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

Stephen Benskin · Aug 27, 2015

Bill Burk said:
Here's another random observation...

I have a selenium cell incident meter, an old Sekonic.

Without the white dome, the meter reads three stops higher than with the white dome.

So even though the dome is colored white...

It's cutting about three stops and the meter cell sees gray.

And thus the reason for the Incident meter's constant "C."

Bill, you should keep in mind that average gradient and how it corresponds to development indication, N, N+1, etc, is reliant upon the enlarger light source and the test parameters of the paper. For an average gradient of 0.50 to equal -1, it has to be based on a diffusion enlarger and normal being 7 1/3 stops with the paper LER being tested using the standard method. The reason why RonC can fit a ten stop range onto a grade two paper with only -1 processing is because his test utilizes the full range of the paper. Normally, if someone had a ten stop range and wanted it to fit in within the 0.04 and 90% Dmax, the gradient would have to be around 0.43. There's a technical reason why 0.04 above Pb+f and 90% of Dmax was chosen, but the following example of two paper curves with the same LER is a good illustration of one of them. Notice the difference in log-H between 90% of Dmax and Dmax.

Let's take a look at what the LER from a full paper range would look like. Conservatively, let's say we add 0.20 from both ends to the 1.15 LER for 1.55. What would the aim CI be for a ten stop range?

1.55 / 3 = 0.517. With flare 1.55 / (3-.30) = 0.574.

The 0.517 is close to what RonC is getting and the 0.574 proves what I was saying about accent black and specular highlights.

Bill Burk · Aug 28, 2015

Stephen Benskin said:
And thus the reason for the Incident meter's constant "C."

Bill, you should keep in mind that average gradient and how it corresponds to development indication, N, N+1, etc, is reliant upon the enlarger light source and the test parameters of the paper. For an average gradient of 0.50 to equal -1, it has to be based on a diffusion enlarger and normal being 7 1/3 stops with the paper LER being tested using the standard method. The reason why RonC can fit a ten stop range onto a grade two paper with only -1 processing is because his test utilizes the full range of the paper. Normally, if someone had a ten stop range and wanted it to fit in within the 0.04 and 90% Dmax, the gradient would have to be around 0.43. There's a technical reason why 0.04 above Pb+f and 90% of Dmax was chosen, but the following example of two paper curves with the same LER is a good illustration of one of them. Notice the difference in log-H between 90% of Dmax and Dmax.

View attachment 115520

True, the "eye match" method leaves a big door open for interpretation.

I don't mention it enough, but you are right to point out that paper and enlarger factor into the Contrast Index that I choose to develop my film to. It's not built into my tests, so I can change them if I want to.

RobC · Aug 28, 2015

Xmas said:
Hi Rob

My meter is the same if you point it at a zone 5 area and tell the meter it is zone 5 it gives the 'correct' exposure for zone 5.
It is however too difficult for me to do that. So I only ever spot zone 1...

I'd merely note that zone 5 is not always not always the mid tone... But it is always zone 5.

Agreed, trying to meter a midtone with with a spot meter is prone to errors of judgment. I already said this earlier in topic.

But what isn't prone to error is metering the brightest highlight which you want to retain full or partial textural detail and placing on zone 7 or 8. Same applies to deepest shadow you want to retain full or partial detail and placing on zone 2 or 3.

RobC · Aug 28, 2015

Bill Burk said:
As far as standards are concerned, there is enough documentation that the shadow 0.10 density point is used to identify the 0.3 x average gradient point... where meters are calibrated to by standards. The standards amazingly passed committee and peer review... shadow calibration stuck.

For a spot meter the forumla is:

EV = Exposure Value ( F1 @ 1s = EV 0 etc)
B = meter reading in cd/m^2
S = ISO Film Speed
K = calibration constant (we'll say 12.5 which = 8%)

2^EV = B*S/K

This formula actually works out (if you use cd/m^2 because 2^EV gives result based on cd/m^2)

There are no other forumlas involved unless they are buried in the meter and we are not told about them.

I have taken a reading which provides me with the cd/m^2 number. The left and and right sides are approximately equal which gives single decimal point accuracy for EV value which is more accurate than you can set on your aperture or shutter speed.

The right hand portion of the formula multiplies up by the film speed to achieve the film sensitivity adjustment and then divides down by K to give the offset from the reading to give the required 0.1 lux.seconds exposure for placement in middle of curve.

How anyone can say the adjustment is upwards I have no idea. Division creates a smaller number not a bigger number. Trying to throw further formulas into the mix which don't exist in the given formula doesn't cut it.

