That was unnecessary, because the equations are included in the article I referenced, just search for 'light meter' on Wikepedia. It's all there.
http://en.wikipedia.org/wiki/Light_meter
Understanding EI???
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That was unnecessary, because the equations are included in the article I referenced, just search for 'light meter' on Wikepedia. It's all there.
http://en.wikipedia.org/wiki/Light_meter
Steve
I'm not sure about that. The full text for FP4 reads:
EXPOSURE RATING
FP4 Plus has a speed rating of ISO 125/22º
(125ASA, 22DIN, EI 125/22) to daylight. The ISO
speed rating was measured using ILFORD ID-11
developer at 20ºC/68ºF with intermittent agitation
in a spiral tank.
Best results are obtained at EI 125/22, but good
image quality will also be obtained at meter
settings from EI 50/18 to EI 200/24.
It should be noted that the exposure index (EI)
range recommended for FP4 Plus is based on a
practical evaluation of film speed and is not based
on foot speed, as is the ISO standard.
They are not just talking about the range. They also list the base speed at EI 125/22, and Mike Gristwood from Ilford told me that Ilford's internal test are not conducted according to ISO. The ISO test is done for the box!
There were three: 1934, 1957 and 1961. Please, take a look at the google link to 'The Manual of Photography' in post #134. It's all in there, including an explanation the the five different methods of measuring film speed. It's a nice overview for all who want to follow or participate to this thread.
ASA/DIN, yes, but ISO/DIN, I don't think so. In any way, it's more precise to say:
The ISO speed rating, printed on the box as in ISO 100/21°, is a combination of the arithmetic and logarithmic speed.
ASA used arithmetic speeds, and DIN used logarithmic speeds. ISO uses both.
OK Stephen, this makes more sense:
Ilford's statements like "It should be noted that the exposure index (EI) range recommended for 100 DELTA Professional is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard."
In plain English that sentence is saying the Ilford ISO standard is based on practical evaluation and is not based on the foot speed. The words "as is" refer back to the "is based".
The data for all films is more informative and for HP5+ is "the developer and conditions used to obtain the ISO". exactly what is required to be listed.
"EXPOSURE RATING
HP5 Plus has a speed rating of ISO 400/27º (400ASA, 27DIN, EI 400/27) to daylight. The ISO speed rating was measured using ILFORD ID-11 developer at 20ºC/68ºF with intermittent agitation in a spiral tank.
Best results are obtained at EI 400/27, but good image quality will also be obtained at meter settings from EI 400/27 to EI 3200/36.
It should be noted that the exposure index (EI) range recommended for HP5 Plus is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard."
Maybe you're reading that last part as "is not based on foot speed as in the ISO standard."
Ian
Ilford's statements like "It should be noted that the exposure index (EI) range recommended for 100 DELTA Professional is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard."
In plain English that sentence is saying the Ilford ISO standard is based on practical evaluation and is not based on the foot speed. The words "as is" refer back to the "is based".
The data for all films is more informative and for HP5+ is "the developer and conditions used to obtain the ISO". exactly what is required to be listed.
"EXPOSURE RATING
HP5 Plus has a speed rating of ISO 400/27º (400ASA, 27DIN, EI 400/27) to daylight. The ISO speed rating was measured using ILFORD ID-11 developer at 20ºC/68ºF with intermittent agitation in a spiral tank.
Best results are obtained at EI 400/27, but good image quality will also be obtained at meter settings from EI 400/27 to EI 3200/36.
It should be noted that the exposure index (EI) range recommended for HP5 Plus is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard."
Maybe you're reading that last part as "is not based on foot speed as in the ISO standard."
Ian
Ralph, sorry I took a while writing and you posted meanwhile.
I seem to remember someone from Ilford saying they now determined their ISO speeds by practical testing, it may have been on the factory tour late 2008.
Ian
I seem to remember someone from Ilford saying they now determined their ISO speeds by practical testing, it may have been on the factory tour late 2008.
Ian
Ian
That is precisely what I was told by Ilford's Technical Support staff. I asked what that meant and was told: "more exposure and better shadow detail". It may also have to do with the fact that Ilford did not use Kodak's CI method to measure the rate of development but their own average-gradient method.
Steve, Ron...
I would appreciate hearing more on this, if anyone can add something.
What do you mean "did poorly" ?
just glancing quickly at one of the curves Ron pointed to, it looks like T-max400 may test out slower in D-76.... Is this what you mean?
Since you have the standards at hand, what developer formula was formerly specified? I would have thought D-76, or something very very close.
Are you saying Kodak was able to "engeenier" a change in the specifications in order to improve the appearent performance of one of their product lines?
Ron,
What aspects of those emulsions caused this, in your opinion?
Was a larger iodide content responsible?
What aspects of T-max developer makes it better suited to T-max film?
???
