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

cowanw · Aug 23, 2015

Pardon my post, this is a complex but absorbing topic, but I wonder if we have a discussion of apples and oranges here.
I had a epiphany regarding RobC's comments about being keyed to the highlights. Are those concepts are keyed to the contents of this thread on FADU by a poster named Argentum?
http://www.film-and-darkroom-user.org.uk/forum/showthread.php?t=9852&highlight=zone+middle
wherein the middle grey of 18% is keyed to 3 stops down from the greatest luminance which, of course, varies with the brightness range. The point being that the recommendation of where middle grey lies (Zone V), that a meter recommends, must vary according to the brightness range of the scene.
Which is a concept not allowed for in other systems of Zone thinking.
Having said this, it will not surprise me if I am quite out to lunch
Bill

RobC · Aug 23, 2015

your spot meter divides the light reading (B) by the Meter K factor. So if you point the meter at an 18% grey card it will divide that reading by 12. That equates to 8.33% of 18%. And that works out to around 3 2/3 stops less than what is being metered. And if you have calibrated to a 10 stop range to fit Grade 2 paper, then that's keyed to about zone 6.3. If you've calibrated to 7 1/3 stops then its the mdidle. So it depends what you've calibrated film dev to and what you meter.

Go figure...

or

just do the test of photographing and printing the zone patches from a piece of hardboard and this will tell you what you're really getting.
(there was a url link here which no longer exists)
You do this becasue it allows you to factor out all the discrepancies between theory and the real world application of actually metering and taking photographs.

All scientific theories have to be proved with experiments otherwise they are worthless. Just call the printing of zone patches an experiment instead of a practical evaluation or ask the theorists to provide you with an experiment which proves their theories (without a densitometer).

Ian Grant · Aug 23, 2015

Stephen Benskin said:
Ian, the daylight and incandescent EIs I believe ended in the 1960s. Now, I'm not sure they were part of the ASA standard, but they were used mainly because of the type of photocells used with exposure meters and the color temperature the exposure meters were calibrated to. An Interpretation of Current Exposure Meter Terminology. I have an old Kodak Reference Handbook and they have different recommended daylight and incandescent EIs for Weston and GE meters.

DIN and ISO (ASA) have been in agreement since (I believe) the 1960 ASA standard, and if not exactly in agreement then, they were with the ISO standard. If the ISO prefix is listed on the box, the manufacturers have adhere to the conditions of the standard. It's comparing apples to apples.

The almost universal 1/2 to 1 stop difference seen in Zone System speed testing from the ISO film speed comes from the use of different ratios between the metered calibration point and the speed point. The Zone System uses four stops. Meter the card and stop down four for Zone I. ISO uses 3 1/3 stops. Hg = 8/Eg and S = 0.80/Hm. The reason why prefixes such as ISO and CI are used is to communicate the testing methodology used. CI 0.58 means one thing and G bar 0.58 or Gamma 0.58 another. Zone System EIs and ASA speeds were in agreement before the 1960 b&w film speed standard which eliminated most of the safety factor effectively increasing film speeds by 2/3 to 1 stop. Now ISO speeds and Zone System speeds are off by that same amount.

You wouldn't expect the same speeds as the ISO from Scheiner speeds or Hurter and Driffield's inertia speeds, or the old DIN and old ASA because they use different testing methods. Why expect it from Zone and ISO?

If you are looking for a spot that's 3 1/3 stops down from the meter reading at 4 stops down, then you will need to adjust the EI by 2/3 of a stop to find it. This can be considered the same thing as changing the speed constant as it changes the ratio between the speed point and the metered exposure point.

As for Sexton, he found his EI to be lower than the ISO of T-Max 100 just like he did with all other films. Why? Because he uses Zone System testing and its methodology is different from the ISO methodology. Sexton A few ideas on using Kodak T-Max Films Successfully, "As with most black and white negative films I used an Exposure Index (El) that is less than the manufacturers' suggested film speed."

Stephen

Stephen, many companies still published Daylight and Tungsten ISO's right into the 2000's and with some films still do. It's only recently that Ilford stopped lising the Tungsten ISO for FP4.

ASA/BS and DIN are completely different methods of testing, the ISO standard allows for testing by whichever of the two methods the company wants and using the standard conversion between the two for the ISO. That doesn't mean that Tmax100 would be 100 ISO if tested by the DIN method though, it would almost certainly be 50/18, the ISO speed is both the ASA/BS and DIN.