If the argument is that ISO speed has inherent factors built into it then of course it does. But I don't care because the multiply UP of film speed is there to reduce the required exposure obtained by the division by K. So infact both the multiplication and the division are effectively reducing the final exposure required. Remember, the bigger the EV number the less expsoure is required. The calculation is always downwards contrary to what we are being told about how a meter works. Nowhere is the calculation is there an offset upwards from film speed point.

Bill Burk · Aug 28, 2015

Looks like you have an exposure computation formula based on a given amount of illumination.

And I have a meter calibration formula based on a minimum useful exposure.

And we are not meeting on the idea that one works up and one works down.

Stephen Benskin · Aug 28, 2015

RobC said:
For a spot meter the forumla is:

EV = Exposure Value ( F1 @ 1s = EV 0 etc)
B = meter reading in cd/m^2
S = ISO Film Speed
K = calibration constant (we'll say 12.5 which = 8%)

2^EV = B*S/K

This formula actually works out (if you use cd/m^2 because 2^EV gives result based on cd/m^2)

There are no other forumlas involved unless they are buried in the meter and we are not told about them.

I have taken a reading which provides me with the cd/m^2 number. The left and and right sides are approximately equal which gives single decimal point accuracy for EV value which is more accurate than you can set on your aperture or shutter speed.

The right hand portion of the formula multiplies up by the film speed to achieve the film sensitivity adjustment and then divides down by K to give the offset from the reading to give the required 0.1 lux.seconds exposure for placement in middle of curve.

How anyone can say the adjustment is upwards I have no idea. Division creates a smaller number not a bigger number. Trying to throw further formulas into the mix which don't exist in the given formula doesn't cut it.

If the argument is that ISO speed has inherent factors built into it then of course it does. But I don't care because the multiply UP of film speed is there to reduce the required exposure obtained by the division by K. So infact both the multiplication and the division are effectively reducing the final exposure required. Remember, the bigger the EV number the less expsoure is required. The calculation is always downwards contrary to what we are being told about how a meter works. Nowhere is the calculation is there an offset upwards from film speed point.

No idea what your up and down fixation is about, but could you show your work on the 8% and 0.1 lxs as being the exposure for the middle of the curve. BTW, another equation is of determining the value of K.

RobC · Aug 28, 2015

Stephen Benskin said:
No idea what your up and down fixation is about, but could you show your work on the 8% and 0.1 lxs as being the exposure for the middle of the curve. BTW, another equation is of determining the value of K.

Why do I need to work it out from 1st principles. It would all be guesswork unless you actually know the facts about how the meter manufacturer really does it.

And that note I would remind you that YOU posted in your formulas in this topic:

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

from that I'm using

2EV = B*S / K

which is the part for a reflection meter (spot meter).

i.e. exactly what you have posted which is exactly what my meter uses. I know that becasue I have it in writing from Minolta who I asked the question to many years ago. I asked Minolta UK who didn't know so they asked Minolta Japn who reponded with that exact formula (2EV = B*S / K) stating it was done in cd/m² and that their K =14 and that it was done to make the exposure of the middle of the curve 0.1 lux.seconds.

Why would I want to do any work on that. I take Minoltas word on it.

If you want to tell me that Minolta were wrong I'll leave it you to worry about it and tell them what their formula should be. I rekon they would think the same as I do or more probably be as interested as I am in hearing your formulas.

And you know perfectly well that I am making the point about the calculation being done down from the reading becasue you say it is being made up from the speed point of the film. And you are doing that even though the formula you posted is clearly showing it's being calculated down from the reading and then trying to divert attention from that.

I don't need to know any more about it. I know sufficiently well about how my meter works and how to use it. The proof is in the zone patch test and not a formula.

And I would remind you once again that your posted formula is wrong:

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

because following is incorrect

B*S / K = E*S/C

becasue it is only true for a specific reflactance value of the subject being metered by the spot meter which you have ommitted from the equation and failed to acknowledge when prompted to do so.

RobC · Aug 28, 2015

Bill Burk said:
Looks like you have an exposure computation formula based on a given amount of illumination.

And I have a meter calibration formula based on a minimum useful exposure.

And we are not meeting on the idea that one works up and one works down.

where did you get your meter calibration formula from and what is it?

Stephen Benskin · Aug 28, 2015

RobC said:
Why do I need to work it out from 1st principles. It would all be guesswork unless you actually know the facts about how the meter manufacturer really does it.

