pentax and related Minolta spot SPD meters. It's a symmetrical bell curve distinctly topping at 550nm green. Since these meters were, and still are, targeted to not only still photographers, but cinema and TV applications, it was important to publish the sensitivity curve, especially in comparison to older CDS meters which had different sensitivity. Via testing, I figured out that the SPD sensor in my Nikon FM2n has essentially the same characteristics as the other SPD's.

Ice-Racer - I don't see how this is necessarily related to some ancient comparison article at all, imagining some kind of unrealistic compromise between between daylight and tungsten sources. Kelvin is kelvin. And if you want a deliberately skewed reading for a particular light balance, there is a filter thread on some of these spotmeters as well.

Brian - therefore SPD readings aren't wonky at all. They're ideal in the sense that they correspond to peak human visual sensitivity itself, right in the middle of the visible spectrum. And yes, the given distinctions are quite significant in real world photography. You need to know, or at least be accustomed to, your own meter's specific personality, with its own spectral sensitivity.

Mark - those Zone VI conversions were calibrated to Tri-X Pan 320 film. Not all pan films are the same in terms of actual spectral sensitivity, and then you've got entirely different films too - othopan, ortho, and blue sensitive, plus different kinds of color film. And the conversion filter installed in those modified units fade anyway over time. The pros and cons can be looked up elsewhere.

Manufacturers of digital cameras no doubt have their own results tucked away somewhere, or otherwise obvious to engineers. The lack of that up front for consumers is probably just a marketing choice. But who knows? ... maybe its in subsection 392.5a.1 on p. 792 of the owner's manual, after you've already read through the first 791 pages just trying to learn how to turn off all the silly apps in order to take a basic picture.
 
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ags2mikon

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I did have a Zone VI Pentax spot meter at one time and it supposedly was calibrated to show a response closer to what black and white sees. How they did it I'm not sure. I can say that when zone testing film it was very close to box ISO. None of my other meters did that. Gossen, Sekonic or Nikon built in meters needed close to 1 stop extra. Anybody know?
 

BrianShaw

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Hmmm. I don’t ever recall any exposure deviation as much as 1 stop with either Gossen or Sekonic meters. Deviations of 1/3 stop or so are not uncommon and rather insignificant in my experience.
 

Sirius Glass

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Hmmm. I don’t ever recall any exposure deviation as much as 1 stop with either Gossen or Sekonic meters. Deviations of 1/3 stop or so are not uncommon and rather insignificant in my experience.

And if they are all properly calibrated they should be the same. The question is for the same type of sensor how does the spectral response compare?
 

ic-racer

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Hmmm. I don’t ever recall any exposure deviation as much as 1 stop with either Gossen or Sekonic meters. Deviations of 1/3 stop or so are not uncommon and rather insignificant in my experience.

Per ISO standard, a meter can be calibrated with any “K” consonant that yields good pictures.
 

DREW WILEY

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ags - Zone VI (Fred Picker) installed certain extra filters to make the sensitivity allegedly match that of Tri-X. But doing so, it inherently mismatched it toward significantly different films. Meanwhile, personal Zone System tests involve all sorts of variables which might or might not be related. A lot also has to do if with the highly questionable advice of Picker to take readings with your chosen contrast filter, like yellow or red, in place. In other words, someone else attempting to standardize their own ZS protocol might come up with results quite different from your own.

Brian - this has nothing to do with meter to meter variation in terms of manufacturer quality control. I've had a number of unmodified Pentax spot meters which identically matched over their full range, and even precisely matched the response of a Minolta Spotmeter F. But the Zone VI modification was exactly that - an internal modification to render a different kind of spectral response. Some people liked it, some didn't. Those can still be serviced by Richard Ritter; but otherwise, they are an evolutionary dead end.
 

Sirius Glass

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There is no ISO standard “K” value. Meters don’t all read the same.

There are standard light sources and standard calibration processes. That does not include a unique ISO standard K value. Some light meters, CDS, were more blue sensitive than non CDS meters, hence no specific K value.
 

DREW WILEY

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There are IRE standards, which Pentax and Minolta adhered to, and which periodic serving matched itself to as well. But if you want to understand what that old Weston meter I inherited is thinking, you need to dig up Merlin, King Arthur, and a dragon or two.

My first camera was an early Honeywell Pentax H1 with an external-coupled Cds averaging meter. I got so accustomed to it that I never botched even a Kodachrome 25 exposure. But when I switched to 4x5 and bought a Pentax spot meter, there was a whole new learning curve. Now after 45 years of using those, they have become the familiar friends instead.
 
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BrianShaw

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Per ISO standard, a meter can be calibrated with any “K” consonant that yields good pictures.

Yep. I believe the ISO standard recommends a range for K (and C) constant but leaves it to the meter makers to pick whatever they so choose. If there are meters that allow user to change those values. I read in several places that the "meaningful difference" between the K/C constant value range recommended by ISO standard only amount to about 1/6 stop. Not sure if that's authoritative or urban legend, though.

I'm well aware of other offsets that some meters offer as exposure compensation aids.

I'm not sophisicated enough to have investigated the value/need to ever do so. Heck... I'm not even sophisticated enough to even care about spectal sensitivites or arithmetic constants, and pay more attention to understandning the metering pattern differences when seeing any possible light reading deviations between meters. I read in several places that the "meaningful difference" between the recommended K/C constant values recommended by ISO standard onlyu amount to about 1/6 stop.
 
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