18% Grey Card. To use it or not.

Alan Edward Klein said:

I appreciate your hard calculations. Could you sum up in layman's terms what it all means?

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It’s about the relationship between the camera setting and what the meter reads.

The equation below is the reflected and incident exposure meter calibration equation. On the left is the camera setting, the middle is the calibration equation for the reflective exposure meter that is concerned with the Luminance of a subject and film speed, and the right equation is the calibration equation for the incident exposure meter with the scene Illuminance and the film speed.



By using the Sunny 16 rule where f/16 at 1/film speed is the exposure, it’s easy to determine the Luminance and Illuminance values, since under the Sunny 16 rule, the shutter speed and film speed cancel each other out, we are only left with a few variables. The variables K and C are known for their base value of 1.16 cd/ft2 and 30 footcandles. To convert to meters, just multiply by 10.76.

What we get is the Luminance value for f/16 at 1/film speed is:

297 cd/ft2 or 3197 cd/m2

The illuminance is:

7680 footcandles or 82,667 lx

Illumination falls on a subject and is reflected toward the subject. By knowing the value of the light falling on the subject and the value of the light reflecting off it, we can determine the Reflectance of the subject. Reflectance implies a Lambertian surface which is considered a theoretically perfect reflector. Its formula is:


What would the Reflectance need to be to produce a value of 297 cd/ft2 from an Illuminance of 7380 footcandles? Plug in the values determined from the calibration equations and


The Reflectance of the subject that will produce a Luminance of 297 cd/ft2 from an Illuminance of 7680 footcandles is 12%. This is good for a metered subject of a single value. Averaging exposure meters will vary depending on the distribution and Reflectance of the values in the scene. Theoretically, under the same Illuminance, the reflected meter will have the same settings as the incident meter when the subject is at 12% Reflectance. To be clear, the meter is reading the Luminance coming off the subject, not the Reflectance. There are a number of variables that can affect the correlation. The meter doesn't care what the Reflectance of the object it is pointed at. For f/16 at 1/film speed, it is looking for 297 cd/ft2.

A short cut is to just use the constants.

DREW WILEY said:

Now factor the influence of intervening atmosphere at any given time and place, and you'll realize why the effective luminance of the moon often differs quite a bit from the theoretical version.

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That might explain the 160 candles per square foot that I am getting.

Chan Tran said:

I think you're mixing foot candle and candela per square foot. As stated sunny 16 is an illuminance measurement in foot candle the refectance is not a factor. The brightness of the moon as seen from earth is a luminance meauserment in cadela per square foot and it does depends on the reflectance of the moon surface. The moon is said to have the reflectance of about 12%.

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I’m calling Sunny 16 the exposure meter dial position which indicates 1/film speed at f/16. Then mentioning the candles per square foot that the arrow points at. 250 on a Weston meter (after adjusting emulsion speed from Weston to ASA).

Also using the exposure formula, taking square root of film speed as the f/stop and the candles per square foot. f/10 at ASA 100 then 1/250 shutter is indicated for 250 candles per square foot.

Bill Burk said:

That might explain the 160 candles per square foot that I am getting.


I’m calling Sunny 16 the exposure meter dial position which indicates 1/film speed at f/16. Then mentioning the candles per square foot that the arrow points at. 250 on a Weston meter (after adjusting emulsion speed from Weston to ASA).

Also using the exposure formula, taking square root of film speed as the f/stop and the candles per square foot. f/10 at ASA 100 then 1/250 shutter is indicated for 250 candles per square foot.

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f/10, ASA 100, 1/250 or EV 14.6 at ISO 100 on your meter means 290 candela per square foot. But you got f/8 instead so it's 190 candela per square foot. If you check the luminance table from the manual you will get that value. It doesn't list candela per square foot but if you take the either the candela per square meter or foot lambert and do a conversion you should get 190 for f/8, 1/250, ASA100. Note the table is rounded off to 2 significant digit. The value should be 2048 rather than 2000.

I think the difference between calling Sunny 16 at 297 candles per square foot and calling it at 250 candles per square foot can be tolerated as estimates or corrected by choosing K=1 (old Weston) or K=1.16 (in candles per square foot units).

My interest in the old units is because it fits Weston meters and the exposure formula. I want to be able to do this stuff in my head. And candles per square foot are the light units of old Westons

To perform the exposure formula in your head, you need to estimate the luminance of your subject in candles per square foot.

I’ll be trying to compare to a 9 foot lambert and a 100 foot lambert standard light source. And I didn’t find Corning 5900 filters, but I did find Kopp 5900 filters.

Chan Tran said:

f/10, ASA 100, 1/250 or EV 14.6 at ISO 100 on your meter means 290 candela per square foot. But you got f/8 instead so it's 190 candela per square foot. If you check the luminance table from the manual you will get that value. It doesn't list candela per square foot but if you take the either the candela per square meter or foot lambert and do a conversion you should get 190 for f/8, 1/250, ASA100. Note the table is rounded off to 2 significant digit. The value should be 2048 rather than 2000.

