help understanding ISO settings?

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stradibarrius

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Here is an example of a shot I took of a violin scroll. 35 mm, Nikon F100, using Ilford XP2+ (C-41) which has a box speed of ISO 400. It was shot using spot metering but is overexposed by about 2/3rds of a stop. I have shot several rolls of this and have learned that I need to underexpose between 1/2 and 1 stop.
So would I just change the ISO setting in the camera to a different value, or just manually underexpose?

 
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Mark,

Don't let the terminology fool you. Exposure Index can mean one of two things. It can be referring to the effective film speed which is the speed of the film under the conditions of use (ie developer/film combination). It can also be referring only to the setting of the camera. With TMZ (P3200) and in most cases, it's the latter.

Steve


The setting of the camera explanation sits well with me. With Velvia / Provia or Delta 100, any speed other than what's on the box is jotted down as EI xxx i.e. Velvia 50 at EI 40 (+0.3) or EI 32 (+0.6). I strictly over- or under-expose slightly: there is no push/pull processing involved.
 

Bruce Osgood

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So would I just change the ISO setting in the camera to a different value, or just manually underexpose?

Yes, that's certainly a good way to do it if you find that you repeatedly need to under-expose by 1/2 or 1 stop.

XP2 is an ISO 400 film so set the camera to EI 640* or 800 respectively and you will get the under-exposure you desire.


*Is my maths right for 1/2 a stop or is that 1/3 (or 2/3)?



Steve.
 
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stradibarrius

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Thanks for the information. Bruce, I went to the link and read it. I was able to understand some of it so I learned something.
Steve, some of the shots on this roll of XP2 that I manually under exposed by about 1/2 stop turned out fairly good. I like the tones and contrast of the XP2 better than Kodak CN400BW.
I currently have Kodak TMax 100 loaded in my camera and maybe by the end of the weekend I will be trying to process my first roll of film!
 

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Mark,

Don't let the terminology fool you. Exposure Index can mean one of two things. It can be referring to the effective film speed which is the speed of the film under the conditions of use (ie developer/film combination). It can also be referring only to the setting of the camera. With TMZ (P3200) and in most cases, it's the latter.

Steve

Steve,

The point I'm making is that if you are only changing the camera ISO setting, and not changing the development process, the effect is simply an exposure change.
 

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So would I just change the ISO setting in the camera to a different value, or just manually underexpose?

Either works. I prefer the manual method. The reason is that the film doesn't lie. Typically if I get a bad exposure, a somewhat regular occurrence :rolleyes: , it's me not the camera or the film.

If your negative is truly overexposed it will be very, very dark, is it?

Also if you have a Kodak "grey card" that's a bit larger than the scroll try metering off that card. I'd place the card right against the adjusters flat to the side of the scroll.

No card, try using some denim.

No foul in using your DSLR as a meter either.
 
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stradibarrius

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I think I just had an epiphany! EI is not absolute...for the same film it may very well be different for me than for you. How I expose, process and even print the film comprises "my" EI for a given film??? Is that correct?
 

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I think I just had an epiphany! EI is not absolute...for the same film it may very well be different for me than for you. How I expose, process and even print the film comprises "my" EI for a given film??? Is that correct?

You are correct, but there is one further wrinkle that may start to apply as you gain experience and confidence.

A photographer varies the EI in order to maximize the usability of the information gained from a light meter.

The EI you choose depends on the film, but it also depends both on your technique and equipment, and on some external factors too.

For example, if you have two different cameras, their shutters may behave differently, and their metering patterns may be different, so you may end up with two different usable EIs for the same film.

You also might decide to customize EIs based on the type of shooting you are doing - for example, use one for flash or harsh and contrasty lighting, and another for dull overcast days.

The developer you choose and your agitation regime can affect EI as well.

Many of these differences are small or subtle, so it probably isn't necessary to worry too much about them at this stage, but if you keep yourself open to the idea that you can adjust how you meter to take into account varying circumstances, you can learn a lot about how to control the results.

Have fun!

Matt
 

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I think I just had an epiphany! EI is not absolute...for the same film it may very well be different for me than for you. How I expose, process and even print the film comprises "my" EI for a given film??? Is that correct?

In every practical sense you are right! Technically you may get an argument but who cares as long as you get good exposures.

Even the water you use to mix the chems, if you develop yourself, will affect "your" EI.

The ISO rating by contrast is made in a "perfect" lab situation.
 