And that note I would remind you that YOU posted in your formulas in this topic:

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

from that I'm using

2EV = B*S / K

which is the part for a reflection meter (spot meter).

i.e. exactly what you have posted which is exactly what my meter uses. I know that becasue I have it in writing from Minolta who I asked the question to many years ago. I asked Minolta UK who didn't know so they asked Minolta Japn who reponded with that exact formula (2EV = B*S / K) stating it was done in cd/m² and that their K =14 and that it was done to make the exposure of the middle of the curve 0.1 lux.seconds.

Why would I want to do any work on that. I take Minoltas word on it.

If you want to tell me that Minolta were wrong I'll leave it you to worry about it and tell them what their formula should be. I rekon they would think the same as I do or more probably be as interested as I am in hearing your formulas.

And you know perfectly well that I am making the point about the calculation being done down from the reading becasue you say it is being made up from the speed point of the film. And you are doing that even though the formula you posted is clearly showing it's being calculated down from the reading and then trying to divert attention from that.

I don't need to know any more about it. I know sufficiently well about how my meter works and how to use it. The proof is in the zone patch test and not a formula.

And I would remind you once again that your posted formula is wrong:

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

because following is incorrect

B*S / K = E*S/C

becasue it is only true for a specific reflactance value of the subject being metered by the spot meter which you have ommitted from the equation and failed to acknowledge when prompted to do so.

Why can't you just answer a simple question? You said 8%. I'm asking you to prove it. You could be right, but please support the claim.

How can 0.1 lxs work for of Hg for all film speeds? Different film speeds have different target exposures for Hg. My numbers come from Connelly's Calibration Levels of Films and Exposure Devices. It's basically Hg = 8/ISO. 100 = 0.08 lxs, 125 = 0.064 lxs, 400 = 0.02 lxs. Maybe I'm missing something but according to the K equation K`1 = ~ 8 lx. Using the exposure equation q*Lg*t*Sa / A2 = P or 8. The constant q = 0.65.

I'm not blowing off the 0.1 lxs. I'll have to look into because maybe the technician meant something else and I don't like to dismiss things out of hand.
The equation I posted in #156 also included the page from the standard.

BTW, the K equation can be found on Post #213.

And you know perfectly well that I am making the point about the calculation being done down from the reading becasue you say it is being made up from the speed point of the film.

.

Still no cue what you are saying.

And please stop being belligerent.

RobC · Aug 28, 2015

Stephen Benskin said:
Why can't you just answer a simple question? You said 8%. I'm asking you to prove it. You could be right, but please support the claim.

How can 0.1 lxs work for of Hg for all film speeds? Different film speeds have different target exposures for Hg. My numbers come from Connelly's Calibration Levels of Films and Exposure Devices. It's basically Hg = 8/ISO. 100 = 0.08 lxs, 125 = 0.064 lxs, 400 = 0.02 lxs. Maybe I'm missing something but according to the K equation K`1 = ~ 8 lx. Using the exposure equation q*Lg*t*Sa / A2 = P or 8. The constant q = 0.65.

I'm not blowing off the 0.1 lxs. I'll have to look into because maybe the technician meant something else and I don't like to dismiss things out of hand.
The equation I posted in #156 also included the page from the standard.

BTW, the K equation can be found on Post #213.

.

Still no cue what you are saying.

And please stop being belligerent.

Because dividing by 12.5 is the same as multiplying by 8%. I would have thought that for some one producing the formulas you do it would have been obvious to you. It should be obvious to anyone that dividing by 12.5 gives 8% of whatever is being divided. It's elementary shoolboy mathematics and I'm no mathematician.

And K= 12.5 (8%) is what sekonic use for their current spot meter K.

Xmas · Aug 28, 2015

RobC said:
where did you get your meter calibration formula from and what is it?

Meters calibrated for mono are calibrated (at least) for four stops above base fog, flare plus detectable silver?
Most Minolta spots are calibrated for typical slide films?
So as the only currency between mono and slide is for zone 5 ie which 'happens' to be the mean for slide and mono average reflective readings for a 'typical' scene, by 'concesus'.
This calibration did not alter in 1961 the safety factor in mono ASA was discarded, (reduced to 1.2).

I use sunny side f/8 post 1961, I only meter for zone 1 contra jour and high contrast.

If you are using mono you got the wrong meter? It is way easy to get clear film in zone 1.

But you are correct it is not datumed off zone1?

Bill Burk · Aug 28, 2015

RobC said:
where did you get your meter calibration formula from and what is it?

