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

Stephen Benskin · Aug 21, 2015

RobC said:
BUT having measured the light falling on the scene which is a measurement in Lux or lamberts or whatever, they MUST do something with it to convert it to an aperture and/or shutter speed for you to set your camera to. They do this by taking your input parameters which are film speed plus aperture or speed and then spit out a result.

2Ev = A2 / T = B*S / K = E*S/C

Where:

B = Luminance
S = Film Speed
K = Exposure Constant (reflected)
A = Aperture
T = Shutter Speed
E = Incident light
C = Exposure Constant (incident)
Ev = Exposure Value

RalphLambrecht · Aug 21, 2015

Stephen Benskin said:
2Ev = A2 / T = B*S / K = E*S/C

Where:

B = Luminance
S = Film Speed
K = Exposure Constant (reflected)
A = Aperture
T = Shutter Speed
E = Incident light
C = Exposure Constant (incident)
Ev = Exposure Value

View attachment 115187

View attachment 115188 View attachment 115189 View attachment 115190

Steve,Would you please send me the attached jpg to rlambrec@ymail.com?Thanks for that in advance

RobC · Aug 21, 2015

Stephen Benskin said:
2Ev = A2 / T = B*S / K = E*S/C

Where:

B = Luminance
S = Film Speed
K = Exposure Constant (reflected)
A = Aperture
T = Shutter Speed
E = Incident light
C = Exposure Constant (incident)
Ev = Exposure Value

That implies an incident meter will give same reading as a spot meter but it won't unless you point the spot meter at a subject of specific reflectance. So what is that specific reflectance percentage?

I'm asking becasue that percentage will tell you if an incident meter is keyed to a highlight, mid tone or a shadow.

MattKing · Aug 21, 2015

RobC said:
I'm asking becasue that percentage will tell you if an incident meter is keyed to a highlight, mid tone or a shadow.

An incident meter is "keyed" to all three.

RobC · Aug 21, 2015

MattKing said:
An incident meter is "keyed" to all three.

Now we're entering fantasy land.

Xmas · Aug 21, 2015

I thought the incident reading was just like a reflected reading it assumed an average scene?

Good for >99% of shots in experiments.

A spot meter is always zonal?

Note two questions!

MattKing · Aug 21, 2015

RobC said:
Now we're entering fantasy land.

Nope - just pointing out the inapplicability of analyzing the matter that way.

RobC · Aug 21, 2015

Xmas said:
I thought the incident reading was just like a reflected reading it assumed an average scene?

Good for >99% of shots in experiments.

A spot meter is always zonal?

Note two questions!

Spot meter doesn't know if what you're pointing it at is black or white. If thats what you call zonal then yes its always zonal.

Look at this way, if you shine 1024 units of light at something an incident meter will always read 1024 units of light regardless of what the subject is reflecting. But a spot meter will only read what is being reflected from the subject and if the subject is black it will read zero and if the subject is white it will read about 90% of 1024.

That means the formula given above which contains

B*S / K = E*S/C

is wrong becasue the left part is for a spot meter and the right part is for an incident meter and they can only be equal when the reflectance is of a certain percentage. I'm not the scientist or mathematician so we'll leave that to him to tell us what that percentage is. When he knows he'll have learnt something, not least that he can't compare a spot meter reading to an incident meter reading without adding another factor to his formula.

Of themselves they are correct but not when compared for equality as they stand.

RobC · Aug 21, 2015

Xmas said:
I thought the incident reading was just like a reflected reading it assumed an average scene?

What is an average scene? Well the ISO film speed standard suggests its one of 7 1/3 stops or thereabouts. To put the exposure in the middle or towards a highlight of that, a meter must know about it (i.e. be keyed to it).

But I don't think it actually works like that. A meter doesn't measure blackness, it measures lightness (luminance). Knowing the luminance it just subtracts x number of stops via its division by its K(spot) or C(incident) factor to place the exposure somewhere on the curve.

Stephen Benskin · Aug 21, 2015

RobC said:
That implies an incident meter will give same reading as a spot meter but it won't unless you point the spot meter at a subject of specific reflectance. So what is that specific reflectance percentage?

I'm asking becasue that percentage will tell you if an incident meter is keyed to a highlight, mid tone or a shadow.