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I have posted these before, but here is an exposure series of Portra 160 exposed at ISO 25, 160 and 400 for comparison.

With a reversal film, ISO is an exact quantity, but with negative films, you have quite a bit of slop, about 2 stops under and 2 stops over at least.

So, ISO depends on film, desired result and the scene.

PE
 

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Even the water you use to mix the chems, if you develop yourself, will affect "your" EI.

The ISO rating by contrast is made in a "perfect" lab situation.

Actually, the standards are designed to reflect real world use. Why would they be anything else and mean anything? The lab aspect guarantees that the results come from known factors. How many people consider spectral sensitivity of the film or meter photo cell, meter accuracy, or even hold time?

Now, not to be too controversial, there's a difference between the concept of film speed and personal exposure choice. Most people don't know that the speed point from the ISO standards doesn't represent the aim point of exposure. It's a point to determine the speed calculation. In addition, while it is a fixed density point, it really acts as part of an equation that approximates the fractional gradient method. While not in the standard, the scientific paper that was publish at the same time as the ANSI change in the standard states that if the degree of development is different than the parameters stated in the standard, then a formula known as the Delta-X Criterion must be used to determine film speed. This formula, while still using the fixed density method, produces the more desirable results of the fractional gradient method. When you compare the two methods, film speed changes very little with changes in the degree of development using the Delta-X Criterion. We can therefore conclude that for a given film/developer combination, there is basically one film speed.

This is not to say that you can't set the camera anyway you wish. If the negatives come out consistently thin, then increase the exposure. It could be the meter or more likely the way you interpret the meter reading, but it almost certainly isn't about a problem with film speed.

Steve
 
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markbarendt

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Actually, the standards are designed to reflect real world use. Why would they be anything else and mean anything? The lab aspect guarantees that the results come from known factors. How many people consider spectral sensitivity of the film or meter photo cell, meter accuracy, or even hold time?

Steve, we don't disagree about the manufacturer's ISO rating nor about it's value in the real world. But it's just a reference point.

In general terms, us ex digital guys as we a going through recovery :D don't always see things the same way as others might.

We need to be able to wrap our heads around the concept that in situation "A" (say, with tungsten lights indoors), with film "B" (say, TMax 100) I need to shoot at EI "C" (say 50), with meter process "D" (say an F100 on spot), and develop the film in process "E" (Rodinal at the EI100 times) to get the look I want.

I need to know that if I change A, B, D, or E; C may need to be considerably different.

The reason this is important is because of the path from digital to film.

(Not trying to start a digital discussion here, just trying to give context.)

Back in my heavy digital days I saw it over and over with my amateur buddies at the camera club and in forums. Most digital shooters have a tough time wrapping their head around the concept that two different software programs, say Nikon's Capture NX and and Adobe's Lightroom for example, will always render the same RAW image differently.

The expectation is to be able to get the same result. The reality is that the two programs do things as differently as Rodinal and Microphen and require different styles of work.

This is not to say that you can't set the camera anyway you wish. If the negatives come out consistently thin, then increase the exposure. It could be the meter or more likely the way you interpret the meter reading, but it almost certainly isn't about a problem with film speed.

Steve

Again Steve, we don't disagree. Your practical advice is exactly right.
 
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stradibarrius

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I agree with the piont that Mark is making about going from digital to film. Or at least for me, trying to make everything have an exact predictable outcome. If I use film rated at ISO 400, set at a given speed and aperture then I will always get a predictable outcome. But I have come to realize that this is not correct. When I first asked the question about understanding ISO I was hoping there was some concrete rules but now I realize that even though there are rules there are several factors that determine the final outcome. The ISO rating is established by a standards group to be able to distinguish between different film sensitivity to light. The rating is just a reference point and not an indicator of the final image produced. Factors like, the developer used, the dilution, the age of the chemicals, how much agitation etc. so I have to work out an EI for me and a given film and all effect the final image.
Digital has it's on set of variables which are outside the scope of this discussion.
Steve, Mark has a little insight about where I am coming from because we both are part of the Nikon forum and he has helped me with many questions along the way.
This discussion has been extremely valuable for me!! I think that you have helped me get a basic understanding that I can now begin to build on.
At the beginning of this discussion I would have also asked the question about placing a megapixel value on a film photograph. But now I realize the the two do not equate.
 