I get it from Stephen, so you probably already have it.

But you know how when you establish the Exposure Index for the Zone System tests, you look at the Zone I patch? That's what it's about. Finding the exposure that causes the film to meet a specific condition... a really low density that just begins to be useful.

Probably factors into your formula as "S" since it's where the film speed is coming from.

Stephen Benskin · Aug 28, 2015

RobC said:
Because dividing by 12.5 is the same as multiplying by 8%. I would have thought that for some one producing the formulas you do it would have been obvious to you. It should be obvious to anyone that dividing by 12.5 gives 8% of whatever is being divided. It's elementary shoolboy mathematics and I'm no mathematician.

And K= 12.5 (8%) is what sekonic use for their current spot meter K.

Missed that. It happens to me all the time. I think something I've written or graph displayed is obvious and nobody gets it or walks away with something completely different. Thanks for clarifying.

RobC · Aug 28, 2015

Bill Burk said:
I get it from Stephen, so you probably already have it.

But you know how when you establish the Exposure Index for the Zone System tests, you look at the Zone I patch? That's what it's about. Finding the exposure that causes the film to meet a specific condition... a really low density that just begins to be useful.

Probably factors into your formula as "S" since it's where the film speed is coming from.

Yes but that has nothing to do with how my spot meter actually works. It's just adjusting EI to be where I want it and making that match what my spot meter is going to show me. i.e. are we calibrating our spot meter to the film and dev or our film and dev to the spot meter. If you think about it, it's the latter. There is a difference.

RobC · Aug 28, 2015

Xmas said:
Meters calibrated for mono are calibrated (at least) for four stops above base fog, flare plus detectable silver?
Most Minolta spots are calibrated for typical slide films?
So as the only currency between mono and slide is for zone 5 ie which 'happens' to be the mean for slide and mono average reflective readings for a 'typical' scene, by 'concesus'.
This calibration did not alter in 1961 the safety factor in mono ASA was discarded, (reduced to 1.2).

I use sunny side f/8 post 1961, I only meter for zone 1 contra jour and high contrast.

If you are using mono you got the wrong meter? It is way easy to get clear film in zone 1.

But you are correct it is not datumed off zone1?

I believe the current standard for K is 12.5 (8%). My Minolta is 14 (7.14%). There is virtually no difference. Infact from K =12 to K =18 I think its no more than 1/6 of a stop difference.

If you are using mono you got the wrong meter? It is way easy to get clear film in zone 1.

Not the way I calibrate and expose for a highlight. Zone 0 is pushed up the curve a stop or two becasue most of my images aren't a full 10 stop range. I just push back down by increasing print contrast. Starting from G2 I usually end up at at somewhere between G2 and G3.5 but probably most often G3

I heard the same thing about slide calibration for the minolta but I don't think its true. When using the (H)ighlight and (S)hadow buttons. H opens up 2 1/3 stops and pressing S closes down 2 2/3 stops. But I never use these buttons. Not necessary when using the zone system and making your own decisions about how much you want to open or close from what's metered.

It's actually the Spot Meter F and you're right its not perfect for landscape photography but I have found it very dependable and has always worked flawlessly except when the batteries die.
And it's best feature is that you can click once looking through it, say on your highlight or shadow and click and hold down the trigger whilst you scan the subject for the other end of the scale and it gives a real time stop range in the view finder from the first reading so its very very easy to find your SBR without neading to count on your fingers how many stops difference two points are.

markbarendt · Aug 28, 2015

RobC said:
Yes but that has nothing to do with how my spot meter actually works. It's just adjusting EI to be where I want it and making that match what my spot meter is going to show me. i.e. are we calibrating our spot meter to the film and dev or our film and dev to the spot meter. If you think about it, it's the latter. There is a difference.

The math cannot be done without knowing the ISO/EI number, right?

If we switch films and that new film has a different EI we have to recalibrate the meter to match the "new" film, right?

Stephen Benskin · Aug 28, 2015

markbarendt said:
The math cannot be done without knowing the ISO/EI number, right?

If we switch films and that has a different EI we have to recalibrate the meter to the "new" film, right?

The meter's calibration doesn't change. Changing the ISO changes the speed variable in the exposure calculator. This will effectively place the exposure on a different part of the curve. If the film speed is 100 and you set the EI to 50, it will effectively adjust where on the curve the exposure will fall. Another way to think about it is by adjusting the EI, you are changing the ratio between the speed point, Hm and metered exposure point, Hg. If you recall the quote from Safety Factors where they said they could reduce the safety factor simply by changing the speed constant? This changes the ratio. The ratio is about where the speed point is and where you want the metered exposure to fall. Since the exposure meter is designed for color films as well as B&W you can't change the meter. Film speed point is chosen for a reason such as limiting gradient (Delta-X) for B&W. Changing the position of the speed point Is also not a good option. The best way is to keep both Hg and Hm in place and change the exposure constant.