Reflection meters "see" luminance, but it's possible to calculate an equivalent reflectance. It's broken down in detail in the three bottom attachments in the previous post but here is an excerpt with the calculations:

but a quick way to figure equivalent reflectance is with the equation K / C.

K = 1.16 * pi
C = 30

3.664 / 30 = .121 or 12%

E in incident meter for f/16 at 1/ISO is 7680 footcandles. This is also explained in the three attachments which are part of the Defining K document available online.

Exposure meters are calibrated to an luminance Bg (Lg). (subscript g means the statistically average or mean value). The exposure calculator then wants to take that luminance and produce Eg = 8 together with the shutter speed and f/stop reciprocal relationship based on the film speed to equal a single exposure value Hg for a given film speed regardless of the scene’s illuminance. So, where ever I point the meter, it wants to produce an exposure of Eg/ISO or 8/ISO.

Incident meters also want to produce a an exposure of 8/ISO. Let's plug that in for different film speeds.

8/100 = 0.080
8/125 = 0.064
8/400 = 0.020

Here is the breakdown using the camera exposure equation.

So where does the 8/ISO exposure fall? Here is a breakdown of the values of exposure from 100% to 0.6% reflectance using a film speed of 125. Lg is the statistically average luminance. Notice how the two values of C and B from the exposure meter equations work in the exposure equation. Where does the exposure of 8/125 = 0.064 fall?

So where does that place Lg? The RD of 0 (highlight) is 0.91 logs above or 3 stops. The RD of 2.20 (Shadow) is 1.29 logs or 4 1/3 stops below. The entire range is 2.20 or 7 1/3 stops.

I don't expect anyone to be able to digest everything in a single read through. It took me a number of years to figure all this out. With the Defining K document that I eventually put together, with everything spelled out, and examples with everything linked together, it shouldn't take anyone interested nearly as long.

Bill Burk · Aug 21, 2015

If I understand the explanation, both incident and reflective light meters want to make the film gray. In Zone System terms, meters are aiming for Zone V.

You don't use this as the speed point because 30 seconds of development changes the resulting density by about 0.05 in the middle gray - so it would be too difficult to measure the film speed. Better to measure the film speed much farther down the toe which doesn't change as much.

Incident meters use C to relate the light coming in to middle gray - without considering the subject (because it can't see the subject).
-Incident meters have the advantage of knowing how much light is falling on the scene. The correct exposure is easy to relate to the incoming light.

Reflective light meters use K to relate the light coming in to middle gray - but they attempt to come up with correct exposure by assuming user points the averaging cell at a typical scene.
-Insanity. But statisticians found average user takes same kind of picture most of the time... so it works.

If the subject is an average picture within the statistical studies...

When I use an average meter and I aim the meter at the subject...

I would like to get the same recommended shutter speed and f/stop combinations...

That I get when I aim at a shadow and place it on Zone II and then aim at a highlight and place it at Zone IX (in a normal scene).

And if I find myself in an atypical situation, the old black cat in a coal pile or great pyrenees in a snowdrift...

The Zone System will give me a correct exposure but the average meter will be thrown off.

Xmas · Aug 21, 2015

Xmas said:
I thought the incident reading was just like a reflected reading it assumed an average scene?

Good for >99% of shots in experiments.

A spot meter is always zonal?

Note two questions!

Ok so yes twice and helpful thanks.
When I'm taking photos I don't do the sums, to busy.
Finding the dome or not dropping it is major problem.

If you want to sell the zone you need to say how simple and elegant it is.

Ie it removes the average scene assumption.

My three big problems

Bride not liking her nose
Not liking the grooms nose
Not liking her dress colour

I blamed my Ja distorting lens or Kodak...

MattKing · Aug 21, 2015

Here is my take on the matter.

The incident meter will measure the amount of light that hits the subject.

It will report the setting to use on the camera that will result in the subject being well recorded if the range of reflectances on the subject is reasonably average, with the low reflectances and the high reflectances being within that average range.

If that range of reflectances is not average, the photographer needs to adjust the settings on the camera.

If there is a particular part of the subject that matters to us the most, we may want to adjust the exposure setting on the camera, to try to render that part of the subject in a particular way. The other parts of the subject will fall where they fall, subject to any development modifications that may be employed.