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A possible equivalent is that the exposure formula for exposure meters will always aim for an exposure of 8 meter candles before applying shutter speed. In the vernacular, P is the constant, P= 8. P divided by shutter speed will equal the meter candle seconds or exact exposure. The ISO number is part of the equation. P/ISO also equals the exposure necessary to expose at the midpoint. B&W film speed is calculated at a point 10X lower, or 3 1/3 stops, or 1.0 log units, or at 0.8. Some ZS users might notice this doesn't agree with ZS testing (four stops). The implication is that ZS testing, in relation to ISO standards, underrates the film.

example

8/125 = 0.064 mcs
0.8/125 = 0.0064 mcs

Exposure is accurate only at the point measured for the film. The rest is assumed. It's called place and fall. The fall part assumes a statistically average scene of 2.20 log units or 7 1/3 stops with a flare value of 0.30 to 0.34. The mean log luminance or mid-tone exposure is 0.95 log units below the highlights and 1.25 log units above the shadow. The shadows will fall at different points depending on the actual luminance range of the scene.

As you might have noticed, the shadows fall below the speed point. This is where flare comes in. Flare adds around a stop of speed to the film. Since 80% of flare comes from the subject, two scenes with the same luminance range but with different distribution of tones will produce different levels of flare and consequently different amounts of shadow exposure. In general, scenes with lower luminance ranges will produce lower flare values and scenes with higher luminance ranges will produce higher flare values. This, of course, means that it is near impossible to predict precise shadow placement.

Hand held meters must assume all of the influence on exposure from the camera. Through the lens metering doesn't (although photo cell spectral sensitivity is always a factor). BTW, Digital cameras utilize the same basic exposure formulas and scene statistics such as stated above.

None of this tells you how to expose, but I hope it helps fill in a few conceptual gaps.

This is a good paper:

Nelson, C.N., Safety Factors in Camera Exposures, Photographic Science and Engineering, Vol 4, No. 1, Jan-Feb 1960.

Hard to find but also excellent

Holm, Jack, Exposure-Speed Relations and Tone Reproduction, IS&T 47th Annual Conference, ICPS 1994.

Also good

Todd, H.N., Zakia, R.D., A Review of Speed Methods, Photographic Science and Engineering, Vol. 8, No. 5, Sept-Oct 1964.

Steve
 
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If I may make another point. According to Todd and Zakia, "The speed of a photographic material is not a fundamental concept, but provides an index number useful for calculating camera settings." Film speed isn't like miles per hour. It's based on the amount of exposure that will produce a quality print as defined in Jones, L.A., The Evaluation of Negative Film Speeds in Terms of Print Quality, Journal of the Franklin Institute pt 1, Vol 227, No. 3, March 1939, and pt 2 in April 1939.

The concept of quality is an interesting one. As Jones points out, "The problem therefore involves the consideration of subjective factors in addition to those of a strictly objective or physical nature." The tests involved a group of observers determining from a set of prints made from differently exposed negatives. They were to place them in order of worst to best. Jones found that after a certain point, there wasn't a noticeable change in quality. He chose the exposure that made the print that first exhibited a high degree of quality. This psychophysical test is frequently referred to as The First Excellent Print Test.

As part of the evaluation, a set of instructions is needed to help set the judging parameters. Jones considers the expression photographic quality as, "reasonable to assume that this aspect of photographic quality is dependent largely upon the fidelity with which the subjective evaluation of brightness and brightness differences produced in the mind of the observer when viewing the original is reproduced in his mind when he views the reproduction. However, it is not safe to assume, in all cases, that a precise reproduction of the original subjective impression is that which the observer would accept as the best possible photographic quality, for he might be more pleased with a reproduced subjective impression into which some distortion has been introduced."

From my interpretation, if the photographer's intention is to produce an image that does not intend to adhere to the practices defined in the first excellent test, then the very concept of absolute or correct exposure no longer applies.

There's also the concept of "quality" which varies from person to person and within the same person. This is why there are so many "systems" out there that seem to work. So, in effect, you can have a system that is technically wrong or mistaken in how photography functions, but still works. As with all natural phenomenon, one can think of it as a normal distribution curve or bell curve. Most will generally fall in the middle of what can be considered quality, and then it tappers off in both directions but never falls to 0. Think about disposable cameras or the old instamatics. They have a fixed shutter and aperture, but produced acceptable images most of the time. The whole idea of pushing for speed comes down to what is considered an acceptable result.

