Why is Zone System EI often about half rated ISO/ASA?

Bill Burk · Aug 30, 2015

Got my answer. We can trust the meter and chart...

Bill Burk said:
Probably would be difficult - I won't propose to do that since I have an incident meter.

But then incident meter applies C to give the film plane exposure recommendation.

When using an incident exposure meter to try to evaluate incident light... Do published charts that convert EV[100] to incident light values... back out C? If they back out C I think they would have a better chance of correctly telling how much light is falling on the scene.

Wikipedia article on EV shows that the chart includes C just like we have to include the 100... So the chart is correct for incident meters with 100 speed and C is 250:

EV[100][C=250]

Bill Burk said:
Or is EV[100] a film plane exposure that incorporates C... making those published charts slightly wrong?

The charts are correct, and just as they are incomplete without the film speed assumption [100], they are incomplete without the C assumption [250].

So knowing both speed 100 and C 250, EV 0 is 2.5 lux.

Stephen Benskin · Aug 30, 2015

In ISO 2720 Photography - General purpose photographic exposure meters (photoelectric type) - Guide to product specification 1974.05.15, the calibration formula for K1 is L*t*S / A2. What happened to the detailed formula from the ANSI 1971 standard? It's not needed. The purpose of this section of the standard is for the calibration of the K factor. That doesn't mean the other equation is no longer relevant. It's just not necessary for the purposes of the standard. I suspect all the variables are still considered when designing meters.

The calibration equation is very close to the camera exposure equation. The only variable missing is the constant q. The equations for q and K utilize many of the same variables as they are both basically light loss constants. K has a few additional variables relating to the meter and one for the exposure constant (Eg).

ISO calibration formula: 3195.7*100*1/125 / 162 = 12.48
Camera Exposure Equation: .65*3195.7*100*1/100 / 162 = 8.11 (this is Eg also known as P from 8/ISO, and is also the exposure constant K`1 from the K equation)

These two equations help to show the interconnection of the different facets of the process and the relationship between the constants can be expressed as:

K = P / q

RobC · Aug 30, 2015

Bill Burk said:
Got my answer. We can trust the meter and chart...

Wikipedia article on EV shows that the chart includes C just like we have to include the 100... So the chart is correct for incident meters with 100 speed and C is 250:

EV[100][C=250]

The charts are correct, and just as they are incomplete without the film speed assumption [100], they are incomplete without the C assumption [250].

So knowing both speed 100 and C 250, EV 0 is 2.5 lux.

You said in earlier post that you are using a flat diffuser and that with a dome it reads 3 stops different. A couple of things occur to me. Firstly that you should be replacing the flat diffuser with the dome and not combining both and also that when using the dome you should be using something more like 340 for C. There should only be a difference of around half a stop between the two and not 3 stops. But I don't know your meter so maybe 3 stops is correct, especially if its an old unit that doesn't have a mode switch for dome or flat or the flat can't be taken out.

C for a dome should be more like 340 and not 250.

Bill Burk · Aug 30, 2015

RobC said:
You said in earlier post that you are using a flat diffuser and that with a dome it reads 3 stops different. A couple of things occur to me. Firstly that you should be replacing the flat diffuser with the dome and not combining both and also that when using the dome you should be using something more like 340 for C. There should only be a difference of around half a stop between the two and not 3 stops. But I don't know your meter so maybe 3 stops is correct, especially if its an old unit that doesn't have a mode switch for dome or flat or the flat can't be taken out.

C for a dome should be more like 340 and not 250.

All correct and I'd use the flat disc when trying to measure lux. I was thinking if we could get a lux reading and try different meter techniques and plans... we would get different results that we could discuss.

The test that I did was a little different than you're thinking.

The thing that meters about three stops is the bare selenium cell exposed by removing all discs and slides... compared to with the cell covered with the flat disc. I tried the dome disc it's also around three stops. This difference isn't part of calibration (to use a reading of the bare cell). But it illustrates the engineering idea that the light coming in is reduced significantly by that disc to bring it to some kind of gray.