Unfortunately there is no definitive ISO testing method for B&W films if there was we wouldn't be having this discussion, it's up to the manufacturers to decide what method they use. As Kodak they were the only B&W US film manufacturer left when Tmax was introduced they were able to have the ASA method of testing changed, without the change Tmax100 wouldn't be 100ISO

DIN and ASA/BS might have been closer in the 60's but the gap widened particularly with Tmax films, I'm talking from practical experience. ISO is only ASA for Kodak, Ilford use the BS (British Standard) which happens to be the same as the ASA system. Continental Europe use the DIN system for speed determination, who knows what Foma use as it's way off like Tmax.

There's to many sweeping statements that Zone system speeds are typical a ½ or full stop less then the box ISO that's not my experience or that of many others, Some films may need half the box ISO exposure, some are spot on particularly Agfa, and some are faster than the box ISO.

If all B&W films were tested exactly the same way then it would be a lot easier,

it's wrong to say Zone system or BTZS testing is wrong as it's a more consistent approach tahn allowing manufacturers to use one of two quite different methodologies and allowing Kodak to change the criteria to make sure a new film becomes 100 ISO when it failed the official testing methodology.

Ian

rbultman · Aug 23, 2015

RobC said:
Yes I do the same. I calibrate for 10 stop SBR and where its greater than that I expose for the shadows instead of a highlight.

What do you include in your 10 stops? Is stop 1 just above fb+f or does it include texture?

Sent from my Nexus 5 using Tapatalk

markbarendt · Aug 23, 2015

cowanw said:
Pardon my post, this is a complex but absorbing topic, but I wonder if we have a discussion of apples and oranges here.
I had a epiphany regarding RobC's comments about being keyed to the highlights.

Bill, the EI/ISO of a film is "found" or "keyed to the film" by a test. In that test, yes there is a reference point from the scene that is a "known".

After the speed test, once the EI is known, then the concept of "Place and Fall" becomes real and because of that all the tones, become knowns, If you know all the tones you can spot meter and place (key to) any subject matter you please in any zone you please and the rest of the zones and tones will fall appropriately.

Use a spot meter to "key" to the shadows or skin or a white dress, apply the appropriate offset to each reading, and all three will end up at the same camera setting. At this point an incident meter should also agree. (If the suggested camera settings disagree, it is almost a given that it is because of a failure of skill with the tools or judgement about the tones being measured by the spot meter.)

So meters can "key" your camera setting to whatever you please.

You may design the standard using a single reference point, but the real world result is a standard that defines a whole range of possible reference or "key" points.

Stephen Benskin · Aug 23, 2015

RobC said:
your spot meter divides the light reading (B) by the Meter K factor. So if you point the meter at an 18% grey card it will divide that reading by 12. That equates to 8.33% of 18%. And that works out to around 3 2/3 stops less than what is being metered. And if you have calibrated to a 10 stop range to fit Grade 2 paper, then that's keyed to about zone 6.3. If you've calibrated to 7 1/3 stops then its the mdidle. So it depends what you've calibrated film dev to and what you meter.

Go figure...

K in the exposure meter equation is basically a light loss factor. Let's look at the the balanced equation again, A2/T = B * S / K. A2/T doesn't exactly work the same way in real life as it does mathematically. It would if ideally there wasn't any light loss (and some other variables involved) as the light passes through the lens and strikes the film. But since there is, the equation has to be balanced with K. Without K, if you plug in the Sunny 16 Rule, F/16 at 1/ISO, A2 and B would both equal 256 or A2=B. But because of the light loss, B needs to be higher to compensate. It's all spelled out in this first example.

The next example is of the K equation. Here are the variables that are considered.

- Distance from lens to object
- Focal length of lens
- Lens transmittance
- Camera Flare Correction factor at Eg
- Angle of image point from axis of lens
- Exposure Constant
- Luminance distribution factor
- Ratio of spectra response between scene luminance and sensitometric illuminance
- Photocell's spectral response

From Dunn's Exposure Manual, 4th ed. discussing integrated or averaging meters, "The two major influences that cause the meter reading to depart from the average conditions to which the calibration of these meters is always related are (a) variations in the subject luminance range and (b) variations in the distribution of the areas of the individual tones in the scene." He then has a number of pages of examples where he shows a scale of integrated values, an image and placement on a film curve.

RobC · Aug 23, 2015

rbultman said:
What do you include in your 10 stops? Is stop 1 just above fb+f or does it include texture?

Sent from my Nexus 5 using Tapatalk

Zone 1 in my tests is just a tad lighter than black so no you can't discern any texture in it. Zone 2 shows some but not full until I get to zone 3.