Ian,
I don't really think you don't understand the above sentences, but something sounds funny.
Maybe in my haste, I misread or misunderstood what you are saying.
Anyway, this is the way I read the section above:
The ISO speed is ISO 400/27º
(This is the same as saying the speed is 400ASA, 27DIN or EI 400/27)
Best results are obtained at EI 400/27,
but good results will also be obtained at meter settings from EI 400/27 to EI 3200/36.
(By international agreement, we cannot claim meter settings at other than ISO 400/27º are ISO speeds... So we call them EI settings. These EI settings are determined based on a practical evaluation of film speed... not by methods set fourth in the ISO standard... which is why we can't call them ISO speeds!)
Now I know you know this, but perhaps it will prove useful for someone....
Ray
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Ray;
I never said that T-max did poorly.
In fact, if you look at Tmax vs TriX in D-76, with about the same contrast, they have the same speed. So, IDK what the comments are all about. The films measure correctly!
As for the developer, T-max performs "better" in a developer tailored especially for T-grain films. IDK what that means, as IDK the exact formula for that developer. I do know that the C-41 developer performs just fine with either K-grans or T-grain emulsions. I've tested that.
Development rate and edge effects would be the biggest things to change as Iodide changes.
PE
Yeah kind of Ray 
Only one of the tests & EI's can be called the ISO speed, what speed you set on the camera is the EI, whether you go by their recommendations or your own personal tests.
You were there as well when Ilford mentioned their speed tests
Ian
Only one of the tests & EI's can be called the ISO speed, what speed you set on the camera is the EI, whether you go by their recommendations or your own personal tests.You were there as well when Ilford mentioned their speed tests
Ian
Full circle...
While it says: Best results are obtained at EI 400/27
The strange thing is...
for many people...
Best results are obtained at EI 320/27
and that is what the OP asked us about...
Going One Step Beyond, I will pose the question...
Why couldn't the standard organzations come up with a system that, for the majority of discerning users, the practical settings actually chosen, actually match their recommendations?
While it says: Best results are obtained at EI 400/27
The strange thing is...
for many people...
Best results are obtained at EI 320/27
and that is what the OP asked us about...
Going One Step Beyond, I will pose the question...
Why couldn't the standard organzations come up with a system that, for the majority of discerning users, the practical settings actually chosen, actually match their recommendations?
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Not you Ron, I think it was Stephen Benskin who said something like:
"T-Max films did poorly in the ISO standard developer, so Kodak [processed] them in [?] and gave them an EI. This prompted the change [in] the 1993 version of the standard where there was no standard developer suggested."
His "did poorly" is what I was thinking of....
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Ray, am I remembering right, didn't the first Tmax films have a box EI rather than an ISO rating, I seem to remember at the time of it's launch that it couldn't pass the ISO tests. I have the original datasheets back in the UK, along with the two seminal articles on Tmax by John Sexton who was working as a consult to Kodak and field testing the films.
Strange I'm sure there's a lost post and Ron replied to you Ray ??? I read it.
Ian
Strange I'm sure there's a lost post and Ron replied to you Ray ??? I read it.
Ian
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The speeds resulting from the use of the ISO standard developer didn't represent real world use. That's all.
Maybe on page 4?
Ron wrote...
In fact, if you look at Tmax vs TriX in D-76, with about the same contrast, they have the same speed.... The films measure correctly!
As far as old boxes, my stack of old data sheets are too near my sleeping wife to safely retrive
I do know that whenever Kodaks color reversal material was off target... they still sold it with an appropriate EI noted in red ink on the spec sheets.
I will look at some old data sheets though when it is morning.
Ray
PE might remember this. I seem to recall the equation for reversal film speed determination changed at one time from 8/Hm to 10/Hm. That's an adjustment of 1/3 stop.
There isn't anything nefarious going on. The use of T-grain emulsions simply highlighted a potential problem with the standards that required an updating. They made a number of other changes while they were at it. They eliminated the range of D. It used to be 0.80 +- 0.05. Now it is only 0.80. They changed the hold time of film from over 1 hour but less than 2 to more than 5 days but less than 10 for general purpose films and not less than 4 hours or more than 7 days for professional films. The change in the hold time was also to better reflect real world use like the change in developers.
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No, I'm reading the part that says "exposure index". While Ilford no doubt does there own practical evaluation too, the films still have an ISO which means they still do the testing. The recommend best EI setting is the same as the ISO setting? It's not a big deal. They haven't abandoned ISO testing. It looks like they are just providing additional information from practical testing. Call back the original poster, we're back on topic. You can't have the ISO prefix unless you adhere to the specifications of the standard. Since Ilford uses the ISO prefix, they then must have adhere to the standard. Maybe we are just talking semantics, but I believe the correct use of the terms facilitates a clearer understanding of the concepts involved, especially for the beginners out there.