Bill Burk · Aug 30, 2015

I was also thinking of drawing some pictures of the fundamentals... and if we could start with lux... at least I could draw the first one.

RobC · Aug 30, 2015

Bill Burk said:
All correct and I'd use the flat disc when trying to measure lux. I was thinking if we could get a lux reading and try different meter techniques and plans... we would get different results that we could discuss.

The test that I did was a little different than you're thinking.

The thing that meters about three stops is the bare selenium cell exposed by removing all discs and slides... compared to with the cell covered with the flat disc. I tried the dome disc it's also around three stops. This difference isn't part of calibration (to use a reading of the bare cell). But it illustrates the engineering idea that the light coming in is reduced significantly by that disc to bring it to some kind of gray.

I expect the meter manufacturer knows what the filter factor of the diffusion disc and dome are and have that factored into the light sensor calculations or adjusted its sensitivity electronically. It figures that they must have otherwise your meter would be useless.

RobC · Aug 30, 2015

Bill Burk said:
All correct and I'd use the flat disc when trying to measure lux. I was thinking if we could get a lux reading and try different meter techniques and plans... we would get different results that we could discuss.

The test that I did was a little different than you're thinking.

The thing that meters about three stops is the bare selenium cell exposed by removing all discs and slides... compared to with the cell covered with the flat disc. I tried the dome disc it's also around three stops. This difference isn't part of calibration (to use a reading of the bare cell). But it illustrates the engineering idea that the light coming in is reduced significantly by that disc to bring it to some kind of gray.

And has your meter got an adjustment dial/screw on it to calibrate it. If it has and has been moved from factory setting, then it will never give you a reading that you can test against an accurate EV reading unless you use an accurate reading from somewhere to re-adjust your meter. If you bought it used then you won't know unless you test it against a known accurate reading.

Still sunny 16 rule should get you close enough and the lux is listed. If the meter says EV 15 then that's 81,920 lux.

If your meter is correctly adjusted then you should be able to ake a reading in EV and convert it to whatever you like.

Bill Burk · Aug 30, 2015

The Sekonic L-28c has a zero adjust, but it's so old that it would not be my primary reference.
-It's the one I can take the discs off of to see how much light is cut.

I only bring up the disc cutting light by about 3 stops because I thought it was interesting. I see a white disc and think "OK it must be reading white" but then I figure that no, it's cutting the light at least a little. I was surprised to find how much and thought it would be worth sharing that thought.

I got the Sekonic L-758DR new and it has adjustments for calibration constant but I have left these at factory settings.

Stephen Benskin · Aug 30, 2015

Here is an illustration of the process.

RobC · Aug 30, 2015

So whats stopping you from drawing your pictures when you have a meter which can probably give you the numbers in lux or cd/m^2 or fl/lb

RobC · Aug 30, 2015

Bill Burk said:
The Sekonic L-28c has a zero adjust, but it's so old that it would not be my primary reference.
-It's the one I can take the discs off of to see how much light is cut.

I only bring up the disc cutting light by about 3 stops because I thought it was interesting. I see a white disc and think "OK it must be reading white" but then I figure that no, it's cutting the light at least a little. I was surprised to find how much and thought it would be worth sharing that thought.

I got the Sekonic L-758DR new and it has adjustments for calibration constant but I have left these at factory settings.

It doesn't read white or grey it reads luminance. Colour is a human perception and not a physical property.

Xmas · Aug 30, 2015

The cell does not read the same as our eye in dim light when we only see mono or the same as ortho or super pan.

So there are a few fudge factors in the ISO, you need to read.

My dome was 1/3 of a stop offset on he first sample as it is clearly designed with an embedded filter but I need to burnish it as it is contimated with a residue, zippo failed to remove. There should be a difference cause no scene is ever average? Unless the calculator has the offset when the indicated EV should be different.