After determining the first excellent print and reviewing the technical characteristics of the negative that produced it, Jones then had to find a way to predictably and consistently reproduce that level of quality without having to perform the tedious psychophysical test with each new film or batch. He looked at all the previous speed methods including the popular inertia, England's 0.10 fixed density, and a 0.20 fixed density, but found that a speed point related to the slope of the curve produced a negative closest to the first excellent negative with the greatest number of different film types under the greatest number of conditions.

According to Jones, "From the standpoint of tone reproduction theory there seems to be no justification for the adoption of any value of density as a significant criterion of the speed of a photographic negative material. The primary function of the negative material is to record brightness (today called luminance) differences existing in the scene. Density, per se, has no significance as an indication of the ability of the photographic material to perform this function. The value of negative density by which any particular object brightness is rendered, as, for instance, the deepest shadow, is of no consequence except insofar as it may have some bearing on the exposure time required to make a print from the negative."

"Tone reproduction theory indicates there is only one characteristic of the negative curve that is significant in expressing the capacity of the material to reproduce brightness differences, and it is upon the way in which brightness differences are reproduced that the quality of the final positive must depend. This characteristic of the D-log E (today D-log H) relationship is the gradient of slope since this determines the magnitude of the density differences by which brightness differences in the object will be rendered in the negative and eventually in the positive made therefrom. The term gradient is used in the previous sentence not in reference to the slope of the curve a particular point, but rather it implies a consideration of the gradient characteristics throughout the entire used portion of the negative curve. There seems to be little doubt that the most logical and theoretically satisfactory solution of the speed evaluation problem lies in the development of a method by which the effective camera speed is expressed in terms of an exposure which is some function of gradient."

This became the Fractional Gradient Method where the speed point is found at a point where the gradient is 0.3 (0.3G) that of the overall gradient. For the record, this is not the point where the exposure is to fall. Part of the formula in calculating film speed includes multiplying it by 4 or in effect adding two stops. Before 1960, films like Tri-X and Plus-X had film speeds one stop lower than today. With the 1960's standard, the one stop safety factor was removed. Side fact, I believe the X in Tri-X, Super XX, and Plus-X represents the value of 50. Plus-X as 50, Super XX was 100, and Tri-X was 200. The one stop slower should also sound familiar to Zone System practitioners. In, at least, the first few editions of The Negative, Adams thanks C.E. Kenneth Mees from Kodak for his assistance. Jones worked under Mees and he was putting on the finishing touches to the Fractional Gradient Method at the same time as Adams was working on the Zone System. A 0.10 fixed density, while also popular at that time, could be used to approximate in may cases speeds similar to the Fractional Gradient Method which included the safety factor. However, when the safety factor was removed in 1960, the Zone System never changed. In effect, it still incorporates the safety factor which is why ZS testing generally produces speeds 1/2 to 1 stop slower than the manufacturer's ISO speeds. But as seen by the first excellent print test, extra exposure doesn't reduce quality (apart from adding grain and reducing sharpness, which isn't too big of a problem with large format). So, technically, the Zone System method is incorrect, but the results are satisfactory.

Some might say that the 1960's standards and all ISO standards thereafter incorporates a fix density method of 0.10. True, but only if the contrast parameters are met. C.N. Nelson, who was Jones' assistant, later found that there is a direct correlation between the fix density method and the fractional gradient method. The fractional gradient method was difficult to do and prone to errors because of the difficulty of finding the exact 0.3 gradient point. By following the contrast parameters of the standard, a speed value will be produced that will be identical to that of the fractional gradient method. For any conditions that fall outside of the parameters, an additional calculation must be made. The 0.8/log-H cannot be applied. In fact, when compared to the fractional gradient method, the fix method without the additional calculation will produce underexposure for contrasts greater than the standard and overexposure for those under. So, the fixed density point of 0.10 isn't an aim nor a goal, but a point of calculation. I've written a paper on this which should be somewhere in APUG titled, Delta-X Criterion.

In a comparison test I did for the paper, Plus-X had a speed of 125 for both the fix speed method and Delta-X method at normal processing. At around a +1 the fixed speed method produced a speed of 144 which rounds to 160 while the Delta-X method produced a speed of 129 which rounds to 125 or no change in film speed. The reason is that as the gradient of the film changes, so does fractional gradient point in relation to any density point. The original standard that used the fractional gradient method didn't state a specific contrast except that it had to be over a gamma of, I believe, 0.50. So in effect, when you consider the fractional gradient method, film speed isn't that sensitive to processing conditions. You could even say there is only one film speed to any film/developer combination.