Ralph,
I believe it was. I looked through the rest of the Conrad article. First, he had an argument about midpoint reflectance based on a flat surface checkerboard where he determined the midpoint reflectance at 18%. While this is correct, for copy work of two dimensional objects, the real world has speculars. According to Jack Holm, Exposure-Speed Relations and Tone Reproduction, IS&T 47th Annual Conference/ICPS 1994, "...a Lambertian scene reflectance of 12% for 100% highlight reflectance or 14% for 128% highlight reflectance."
I think I found where the discrepancy is between your reflectance values for K (or your interpretation of Conrad's) and mine. Conrad says, "K of 12.5 measuring a flat subject with a reflectance of 15.7%," which has a C=250. I believe to have that the ratio apply to scenes, you need to use the C for Cardioid Types or for the dome and not the disk. That would make it C=330 (30 * 10.76). Interesting enough, another article on Wikipedia (actually Conrad's article wasn't really on Wikipedia) for Light Meters had equations too. Just like I told Matt, a really good understanding comes when you spend some time playing with the numbers.
"In a typical scene, many elements are not flat and are at various orientations to the camera, so that for practical photography, a hemispherical receptor usually has proven more effective for determining exposure. Using values of 12.5 for K and 330 for C gives
R = pi * 12.5 / 330 = ~ 11.9% "
Interesting enough, if you read the packaging that comes with the Kodak gray card, it says to open up 1/2 stop when metering a scene and not to when metering for copy (flat vs dome C).
Truthfully, I've never completely had a handle on the ratio of K to C being the average scene reflectance. I understand that because B*S/K = I*S/C it is possible to determine I by knowing B and so the K to C ratio will be the same as the B to I ratio, but I have trouble when in two different standards B is the same, but C has changed. Since reflected light meters don't use C, how would that than change the average reflectance for the reflected light meter if the reflected light meter hadn't changed anything? Aren't you still metering the same luminance value as before? Isn't it still placing it on the same part of the characteristic curve? Conrad made an interesting point on this, "ANSI/ISO 2720-1974 does not suggest a relationship between K and C, and does not suggest that either relate to any specific average subject reflectance." However, Allen Stimson uses it in his paper An Interpretation of Current Exposure Meter Technology.
Anyway, I took the liberty of going back over the old standards. While the first standard is from 1948, there is a joint Army-Navy specification that was released in 1944. It doesn't have a value for C. The value for K = 1.25 or 13.4. In order to approximate what the reflectance might be, I took the value of C from the first standard, C=22.5. 1.25*pi/22.5 = 17.5%. But if you calculate it using the current value of C (30), you get 13%.
For the other standards over the years, I worked out values for the ranges they gave for what it's worth. In order to keep things simple, I'm only listing the results from the range means.
1948 = 16.6%
1957 = 16.6%
1961 = 16%
1971 (ANSI) = 12%
1974 (ISO) = 12%
1994 = 12%
I have that book. People might also be interested in:
H.N. Todd nd R.D. Zakia, A Review of Speed Methods, Photographic Science and Engineering, vol 8, no 3, Sept-Oct 1964.
G.S. Allbright, Emulsion Speed Rating Systems, Journal of Photographic Science, Proceedings Issue.
I believe the Allbright paper was listed in the bibliography for that section in "The Manual of Photography."
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AFAIK, Kodak never sold any film with a "corrected" ISO value penciled in on the box. To us, such film was scrap! It was not sold.
I never worked on reversal films, but the threshold speed was defined as the intercept of the straight line contrast with the straight line Dmax, projected onto the X-axis.
When you talk about giving a 400 speed film a 320 exposure, this goes back to my posts and Ralphs eariler. In that, we commented that the (4)th point on Stephens graph was too far into the toe. Overexposure by 1/3 stop, as I noted, would move this point to the right just enough to make the entire curve fit on the straight line.
In the final analysis, you have all given a lot of theory and speculation, but I think I am the only one that had to do this from start to finish and produce a salable product in both B&W and color. I did it at EK amidst millions of dollars of equipment and many co-workers that were, in many cases, far better at this than I was. Well, their expertise rubbed off on me to some extent. And now, here I am repeating some of this work at home for my emulsion projects. I'm putting it all into practice again, but with no backup.
I've tried to give practical advice rather than theory, because photo companies don't sell H&D curves, they don't sell algebraic formulas or any of that, they sell film to take pictures.
As an afterthought to Ian. I have looked at some of your posted pictures and they are very nice. I particularly liked the ancient "theater" and the stone steps. But in all of them I was struck by what I thought to be low contrast with gray whites and gray blacks. Maybe it is a scanning artifact or maybe it is your personal preference, but just looking at them with no other input makes me think you like low key pictures.
PE
I never worked on reversal films, but the threshold speed was defined as the intercept of the straight line contrast with the straight line Dmax, projected onto the X-axis.