But it will depend on your film and developer. As I mentioned in this or another topic, a shallow toe probably won't show any texture until well into zone 2.

Zone 9 is just a hint of tone and zone 10 is pure white.

Stephen Benskin · Aug 23, 2015

Ian Grant said:
Stephen, many companies still published Daylight and Tungsten ISO's right into the 2000's and with some films still do. It's only recently that Ilford stopped lising the Tungsten ISO for FP4.

ASA/BS and DIN are completely different methods of testing, the ISO standard allows for testing by whichever of the two methods the company wants and using the standard conversion between the two for the ISO. That doesn't mean that Tmax100 would be 100 ISO if tested by the DIN method though, it would almost certainly be 50/18, the ISO speed is both the ASA/BS and DIN.

Unfortunately there is no definitive ISO testing method for B&W films if there was we wouldn't be having this discussion, it's up to the manufacturers to decide what method they use. As Kodak they were the only B&W US film manufacturer left when Tmax was introduced they were able to have the ASA method of testing changed, without the change Tmax100 wouldn't be 100ISO

DIN and ASA/BS might have been closer in the 60's but the gap widened particularly with Tmax films, I'm talking from practical experience. ISO is only ASA for Kodak, Ilford use the BS (British Standard) which happens to be the same as the ASA system. Continental Europe use the DIN system for speed determination, who knows what Foma use as it's way off like Tmax.

There's to many sweeping statements that Zone system speeds are typical a ½ or full stop less then the box ISO that's not my experience or that of many others, Some films may need half the box ISO exposure, some are spot on particularly Agfa, and some are faster than the box ISO.

If all B&W films were tested exactly the same way then it would be a lot easier,

it's wrong to say Zone system or BTZS testing is wrong as it's a more consistent approach tahn allowing manufacturers to use one of two quite different methodologies and allowing Kodak to change the criteria to make sure a new film becomes 100 ISO when it failed the official testing methodology.

Ian

Ian, I have a number of B&W film standards. There is currently only one testing method. Logithmic speeds, which DIN used, are included because of exposure that use those settings. The section of the ISO 6 Photography - Black-and-white pictorial still camera negative film/process systems - Determination of ISO speed that discusses this is section 7 - Product marking and labeling. DIN is not mentioned in the standard. Once again if the ISO prefix is used, the standards were followed.

At one time ASA and DIN were different. One of my favorite things with DIN is that film was to be developed to gamma infinity.

Unfortunately there is no definitive ISO testing method for B&W films if there was we wouldn't be having this discussion, it's up to the manufacturers to decide what method they use. As Kodak they were the only B&W US film manufacturer left when Tmax was introduced they were able to have the ASA method of testing changed, without the change Tmax100 wouldn't be 100ISO.

The ISO standard is in its very nature the definitive testing method. It's the standard. ISO stands for International Organization of Standards. Now the ISO standards tend to be very frugal with language, but the older standards listed the participants. They usually include all the film manufacturers, scientists from related fields, representatives from exposure meter manufacturers, Optics manufacturers, and scientific journals and societies. They also usually included people from the military. For the ISO 6 - 1993 it simply says, International Standard ISO 6 was prepared by Technical Committee ISO/TC 42, Photography. Jones chaired the Subcommittee on Colorimetry in the 30s and early 40s and he writes that it took 10 years for the committee members to agree that color is psychophysical. It's incredibly hard to get a room of experts in their fields to agree on anything.

What happened with Tmax film is that the new type of emulsion didn't respond well to the developer stated in the ISO standard. The decision was made to re-evaluate the standard and changes were made to better reflect real world use. Now manufacturers can use the developer of their choice but they have to note it. This might be what you are thinking of.

it's wrong to say Zone system or BTZS testing is wrong as it's a more consistent approach tahn allowing manufacturers to use one of two quite different methodologies and allowing Kodak to change the criteria to make sure a new film becomes 100 ISO when it failed the official testing methodology.

Not saying they're wrong. Am saying the Zone System has a different testing methodology than the ISO standard so the results can't really be compared. What I do say is much of the Zone System testing procedures are sloppy and ignore important variables. I also think much of the Zone System is vague. But as I've said before, Phil Davis' theory is solid.

RobC · Aug 23, 2015

They say that the really great thing about standards is that there are so many different ones to choose from:confused:

Xmas · Aug 24, 2015

Stephen Benskin said:
From Dunn's Exposure Manual, 4th ed. discussing integrated or averaging meters, "The two major influences that cause the meter reading to depart from the average conditions to which the calibration of these meters is always related are (a) variations in the subject luminance range and (b) variations in the distribution of the areas of the individual tones in the scene." He then has a number of pages of examples where he shows a scale of integrated values, an image and placement on a film curve.