I'm going to stop here because it is getting way too long. Hope this gives some perspective on speed.
 
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I've attached something more specific on exposure. It's a breakdown of exposure using the exposure formula for a statistically average scene of 7 1/3 stops (log 2.20) with average daylight illuminance. The left set of numbers is for Reflection Density beginning at 0 and ending at 2.20. The middle set of numbers is the exposure values for each reflection density not incorporating flare into the equation. The right set of numbers incorporates flare. It should be considered real world values.

The shutter speed is 1/125 and according to Sunny 16, that would indicate a film with a speed of 125. Speed point for a 125 speed film falls at 0.0064. Mid-tone exposure is 1.0 log units higher or an exposure of 0.064mcs. As you can see in the example, without flare, film speed would be over a stop slower. In addition, flare reduces the apparent luminance range of the scene too. Development for a normal 7 1/3 stop scene would be 2.2 - .30 = 1.90 or 6 1/3 stop scene. Depending on the format and lens type, flare for an average scene falls around 1 to 1 1/3 stops. The value of q comes from a rather long equation taking many light loss factors into consideration.

Since 80% of flare comes from the subject, the amount of flare can vary within the same luminance range. This means the shadow values can fall at different places while the mid-tone exposure remains relatively constant. Exact placement of the shadows is impossible without knowing the scene's flare value. (This is something to consider for people who do just black proofing. Also, if you consider the fractional gradient method of film speed, it negates the concept of just black speed determination (except for normal contrast) all together.)

ISO testing is done by contacting using a sensitometer. It introduces zero flare. That's why the speed point is only 1.0 log units down from the mid-point even though the average scene is a stop greater. It assumes flare will bring up the shadow. In this way, we have a controlled test, which has eliminated many of the variables, and which also reflects real world situation.

Now, consider Zone System testing from this point of view. It uses a set-up that produces little flare, a single toned card. Then it stops down 1.2 log units. As I've said before, if you are talking about level of quality, it's not a big deal, but if you are talking about defining film speed, it becomes more important especially when you hear people referring to ZS testing as defining your true speed or even personal speed.
 
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I forgot to mention that in the attachment above, Zone I would fall at 0.0064 mcs for a 125 speed film.

Here are two more papers that help to explain the reasoning behind film speed.

Jones, L.A., Russell, M.E., Minimum Useful Gradient as a Criterion of Photographic Speed, Journal of the Optical Society of America, Vol. 2, Dec. 1935.

This paper predates the first excellent print tests so there's no conclusive results, but it does examine the different photographic speed criteria and is an excellent insight into the strengths and weakness of each approach.

Jones defines three criteria as:
1) The exposure required to produce some specified density.
2) The exposure corresponding to the inertia.
3) The exposure corresponding to the point on the density - log exposure curve at some specified value of gradient.

In the paper, he says, "Since a part of the underexposure region of the characteristic curve of the negative material (he means toe) is very generally used in practical work, at least in the amateur field, it must be admitted that such procedure yields a quality of tone reproduction which is satisfactory or at least acceptable. The question then arises as to how much of the underexposure region can be used and still give acceptable results? It seems obvious that this must be answered in terms of gradient, minimum usable gradient, since it is gradient alone which determines the quality of detail (brightness differences) in reproduction. It does not seem possible that the question of how much of the underexposure region can be used can be answered in terms of density which of itself tells absolutely nothing as to the way in which brightness differences (today luminance differences) will be reproduced. (emphasis is mine).

The other paper is:

Jones, L.A., Nelson, C.N., A Study of Various Sensitometric Criteria of Negative Film Speeds, Journal of the Optical Society of America, Vol. 30, No. 3, March 1940.

This paper comes after the first excellent print test and the corresponding papers. It examines a number of various speed criteria in relation to the results obtain from the first excellent film test to determine which method "will yield results in close agreement with those obtained by the direct psychophysical method" of the first excellent print test.

His conclusion was "from the standpoint of tone reproduction theory there seems to be no justification for the adoption of any value of density as a significant criterion of the speed of a photographic negative material. The primary function of the negative material is to record brightness differences existing in the scene. Density, per se, has no significance as an indication of the ability of the photographic material to perform this function. the value of negative density by which any particular object brightness is rendered, as, for instance, the deepest shadow, is of no consequence except insofar as it may have some bearing on the exposure time required to make a print from the negative."