As a human being, we are better able to subjectively evaluate the apparent reflectance of something than the objective value of any light reflecting off it.

RobC · Aug 21, 2015

So for an incident meter 12% is 3 stops less than whatever light is falling on your subject which if we take the average of 7 1/3 stops means an incident meter is keyed towards the highlights for an average subject brightness range. Only where the SBR is less than 6 stops would it be keyed to wards the shadows.

And for a landscape of say 10 stop SBR it is keyed well into the highlights.

So Matt, I apologise, you were right (for low contrast subjects ).

What am I going to do with this information? Nothing because I use a spot meter which it is of no practical use for.

And the the top formula is still written incorrectly because its only correct for a 12% subject reflectance assuming your 12% calculation is right which I have my doubts about becasue E is not equal to B.

Stephen Benskin · Aug 21, 2015

Bill Burk said:
You don't use this as the speed point because 30 seconds of development changes the resulting density by about 0.05 in the middle gray - so it would be too difficult to measure the film speed. Better to measure the film speed much farther down the toe which doesn't change as much.

An important factor is the ratio between the speed point and the metered exposure point and how the ISO speed setting factors in. Like I said in an earlier post, the metered exposure point can be calculated as 8/ISO. So for a 125 speed film that would be 0.064 lxs. Black and white speed point is 1.0 log-H below this point. Using the film speed equation to determine what the exposure would be for a 125 speed film at the speed point, we get 0.80/125 = 0.0064 lxs.

So if the film has a speed of 125 and you set the exposure meter to 125, then the metered exposure will fall 1.0 log-H above this point. If the film has a speed of 125 but you set the exposure meter to 80, then the metered exposure will fall 1.10 log-H to the right of the speed point changing the ratio.

The ratio was chosen so that the luminance of the average scene would fall within the portion of the film's characteristic curve that will render quality prints. The speed point is where it is because of the importance of maintaining shadow detail with black and white. It defines the limiting gradient (not density and not at 0.10 over Fb+f).

markbarendt · Aug 21, 2015

RobC said:
That implies an incident meter will give same reading as a spot meter but it won't unless you point the spot meter at a subject of specific reflectance. So what is that specific reflectance percentage?

They will agree.

RobC said:
I'm asking becasue that percentage will tell you if an incident meter is keyed to a highlight, mid tone or a shadow.

MattKing said:
An incident meter is "keyed" to all three.

RobC said:
Now we're entering fantasy land.

All any light meter needs to know to calculate a camera setting that will let in "the right amount" of light is: "how much light is falling on the scene" (an incident meter finds the luminance level internally and objectively) and by knowing "how the film will respond" (the EI or ISO of the film in use set by the user).

To find the "luminance level" with a spot meter you need to measure the reflectance from a specific subject in the scene and then judge how far to offset that reading to correct to your tested standard. If a known target is used then finding the luminance with a spot becomes objective but still requires external math.

When both metering styles are done objectively they will agree.

RobC · Aug 21, 2015

markbarendt said:
They will agree.

All any light meter needs to know to calculate a camera setting that will let in "the right amount" of light is: "how much light is falling on the scene" (the luminance level is found by taking a reading) and; "how the film will respond" (the EI or ISO of the film in use set by the user).

To find the "luminance level" with a spot meter you need to measure the reflectance from a specific subject in the scene and then offset that reading by how much the subject differs from your tested standard.

I suggest you go out with an incident meter and a spot meter and see if you can get them to give the same reading.

Bill Burk · Aug 21, 2015

RobC said:
I suggest you go out with an incident meter and a spot meter and see if you can get them to give the same reading.

I can when I point the spot meter at the 12.7% reflectance patch on my gray card.

We can use 12% as the point of agreement, it's close enough for discussion.

RobC · Aug 21, 2015

Bill Burk said:
I can when I point the spot meter at the 12.7% reflectance patch on my gray card.

We can use 12% as the point of agreement, it's close enough for discussion.

I don't make photographs of grey cards. And if you do meter one I think we've shown that it is keyed towards the highlights so the old meter a zone III has just flown out the window and seems to be heading to wards the highlights, say a zone VII if you have a 10 stop SBR which seems to be in line with what have been saying is the way I work (but not using a grey card).