How it's all interconnected is a fascinating subject for me. I did a thread about the exposure constants which didn't go anywhere. The exposure constants represent how it's all connected. The Defining K document also connects it all together.

markbarendt · Aug 28, 2015

Stephen Benskin said:
The meter's calibration doesn't change. Changing the ISO changes the speed variable in the exposure calculator. This will effectively place the exposure on a different part of the curve. If the film speed is 100 and you set the EI to 50, it will effectively adjust where on the curve the exposure will fall. Another way to think about it is by adjusting the EI, you are changing the ratio between the speed point, Hm and metered exposure point, Hg. If you recall the quote from Safety Factors where they said they could reduce the safety factor simply by changing the speed constant? This changes the ratio. The ratio is about where the speed point is and where you want the metered exposure to fall. Since the exposure meter is designed for color films as well as B&W you can't change the meter. Film speed point is chosen for a reason such as limiting gradient (Delta-X) for B&W. Changing the position of the speed point Is also not a good option. The best way is to keep both Hg and Hm in place and change the exposure constant.

How it's all interconnected is a fascinating subject for me. I did a thread about the exposure constants which didn't go anywhere. The exposure constants represent how it's all connected. The Defining K document also connects it all together.

Yes, it is fascinating, as is finding the language to describe the concepts here. I do realize that there is a calibration of the instrument.

Conceptually what I'm trying to get at is that the meter is an outside observer in the process of making a photo, in much the same way a tape rule is to building a house.

"We" build tools to work in the environment they will be used in, to suit a task.

All the math involved, all the design, everything about our meters has been driven by the way light effects film in a camera.

"We" didn't design photography around meters, we designed meters around photography.

Stephen Benskin · Aug 28, 2015

markbarendt said:
Yes, it is fascinating, as is finding the language to describe the concepts here. I do realize that there is a calibration of the instrument.

Conceptually what I'm trying to get at is that the meter is an outside observer in the process of making a photo, in much the same way a tape rule is to building a house.

"We" build tools to work in the environment they will be used in, to suit a task.

All the math involved, all the design, everything about our meters has been driven by the way light effects film in a camera.

"We" didn't design photography around meters, we designed meters around photography.

+1. Well said.

RobC · Aug 28, 2015

Stephen Benskin said:
How can 0.1 lxs work for of Hg for all film speeds? Different film speeds have different target exposures for Hg.

the 0.1 lux.seconds is for 100 speed. Adjusting the meter film speed from that effectively adjusts the result of B*S i.e. Where I said the film speed multiplies UP the light reading to adjust for film sensitivity. The Film speed is used as a Factor to increase or decrease the EV number and an increased EV number means a reduced exposure amount and a decreased EV number is an increased exposure amount. That's how it works according to Minolta and as I understand it. And how the formula ( 2EV = B*S / K ) says it happens.

The Minolta manual for my meter lists the EV to cd/m^2 values for ISO speed 100. ( also the ft candles values but don't use those because 2EV outputs in cd/m2 )

And

0.1 lux.seconds is only 0.02 different from your 0.08 so 2/100 of a lxs. Is that worth arguing about, I think not.

markbarendt · Aug 28, 2015

RobC said:
... The Film speed is used as a Factor to ...

For me the word factor is problematic.

As part of an equation the word factor is fine.

Conceptually though the film speed and measured luminance are the foundations upon which everything else rests.

RobC · Aug 28, 2015

markbarendt said:
For me the word factor is problematic.

As part of an equation the word factor is fine.

Conceptually though the film speed and measured luminance are the foundations upon which everything else rests.

Reading between the lines it seems to me that the foundation of how a reflection meter works (well specifically my meter which seems to use the standard formula) is that it is based from a single film speed of 100 requiring 0.1 lxs to place the exposure in the middle of the curve. Well actually 8% of the light reading which is approx 3 1/2 stops down from the metered value which indicates an assumption of approx 7 stop range. The 0.1 film speed point doesn't come into it except in that its embedded into the ISO film speed calculations which give the 7 1/3 stop range. But its not used directly to do any calculations in the meter becasue the meter is basing calculations on 0.1 lxs for 100 speed (according to minolta).