"Tone reproduction theory indicates that there is only one characteristic of the negative curve that is significant in expressing the capacity of the material to reproduce brightness differences, and it is upon the way in which brightness differences are reproduced that the quality of the final positive must depend. This characteristic of the D-log E (today D-log H) relationship is the gradient or slope, since this determines the magnitude of the density differences by which brightness differences in the object will be rendered in the negative and eventually in the positive made therefrom."

For the record, Jones was chairman for the ASA photographic standards in the 1940s, and was instrumental in establishing the first ASA film speed standard and I believe around 40 more.

I found all the papers listed in various posts in this thread at my local university. It got a little costly photo copying everything, but it's been well worth it.

While browsing through the folders where I keep all the research papers, I stumbled across a few more that merit mention for those interested.

Simple Methods for Approximating the Fractional Gradient Speeds of Photographic Materials, JOSA, Vol 46. No. 5, May 1956.

This paper explains a number of different methods including the Delta-X method which is the underlying method used in post 1960 ISO speed standards.

Dunn, Jack, Expose for the Middle Tones, Photographic Journal, Vol 111, 1963.

Roberts, C.J.V., Photography Before Hurter and Driffield, Journal of Photographic Science, Vol. 39, 1991.

Allbright, G.S., Emulsion Speed Rating Systems, Journal of Photographic Science - Proceedings Issue.

Dunn, Jack, Developments and Trends in Practical Exposure Determination, Journal of Photographic Science, Vol. 6, 1958.

And one I forgot I had and have set out to reread.

Cords, Paul, Fatora, David, Relating the Ansel Adams Zone System to the ASA Criteria for the Determination of Film Speeds, S.P.I.E. Journal, Vol. 2, 1963.
 
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Attached is a graph showing the relationship between the ISO standard and Delta X. It includes the Delta X equations for determining film speed.

Hfg = Calculated fractional gradient speed point
Delta X = difference between the 0.10 over fb+f and Hfg
Hm = ISO speed point - 0.10 over fb+f

Delta X = 0.83 - (0.86 * Delta D) + (0.24 * Delta D^2)

example
0.83 - (0.86 * 0.80) + 0.24 * 0.80^2 = 0.296

Delta X = 0.296

This means the fractional gradient speed point is 0.296 logs below the ISO speed point when the contrast conditions (Delta D) equal 0.80. The stipulation of 0.80 as Delta D is how Delta X, and consequently the fractional gradient method, is built into the ISO equation without having to apply the extra Delta X equation. However, this is only when Delta D equals 0.80. Otherwise, you have to use the equations, which continue as:

Finding the value of Hfg. Let's say that Hm is 0.0064 which is the exposure for 125 speed film. You need to subtract the value of Delta X, but first you will need to convert it from logs.

Hfg = Hm / 10^Delta X

example
Hfg = .0064 / 10^.296 = 0.0032 mcs

Delta X speed equation is:

Speed Delta X = 0.4 / Hfg

example
0.4 / 0.0032 = 125

Notice how the numerator is different from the ISO numerator. This illustrates how the actual speed point isn't necessarily the target point for the shadow placement when taking an exposure. Why go to all this trouble when you can just use 0.10 density point and 0.8 / Hm to calculate speed? The reason is that the fractional gradient point isn't a fixed point. Delta X isn't always going to be 0.296. It's only that value when Delta D is 0.80. Increased or decreased development will change the value of Delta D and the value of Delta X which represents the difference between Hm and Hfg.

Let's say that Delta X is have the 0.296 value but Hm is the same 0.0064 mcs value.

Hfg = 0.0064 / 10^0.148 = .0045

Delta X Speed = 0.4 / .0045 = 89

In this situation, while the value of Hm would indicate a speed of 125, the Delta X Criterion would indicate the speed is 89 which rounds up to 100. This would indicate a 1/3 stop error would occur in this situation with the 0.10 fixed density method.

The fixed density method tends to underrate (overexpose) film that is developed to a point lower than the ISO contrast specification and to overrate (underexpose) films that is developed to a point higher than the ISO contrast specifications. According to C.N. Nelson in his paper Safety Factors in Camera Exposure, "Delta X speed criterion will continue to be useful as a supplement to the fixed density speed criterion when an evaluation is desired of the effective picture taking speeds of films that have been developed to average gradients higher or lower than the proposed standard average gradient."