Hi Stephen

It is (a) & (b) that will get you.

Kodak did samples and said an average reflective reading was ok >99% of time? But that is sample dependent.

If you are shooting contra jour with a low sun (a) is kaput.

Eg a Nikon FG has a back lit button, you need to be more careful with an OM1...

Rob says that reflective and incident are keyed of highlights well that is dependent on how you calibrate and measure the scene. I calibrate EI for zone 1 and assume the scene average is metered at zone 5. If I have time I check the assumption, eg by as well measuring a zone 1 spot as well as the 'average'.

frequently it is obviously not average and you need a zone1 or go at risk with a fudge factor...

I know this is how I work cause the westons zone1 calculator is four steps away from zone5.

One two buckle my shoe
Three four pick up sticks

Maths is not my favorite subject.

Noel

baachitraka · Aug 24, 2015

Its, five six pick up sticks...

Stephen Benskin · Aug 24, 2015

Xmas said:
Kodak did samples and said an average reflective reading was ok >99% of time? But that is sample dependent.

Rob says that reflective and incident are keyed of highlights well that is dependent on how you calibrate and measure the scene. I calibrate EI for zone 1 and assume the scene average is metered at zone 5. If I have time I check the assumption, eg by as well measuring a zone 1 spot as well as the 'average'.

The average scene luminance range is 2.20 with a standard deviation of 0.76, which means 68% of scenes fall within a 2 1/2 stop range and 95% fall within a 5 stop range.

RobC has yet to offer any proof to support that statement.

Xmas · Aug 24, 2015

baachitraka said:
Its, five six pick up sticks...

You did not read the following sentence?

RobC · Aug 24, 2015

Stephen Benskin said:
The average scene luminance range is 2.20 with a standard deviation of 0.76, which means 68% of scenes fall within a 2 1/2 stop range and 95% fall within a 5 stop range.

RobC has yet to offer any proof to support that statement.

I don't do average. I'm not interested in theory. I do practical photography.

Stephen Benskin · Aug 24, 2015

RobC said:
I don't do average. I'm not interested in theory. I do practical photography.

Well you have to have some sort of theory (idea of why) if you make a statement on what meters are keyed to. You must have a reason why you think that meters are keyed to highlights.

RobC · Aug 24, 2015

Stephen Benskin said:
Well you have to have some sort of theory (idea of why) if you make a statement on what meters are keyed to. You must have a reason why you think that meters are keyed to highlights.

Because you told me that a spot and incident reading would match on a 12% reflectance. Remember?

markbarendt · Aug 24, 2015

RobC said:
Because you told me that a spot and incident reading would match on a 12% reflectance. Remember?

You need more than one piece of the puzzle to get the whole picture.

Xmas · Aug 24, 2015

Stephen Benskin said:
The average scene luminance range is 2.20 with a standard deviation of 0.76, which means 68% of scenes fall within a 2 1/2 stop range and 95% fall within a 5 stop range.

RobC has yet to offer any proof to support that statement.

I think Kodaks 99% was family group in a line up...

My shooting is either dead flat battleship grey cloud, or low sun in frame need to clean eye glasses.
But I do accept you are statistically well founded.

Stephen Benskin · Aug 24, 2015

Xmas said:
I think Kodaks 99% was family group in a line up...

My shooting is either dead flat battleship grey cloud, or low sun in frame need to clean eye glasses.
But I do accept you are statistically well founded.

Of course it's more nuanced than it sounds. The information comes from the seminal research by Loyd Jones in the late 30s and early 40s. The paper on scene luminance ranges and distribution is The Brightness Scale of Exterior Scenes and the Computation of Correct Photographic Exposure, JOSA, 1941. It's 27 pages long and not a single mention of average reflectance.

RobC · Aug 24, 2015

markbarendt said:
You need more than one piece of the puzzle to get the whole picture.

Not if its only a two piece puzzle, you can intuitively see what shape the other half is. But if you try and turn it into a Rubik cube when it doesn't need to be it becomes anything but intuitive.

MartinCrabtree · Aug 24, 2015

Now children..............

Sirius Glass · Aug 25, 2015

RobC said:
I don't do average. I'm not interested in theory. I do practical photography.

I use matrix readings with the Hasselblads and Nikons most of the time but unusual lighting or LF photography means that I take an average reading and several spot readings and calculate the setting.