Bill Burk · Aug 28, 2015

RobC said:
the 0.1 lux.seconds is for 100 speed. Adjusting the meter film speed from that effectively adjusts the result of B*S i.e. Where I said the film speed multiplies UP the light reading to adjust for film sensitivity. The Film speed is used as a Factor to increase or decrease the EV number and an increased EV number means a reduced exposure amount and a decreased EV number is an increased exposure amount. That's how it works according to Minolta and as I understand it. And how the formula ( 2EV = B*S / K ) says it happens.

The Minolta manual for my meter lists the EV to cd/m^2 values for ISO speed 100. ( also the ft candles values but don't use those because 2EV outputs in cd/m2 )

And

0.1 lux.seconds is only 0.02 different from your 0.08 so 2/100 of a lxs. Is that worth arguing about, I think not.

If you are rounding, then it's pretty significant because you are taking us from 100 speed film to 80 speed film.

markbarendt · Aug 28, 2015

RobC said:
Reading between the lines it seems to me that the foundation of how a reflection meter works (well specifically my meter which seems to use the standard formula) is that it is based from a single film speed of 100 requiring 0.1 lxs to place the exposure in the middle of the curve. Well actually 8% of the light reading which is approx 3 1/2 stops down from the metered value which indicates an assumption of approx 7 stop range. The 0.1 film speed point doesn't come into it except in that its embedded into the ISO film speed calculations which give the 7 1/3 stop range. But its not used directly to do any calculations in the meter becasue the meter is basing calculations on 0.1 lxs for 100 speed (according to minolta).

For a guy that's not into geekery you sure spit out a lot of jargon and numbers.

That is meant in fun Rob.

There seems to be a "not seeing the forest for the trees" thing going on though. The big ideas seem to be getting lost in the focus on the minutia of the formulas.

There are two real world things a meter doesn't have when you first turn it on, what are they?

Stephen Benskin · Aug 28, 2015

RobC said:
the 0.1 lux.seconds is for 100 speed. Adjusting the meter film speed from that effectively adjusts the result of B*S i.e. Where I said the film speed multiplies UP the light reading to adjust for film sensitivity. The Film speed is used as a Factor to increase or decrease the EV number and an increased EV number means a reduced exposure amount and a decreased EV number is an increased exposure amount. That's how it works according to Minolta and as I understand it. And how the formula ( 2EV = B*S / K ) says it happens.

The Minolta manual for my meter lists the EV to cd/m^2 values for ISO speed 100. ( also the ft candles values but don't use those because 2EV outputs in cd/m2 )

And

0.1 lux.seconds is only 0.02 different from your 0.08 so 2/100 of a lxs. Is that worth arguing about, I think not.

You didn't happen to mention he was talking about 100 speed. My impression was you were claiming 0.1 lxs was for all film speed. Now 0.1 and 0.08 are a third of a stop apart. But meter manufactures set Sx at 100, plus I can also see the tech thinking about the reversal speed equation, which is 10/Hr reducing the difference. We are actually in agreement on much of this. What I don't understand is why you were such an ass when I presented the same information earlier. Instead of quibbling over minor details, you dismissed most of it out of hand.

The standards tend to exclude most theory because it is superfluous to the purpose of the standard. The 1974 standard was special in that it had a very nice appendix that did explain a lot of theory but the standard also makes it clear the appendix falls outside of the standard. The ISO b&w speed standard doesn't mention the method for determining speed is actually the Delta-X Criterion which has it's own set of equations. It also doesn't say that if the development is above or below the parameters of the standard that the resulting speeds using the fixed density method would be progressively off. However it does use the Delta-X Criterion and is explained elsewhere. So just because it's not in the standard doesn't mean something isn't relevant.

Using the equation A2 / T = B*S / K, it's possible to find the calibration level of B. K has been found to be 1.16 cd/ft2 or 12.5 cd/m2. I've found that different units of measure tend to work better with certain equations. I'm going to use candela per square foot for what I hope will be obvious. Solving for B, the equation becomes (A2 / (T*S)) * K = B. Using the Sunny 16 rule with f/16 at 1/ISO, T and S = 1. The equation becomes 256 * 1.16 = B. B = 297 cd/ft2 or 3197 cd/m2.

To verify this, it can be plugged into the exposure equation (q*Lg)/A2 = Eg. Eg = 8. The constant q has been determined to equal 0.65.

(0.65 * 3197) / 162 = 8.11.

The value for I can also be determined in a similar manor.

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

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