Stephen Benskin · Aug 21, 2015

RobC said:
I suggest you go out with an incident meter and a spot meter and see if you can get them to give the same reading.

They will if used correctly, eg angle, target, sun, and all things being equal. This is not an easy task.

And the the top formula is still written incorrectly because its only correct for a 12% subject reflectance assuming your 12% calculation is right which I have my doubts about becasue E is not equal to B.

You need to be more specific. I used a lot of equations. But with E and B or with current nomenclature E and L, illuminance (E) falls on a surface and the amount reflected (L) Luminance is determined by the reflectance. Reflectance being Luminance /Illuminance. To find Luminance it becomes L = R * E. Because we need to change the unit of measurement for L so that E and L are the same we need to divide by pi. From the earlier post E = 7860 and R = 0.121.

So L = (0.121 * 7860) / pi
L = 296 cd/ft2. From the earlier post L (B) = 297.

This is how the Illuminance and Luminance relate in the incident and reflection exposure meters. The Luminance value plugs into the exposure equation and produces the predicted exposure value. So we can go from the light falling on an object, to metering the object, to the reflected light passing through a lens and striking the film at the predicted point based on film speed.

RobC · Aug 21, 2015

Stephen Benskin said:
They will if used correctly, eg angle, target, sun, and all things being equal. This is not an easy task.

But the point is you can only make them match with a specific reflectance percentage subject. And that means in normal usage an incident and spot meter won't match unless you are using a reference subject for your metering which really is of no use because if you are then you may as well just use an incident meter and forget the zone system all together.

The major advantage of a spot meter and calibrating the way I do which is in line with what the zone says its trying to achieve is that you can put any part of the subject on any value you want. It isn't about direct one to one tone reproduction, it's about making creative decisions at the taking point about the way you want to represent the subject in the print.

Bill Burk · Aug 21, 2015

The spotmeter is keyed to middle gray and you choose how the reading relates to the exposure you want.

You can meter and open up three stops to get highlights placed high. I can meter and stop down three stops to place shadows low.

If you want a spotmeter that can be keyed to highlights, then you need an SEI Photometer.

There's a black spot on the other side for me.

Stephen Benskin · Aug 21, 2015

RobC said:
But the point is you can only make them match with a specific reflectance percentage subject. And that means in normal usage an incident and spot meter won't match unless you are using a reference subject for your metering which really is of no use becaause if you are then may as well just use an incident meter and forget the zone system all together.

That's not what it means. They are points of calibration, then there's the reciprocal relationship between the f/stop and shutter speed. The equation balances with A2/T on one side. If the value of L or E changes then A or T also have to change. Let's not forget the Ev part on the far left that comes with the whole Additive System extravaganza.

I can not stress this point enough. All meters want to create a film plane exposure of 8 lx * 1/ISO or 8/ISO = Hg.

Bill Burk · Aug 22, 2015

markbarendt said:
Just meter the shadow using an EI that gives you the camera setting, done.

The SEI Photometer is good for that, you set the ASA opposite a black mark for shadow readings, or opposite the white mark for highlight readings.

As to the general idea, I find it more convenient to have the meter set at an exposure index that I believe is correct, and then use Zone System terminology to place the reading on the Zone that I want the spot to be. I like being able to confirm my readings by switching to incident mode and taking a quick reading.

I find it inconvenient enough to change ISO when I place a filter on my lens, because that's the safest way for me to incorporate filter factors. If you know a better way to deal with filter factors (I know about the Hutching's method to meter through the filter and apply yet another factor from a chart).

RobC, I don't mind if you want to place high readings on Zone VIII and allow shadows to fall where they may. Fred Picker, one of the strongest proponents of the Zone System, shifted his own philosophy away from placing shadows towards placing highlights.

I couldn't find the article that marks when he made the change because I think I'm missing the issue. But I did find a later newsletter where he explained it...

In the Zone VI Newsletter Number 63, July, 1990 Fred Picker wrote

I want... a rich, fat, fully exposed, fully developed negative. (Maximum Printable Density.) That's the reason I "place the high value on VIII and take the picture." This places the low values as far up the film's straight line as possible which gives me all the low value separation possible.

markbarendt · Aug 22, 2015

RobC said:
I suggest you go out with an incident meter and a spot meter and see if you can get them to give the same reading.

Been doing that for years, it works.

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

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

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