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If you were looking at my photo, that’s f/10. I got f/8.6 where the decimal unit of the Sekonic is tenths of a whole stop, (f/8.3 = f/9 and f/8.6 = f/10).

Bill Burk said:

If you were looking at my photo, that’s f/10. I got f/8.6 where the decimal unit of the Sekonic is tenths of a whole stop, (f/8.3 = f/9 and f/8.6 = f/10).

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Yes the meter reads f/10 which corresponds to 290 candela per square foot but you said the moon is 2/3 stop darker than your ligh source which makes it f/8 and that would be 190 candela per square foot.

Chan Tran said:

Yes the meter reads f/10 which corresponds to 290 candela per square foot but you said the moon is 2/3 stop darker than your ligh source which makes it f/8 and that would be 190 candela per square foot.

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Right.

I’m conforming my readings to the Weston light markings. Rounding to nearest. Like ASA speeds are rounded to speeds like 64, 80, 100, 125

And the light marking nearest Sunny 16 is 250 candles per square foot.

The moon being nearest 150 candles per square foot.

Unless you want to break with tradition and say that exposure for ASA 100 at Sunny 16 should be 1/130 second.

Or maybe just choose a K value that brings us to 1/100 second.

Bill Burk said:

Right.

I’m conforming my readings to the Weston light markings. Rounding to nearest. Like ASA speeds are rounded to speeds like 64, 80, 100, 125

And the light marking nearest Sunny 16 is 250 candles per square foot.

The moon being nearest 150 candles per square foot.

Unless you want to break with tradition and say that exposure for ASA 100 at Sunny 16 should be 1/130 second.

Or maybe just choose a K value that brings us to 1/100 second.

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Hi Bill. Forgive my ignorance, but how are you measuring the moon with a Weston meter?

By transferring SEI photometer readings to a variable light source and then reading the light source.

Really I use the Sekonic L-758DR for the readings. (This Weston isn’t yet calibrated, I just cheated and dialed it to 250 for purpose of illustration).

Bill Burk said:

Right.

I’m conforming my readings to the Weston light markings. Rounding to nearest. Like ASA speeds are rounded to speeds like 64, 80, 100, 125

And the light marking nearest Sunny 16 is 250 candles per square foot.

The moon being nearest 150 candles per square foot.

Unless you want to break with tradition and say that exposure for ASA 100 at Sunny 16 should be 1/130 second.

Or maybe just choose a K value that brings us to 1/100 second.

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I determine the luminance based on how YOUR meter is calibrated. My meter will be a little different. Your Sekonic 758 is calibrated so that EV0 @Iso 100 is 0.125 Candela per square meter.

I’m seeing EV0 at ISO 100 is 1/15 f/4 (Wikipedia EV article) and that shows 2.5 candles per square foot on a Weston dial or 26.9 candles per square meter. (Multiplying by 10.76 referring to Stephen’s post 201).

If I take 1/8 Candela per square meter and divide by 10.76 to convert, I arrive at about an eighty-sixth of a candle per square foot. It doesn't sound right.

Bill Burk said:

I’m seeing EV0 at ISO 100 is 1/15 f/4 (Wikipedia EV article) and that shows 2.5 candles per square foot on a Weston dial or 26.9 candles per square meter. (Multiplying by 10.76 referring to Stephen’s post 201).

If I take 1/8 Candela per square meter and divide by 10.76 to convert, I arrive at about an eighty-sixth of a candle per square foot. It doesn't sound right.

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EV 0 is 1 sec and f/1.0. EV0 at ISO 100 is .125 candela per square meter and it's 0.0116 candela per square foot. Check you Sekoni meter manual. 1/15 and f/4 is EV8. and yes it's 2.5 candela per square foot on the Weston but it's 2.97 candela per square foot on the Sekonic.

I put together a couple of tables with f/stops, Evs, and Luminance based on 100 film speed. The shutter speed is 1/100. It might be helpful if it's error free.

Note: The pdf has been modified since the initial upload.

Ok switched the Sekonic to EV mode. I can’t read 0 with it but can get it to 1.
I was looking at the wrong context of EV.

You may have given me the break I was looking for. K is different between Weston and Sekonic. I believe Weston is 1 in candles per square foot.

That’s how 250 candles per square foot can be f/10 at 1/250 for ASA 100.

But when the Sekonic says f/10 at 1/250 for ASA 100 it’s reading 290 candles per square foot and applying a different value of K.

The tables will help. Looks like the columns without K are same as when K=1

Bill Burk said:

The tables will help. Looks like the columns without K are same as when K=1

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That's basically what I did, but now I'm thinking it's potentially confusing, so I've uploaded a modified version with just the normal value of K to determine L.