This method can work for relative speeds as well. Once you have a curve that fits the ISO criteria, you can assign it an EI (that will have a corresponding exposure value in mcs for Hm. After that, any other tests at different Delta Ds can be evaluated in relation to the EI of the initial test.

Relative Hm = 0.8 / EI
 
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Photo Engineer

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What I posted previously is what Stephen posted but without the math.

Curves and data are posted by Kodak for each product on their web site.

PE
 

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Stephen;

My post is earlier in this thread, but here is a Kodak data sheet:

http://www.kodak.com/eknec/documents/4c/0900688a8088594c/E7023.pdf

See page 4.

They have data sheets for every film showing curve shape and speed as a function of exposure in Lux Seconds.

Siimply stated, speed is the straight line portion of the curve just above the toe that encompases the entire gamut of brightness to darkness in your scene. You may move up that scale until the shoulder curve begins and that is the limit to the speed of your film for all practical purposes, and the distance between is the total latitude of the film.

You cannot use this method for reversal films. In those films, speed is also dependant on Dmax. So, changing Dmax changes camera speed of identical films. This is not so with negative films.

PE
 
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Stephen;

My post is earlier in this thread, but here is a Kodak data sheet:

http://www.kodak.com/eknec/documents/4c/0900688a8088594c/E7023.pdf

See page 4.

They have data sheets for every film showing curve shape and speed as a function of exposure in Lux Seconds.

Siimply stated, speed is the straight line portion of the curve just above the toe that encompases the entire gamut of brightness to darkness in your scene. You may move up that scale until the shoulder curve begins and that is the limit to the speed of your film for all practical purposes, and the distance between is the total latitude of the film.

You cannot use this method for reversal films. In those films, speed is also dependant on Dmax. So, changing Dmax changes camera speed of identical films. This is not so with negative films.

PE

P.E.

Where in the preceding 3000 or so word posting I've just made has lead to you think you have to explain speed to me with such a general manor? I've listed almost a dozen scientific papers on exposure. Do you believe that I've listed these papers without actually having read them?

The only post I've found earlier in this thread by you had only three lines. Delta-X Criterion and its relationship between the fixed density method was not stated. Nor was there anything having to do with the psychophysical method of determining film speed (which btw, as of 2005, has a three part ISO standard). How can you state that you already covered what I have written?
 
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Ian David

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If, for example if I were shooting Tri-X 400 and set the ISO in the camera at 200 would I still meter the same and set the shutter speed the same as if I were actually shooting at ISO400?

This question is a bit confusing. I am sure you are not confused, and apologies if the following is really obvious! It probably now comes out of all the other comments above anyway, but I remember finding a very basic point helpful to keep in mind when I was first getting to grips with manual cameras and exposure, and experimenting with film speed:
It is your metering device that needs to be set to whatever EI you are shooting at. If you are using a manual camera, ask where you are getting your shutter speed and aperture settings from. If they are coming from the camera's own internal meter, then that meter needs to be set to the appropriate EI (eg 200 if that's how you want to rate a particular film). If the settings are coming from a separate hand-held meter, then that meter needs to be set (or compensated) to the appropriate EI. Setting a dial on your camera to ISO 200 won't help if you are actually taking your exposure settings from some other device. For example, the film speed dial on the side of a Hasselblad 503 has no effect on what your camera does at all (unless you are using a plugged-in flash).
 

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Stephen;

I am not criticizing what you say, I am merely pointing out that it can be explained tersely and without recourse to all of the math. As you say, you have read the articles. I have had to do this all in practice from the ground up, building the product and testing it to prove its speed or measure it.

Either way can be acceptable, but the math and means may be beyond some people considering either approach.

Speed is a general matter and can be explained in a general manner. You get good pictures if you get the speed right. The speed is right if the film is not exposed on the toe or shoulder. You can determine that by exposing a straight line reflective neutral scale with your camera! There it is in a nutshell. All of your math still applies, but practially, you have to go out and do the "field work" and that is just as necessary, if not more so than the math.

As a note here, I have been criticized in my emulsion and chemical posts for doing what you are doing but with chemistry to make points and not everyone can follow it. I understand that and that is why I posted a simple methodology that can be done in-camera without math.

No offence or criticism meant.

PE
 
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