When you talk about giving a 400 speed film a 320 exposure, this goes back to my posts and Ralphs eariler. In that, we commented that the (4)th point on Stephens graph was too far into the toe. Overexposure by 1/3 stop, as I noted, would move this point to the right just enough to make the entire curve fit on the straight line.
In the final analysis, you have all given a lot of theory and speculation, but I think I am the only one that had to do this from start to finish and produce a salable product in both B&W and color. I did it at EK amidst millions of dollars of equipment and many co-workers that were, in many cases, far better at this than I was. Well, their expertise rubbed off on me to some extent. And now, here I am repeating some of this work at home for my emulsion projects. I'm putting it all into practice again, but with no backup.
I've tried to give practical advice rather than theory, because photo companies don't sell H&D curves, they don't sell algebraic formulas or any of that, they sell film to take pictures.
As an afterthought to Ian. I have looked at some of your posted pictures and they are very nice. I particularly liked the ancient "theater" and the stone steps. But in all of them I was struck by what I thought to be low contrast with gray whites and gray blacks. Maybe it is a scanning artifact or maybe it is your personal preference, but just looking at them with no other input makes me think you like low key pictures.
PE
-
- Stephen Benskin
- Deleted
I think I found the source for Dunn's graph. C.B. Neblette's book Photography, It's Materials and Processes, 6th Edition, chapter 8 Exposure and Exposure Meters by Allen Stimson.
But when you tell people, for instance, to use the straight line portion of the curve, isn't that based on a theory that you just aren't defining? "Expose for the shadows" - practical advice or theory?
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Stephen;
If you refer to the graph with point (4) on the toe, then I would have to say that just copying Neblette's work does not make it right. Neblette himself was Director of George Eastman House and did many wonderful things, but he may have gotten this wrong as well. IDK. I do know that in practice, if you have a curve such as you showed, and use it to take pictures, there will be blocking in shadow detail unless you give it 1/3 stop more exposure given the speed point you describe.
In Kodak work, we tried to take this into account and move the speed point up by that tiny bit.
PE
If you refer to the graph with point (4) on the toe, then I would have to say that just copying Neblette's work does not make it right. Neblette himself was Director of George Eastman House and did many wonderful things, but he may have gotten this wrong as well. IDK. I do know that in practice, if you have a curve such as you showed, and use it to take pictures, there will be blocking in shadow detail unless you give it 1/3 stop more exposure given the speed point you describe.
In Kodak work, we tried to take this into account and move the speed point up by that tiny bit.
PE
Steve
That is one point we totally agree on. Exposure meters are not necessarily calibrated for 18% reflectance, and more likely, they are calibrated for 12%. I just don't think that it makes a big difference, and in fact, can only lead to 'beneficial' overexposure with negative B&W film.
That is one point we totally agree on. Exposure meters are not necessarily calibrated for 18% reflectance, and more likely, they are calibrated for 12%. I just don't think that it makes a big difference, and in fact, can only lead to 'beneficial' overexposure with negative B&W film.
One, it was Stimson who wrote it. Two, I'm showing it because it represents the standard model for which film speed and light meter calibrations are based upon. I'm not advocating anything. BTW, point 4 is the 0.10 fixed density speed point. You should re-read my posts about how exposure actually falls at least 1/3 stop over speed point according to exposure theory.
Steve
All the theory aside, what PE explains here matches very well with practical B&W photographic experience, and years of film and paper testing.
As you well know, B&W negative film, exposed at ISO speeds, is not very forgiving to underexposure, but it needs excessive overexposure before highlight compression becomes a real problem. So, the difference between exposure-meter calibration and Zone V just helps to move critical shadow densities up the curve without pushing highlight densities too far onto the shoulder. Does it matter to understand the difference? Yes, but how much gets lost in the subjectiveness of Zone System visualization and estimating tonal values? It's helpful to understand the tonal reproduction cycle, the influence of flare and individual material characteristics, but at the end of the day, it still comes down to:
When in doubt, overexpose and underdevelop!
Nevertheless, I truly appreciate your scientific approach, because only the one who fully understands the underlying principles can knowledgeably chose what is, and what is not, important for a successful imaging process.
And, when I use the term 'fully understand', I really mean 'as well as we can', because after 180 year of photography, there is still nobody who fully understands what really happens when light strikes silver salts and development agents 'reduce' them to metallic silver.
This was a great thread!
A big difference to me is when people think they are getting one thing when they are not. I like to understand what is really going on so I can make the decisions myself and not just follow what I've been told to do. That makes a big difference to me.
We agree on the part about the potential benefits of additional exposure. For some reason, people tend to assume that just because I am explaining how ISO and exposure is designed to work I am a stickler for the principles in practice. Just the opposite, I believe a thorough understanding of the concepts involved frees you up.
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