Bill Burk · Aug 25, 2015

I looked at Ansel Adams The Negative to see if I could tell how... what he considers "Normal" development... translates to Contrast Index.

His example of Tri-X in Appendix 2 translates to CI 0.55

When I use the charts to match CI 0.55 to Grade 2 paper, it works out backwards to a Subject Luminance Range of 7 2/3 stops.

(His N+2, graphs as CI 0.8 which works out to 5 2/3 stops SLR - this confirms a 2 stop expansion - which makes sense).

So getting back to the original question of why Zone System EI is often half rated ISO...

I estimate that the "impact" on film speed from the reduced development.

Is approximately 1/6 stop speed loss.

So we have the 2/3 stop reduced speed thanks to the different number of stops that you "stop down" from the metered point to hit the 0.1 density speed point.

Then 1/6 stop reduced speed thanks to reduced development compared to ISO 0.62 CI.

Gives us 5/6 stops reduced speed for Zone System tests compared to ISO.

Stephen Benskin · Aug 25, 2015

Bill Burk said:
I looked at Ansel Adams The Negative to see if I could tell how... what he considers "Normal" development... translates to Contrast Index.

His example of Tri-X in Appendix 2 translates to CI 0.55

When I use the charts to match CI 0.55 to Grade 2 paper, it works out backwards to a Subject Luminance Range of 7 2/3 stops.

(His N+2, graphs as CI 0.8 which works out to 5 2/3 stops SLR - this confirms a 2 stop expansion - which makes sense).

So getting back to the original question of why Zone System EI is often half rated ISO...

I estimate that the "impact" on film speed from the reduced development.

Is approximately 1/6 stop speed loss.

So we have the 2/3 stop reduced speed thanks to the different number of stops that you "stop down" from the metered point to hit the 0.1 density speed point.

Then 1/6 stop reduced speed thanks to reduced development compared to ISO 0.62 CI.

Gives us 5/6 stops reduced speed for Zone System tests compared to ISO.

Bill, that graph has grid lines at 0.30 intervals. I read Zone Vlll falling around 1.30. This brings the average gradient to 0.57. It makes more sense for it to fall around 1.30 because Adams uses 1.25-1.35 for the aim value in the table on page 220.

Your different speeds from the different CIs only works if both methods use the fixed density point for speed determination in the same way. Remember the ISO standard uses Delta-X Criterion. Did you know that the speed equation was adjusted from 0.95/Hm to 0.80/Hm because the light source used in sensitometers changed from sunlight to daylight (sunlight plus skylight). The increased blue light would produce slightly higher speeds, so the speed constant needed to be adjusted. While Adams suggests placing the exposure target in daylight, how many Zone System testing procedures recommend placing it in the shadows?

Mostly, people who do a film speed testing with their cameras would be lucky to be within 1/3 stop.

Bill Burk · Aug 25, 2015

Stephen Benskin said:
Bill, that graph has grid lines at 0.30 intervals. I read Zone Vlll falling around 1.30. This brings the average gradient to 0.57. It makes more sense for it to fall around 1.30 because Adams uses 1.25-1.35 for the aim value in the table on page 220.

Your different speeds from the different CIs only works if both methods use the fixed density point for speed determination in the same way. Remember the ISO standard uses Delta-X Criterion. Did you know that the speed equation was adjusted from 0.95/Hm to 0.80/Hm because the light source used in sensitometers changed from sunlight to daylight (sunlight plus skylight). The increased blue light would produce slightly higher speeds, so the speed constant needed to be adjusted. While Adams suggests placing the exposure target in daylight, how many Zone System testing procedures recommend placing it in the shadows?

Mostly, people who do a film speed testing with their cameras would be lucky to be within 1/3 stop.

I interpret the same Zone VIII density. But I used the Contrast Index meter overlay to read CI from my hand-drawn chart where I transcribed the numbers. I see Ansel Adams didn't subtract Base plus Fog from the graphs, and when I came up with the lower CI, I had set the Contrast Index meter overlay on baseline zero. When I lift the CI meter to about 0.05 density (which is where I believe his Normal curve would zero out), I get close to 0.57 CI like you do.

I agree, Delta-X criterion justifies holding the same speed rating (so I will not include this specific 1/6 stop adjustment in my own personal EI), but most Zone System EI are taken from the 0.1 density mark, and that would definitely move for people reading Zone System tests.

The reason for working out factors of 1/6 stop is not to achieve a false sense of precision... I would round to the nearest 1/3 stop. For now, for me, that is 2/3 stop.

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

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