Sirius Glass · Aug 30, 2015

RobC said:
It doesn't read white or grey it reads luminance. Colour is a human perception and not a physical property.

Light meters do not measure the light with a spectrum that is not the same as the human eye. Each type of sensor has its own spectrum; for example CdS sensors see more blue than other sensors.

Sirius Glass · Aug 30, 2015

The diffusion disc and dome measure different types of light. A diffusion disc measures luminance, while a dome measures incident light.

Stephen Benskin · Aug 30, 2015

RobC said:
It doesn't read white or grey it reads luminance. Colour is a human perception and not a physical property.

Color is actually psychophysical. I'm interpreting your use of human perception as being psychological.

Stephen Benskin · Aug 30, 2015

Xmas said:
So there are a few fudge factors in the ISO, you need to read.

Would you mind elaborating on these factors?

Bill Burk · Aug 30, 2015

Stephen Benskin said:
Here is an illustration of the process.

View attachment 115673

That's a great diagram Stephen...

Xmas · Aug 30, 2015

Stephen Benskin said:
Would you mind elaborating on these factors?

There was a compensation mentioned in Nelsons '59 paper for colour temperature of sunlight, the test light and the meter cell? Cine mono stills has an ISO for standard day and in cal photo flood or my current can has (5222).

I was not being pedantic some of us still use selenium or CdS where you need to be aware of colour temperature.

Stephen Benskin · Aug 30, 2015

Xmas said:
There was a compensation mentioned in Nelsons '59 paper for colour temperature of sunlight, the test light and the meter cell? Cine mono stills has an ISO for standard day and in cal photo flood or my current can has (5222).

I was not being pedantic some of us still use selenium or CdS where you need to be aware of colour temperature.

Agreed. The K equation has a variable for the "actual spectral response ratio of light detector."

If you're referring to Safety Factors, that was an adjustment to the speed constant to compensate for a change in the color temperature of the light in the sensitometer. They changed it from sunlight to daylight. The Stimson paper An Interpretation of Current Exposure Meter Technology available HERE, has a very nice section on the spectral sensitivity of photo cells and calibration color temperature.

Photo Engineer · Aug 30, 2015

You know, photography is an art, and all of this folderol will NOT necessarily take a good photography.

I've said before that all of my PWs were taken by just metering or by guess and by golly (sunny 16).

PE

Xmas · Aug 30, 2015

Hi Ron

I use sunny f/8 since '61 unless the scene is contrasty when I meter unless I was using Kodachrome when it was dome on brides nose from 1st shot till last.

Noel

Photo Engineer · Aug 31, 2015

Typo in my last post. Sorry, but the meaning is still clear.

As for imaging, this discussion and my comment are all related to color negative or B&W negative imaging. Reversal is a whole different thing!

PE

SodaAnt · Aug 4, 2023

Bill Burk said:
As Michael R 1974 says, these are important.

ISO

-Difference between the metered exposure point and the ISO speed point is 1.0 log-H (3 1/3 stops)

Sorry to resurrect this ancient zombie thread, but I'm trying to understand something here, so bear with me.

I don't understand the nomenclature of "1.0 log-H (3 1/3 stops)". In standard mathematical notation this would be (1.0 times the log of something) - (H times a constant) (3-1/3 stops in this case). The log of what? That part seems to be missing, and is it actually H times 3.33 stops or is the "3 1/3" the value of H itself and that's what gets subtracted from the first term? I'm assuming that "log" here is the common log (base 10) and not the natural log (base e)--is that correct?

Stephen Benskin · Aug 4, 2023

log-H means log exposure at the film plane, and it does use base 10.

SodaAnt · Aug 4, 2023

Stephen Benskin said:
log-H means log exposure at the film plane, and it does use base 10.

That makes sense. I guess you just have to be familiar with the notation used. What does the "(3 1/3 stops)" part mean?

Why is Zone System EI often about half rated ISO/ASA?

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