Have I Misunderstood ISO?

[Bill Burk]

You could use XTOL if you want since you have it mixed up. I don’t want to waste your supplies. If you can find a time contrast chart, choose a time that aims for 0.62

 

[focus_on_infinity]

You could use XTOL if you want since you have it mixed up. I don’t want to waste your supplies. If you can find a time contrast chart, choose a time that aims for 0.62

Kodak's is 0.56 for 400/800, which is what I used. But I'm curious, how much of a difference is 0.62, and where would I find a chart like that? Or, would one just linearly interpolate the development time from the 400 and 1600 values? At a glance, it looks like there are at least some values on the Kodak sheet that were derived this way.

 

[Anon Ymous]

Kodak's is 0.56 for 400/800, which is what I used. But I'm curious, how much of a difference is 0.62, and where would I find a chart like that? Or, would one just linearly interpolate the development time from the 400 and 1600 values? At a glance, it looks like there are at least some values on the Kodak sheet that were derived this way.

The Tri-X datasheet has the chart you need. For several developers actually.

 

[Bill Burk]

The Tri-X datasheet has the chart you need. For several developers actually.

Right! The graphs you want are labeled "Contrast Index Curves" and you pick the one for your film size that best fits how you develop your film. For example the time I use ( 13:30 for D-76 at 1:1) best fits the graph "Contrast Index Curves KODAK PROFESSIONAL TRI-X 400 Film / 400TX, 35 mm and 120-size"

Aiming for 0.62 meets the contrast parameters of ASA/ISO. If you develop to that contrast, the reference point for measuring speed is where the curve crosses 0.1 above base plus fog. If you develop to 0.56 contrast and want to find speed... you'll have to locate the 0.3 gradient and then determine the appropriate index based on that (I use a Delta-X transparency for the purpose). But you should probably develop a test roll to 0.62 to keep things simple.

 

[pentaxuser]

But you should probably develop a test roll to 0.62 to keep things simple.

In that case then TMY -400 the times in Xtol for both 400 and 800 are the same but the CI jumps from 0.56 to 0.62 In all other cases both times and CI increase so why doesn't this make TMY -400 an 800 speed film? Its the only film on the Xtol sheet that appears to be able to square this circle. So 2 TMY-400s exposed at 400 and 800 respectively and developed for the same time results in 2 different CIs of 0.56 and 0.62. I am having difficulty getting my head around thIs. Would you be able to see this difference on the negatives and how would this difference appear compared to TMY-100 where the times increase to get the same increase in CI?

Thanks

pentaxuser

 

[Bill Burk]

pentaxuser,

I think they are saying that if you develop TMAX-400 to 0.56 Contrast Index you can get excellent prints with Exposure Index 800.

I think they’re saying that TMAX-400’s characteristic curve breaks away from the traditional shape that the ASA parameters were derived from. ASA parameters were determined by the studies that found excellent prints could be made with negatives that had the deepest shadows at “0.3 gradient” (and that falls 0.29 LogE “to the left” of 0.1 above base plus fog when you develop the film to the ASA contrast parameters).

I think they’re saying the straight line extends all the way to base plus fog. I believe it. But that point is about 0.3 “to the left” of 0.1 above base plus fog... so that can’t be the whole answer.

Maybe the comparison to traditional curves is long toe film starts to degrade once it hits 0.1 above base plus fog, while TMAX-400 gives detail all the way to zero. I believe that too. In that case maybe TMAX-400 gives you a full extra useful stop... but that’s silly.

Tri-X’s toe isn’t “that much” different than TMAX-400’s... someone else may have a better explanation.

But no, they’re not saying the same development time gives you two different Contrast Index results. They’re saying you can get away with 800 without doing anything special.

 

[pentaxuser]

Thanks Bill I am sure I haven't fully understood your explanation and not because of your inability to explain but my lack of knowledge of such matters as H&D type graphs for films. However what I can say is that in the Kodak Xtol leaflet j-109 March 2008 it gives two column for EI and CI. In the case of TMY-400 the CI changes from 0.56 at EI 400(box speed) to 0.62 at EI 800 but the development time is the same so my conclusion based on this is that in the case of Xtol and TMY-400 increasing film speed alone changes the CI since the development time remains at 6.5 mins in both cases

In the case of all other films I can see on the list the CI also increases when you double the film's speed but the development time increases as well

So my conclusion is that in the case of TMY-400 a CI increase results from a change of film speed from 400 to 800 but this only occurs in the other films' cases by increasing development time as well.

As I recall matters from other Photrio posts some years ago one of the advantages of TMY-400 is that in the event to needing to double the film speed on the same film you can do this without having to compromise on development time. The same development time is right for both speeds. I had assumed that the implication was that in the case of TMY-400 the negatives at both 400 and 800 would turn out the same. Based on a closer reading of the j-109 sheet this isn't in fact the case. The CI does rise as a result of "underexposing" the film by one stop so my second assumption was that Kodak was saying that while the CI may rise from 0.56 to 0.62 it is safe to develop the film shot at both speeds for the same time because the resulting negatives would produce prints that were indistinguishable from negatives at 400 and 800?

It was the last assumption on my part, based on what the Kodak leaflet said, that made me want to ask what difference could we expect to see between TMY-400 negatives shot at both 400 and 800 and what recognisable differences might we expect to see in prints

In my own very limited experience of having shot TMY-400 at 800 then later another TMY-400 at 400 and using the same Kodak Xtol development time I have to say I could see very little difference in prints. So much so that if my budget allows me to buy more TMY-400 after the price increases then I'd probably treat TMY permanently as a 800 film.

However I am assuming that a change of CI from 0.56 to 0.62 must result in negative changes even if I can't see any difference in the resulting prints so the question is: what are the differences even if, in my case, I am having difficulty in seeing the differences

Thanks

pentaxuser

 

[MattKing]

I think you have the cart before the horse.
With TMY-400, if you:
1) expose it normally at an EI of 400, you will achieve the necessary speed point density, accompanied by contrast behavior measured as a contrast index of 0.56; or
2) you can under-expose it at an EI of 800, in which case you will achieve the necessary speed point density, accompanied by contrast behavior measured as a contrast index of 0.62.
The lower contrast index will give you a longer straight line portion of the characteristic curve, and potentially better highlight detail separation than you are likely to obtain from a contrast index of 0.62.
Kodak isn't saying that the prints will be identical between the two EIs if you don't change the development. They are saying that the EI 800 exposure will be better if the development time doesn't change because, while you will lose some shadow detail, you will retain better highlight detail, and the damage to the highlight detail when the development is "pushed" is more important than the damage to the shadows.

 

[focus_on_infinity]

Kodak isn't saying that the prints will be identical between the two EIs if you don't change the development. They are saying that the EI 800 exposure will be better if the development time doesn't change because, while you will lose some shadow detail, you will retain better highlight detail, and the damage to the highlight detail when the development is "pushed" is more important than the damage to the shadows.

Yeah, I think we have the same interpretation. My take on this is merely that Kodak is saying they believe they have an extra stop of latitude, and that you can underexpose and still have enough detail to bring it up without resorting to extended developing. Which, is different from pushing for effect.

 

[138S]

Yeah, I think we have the same interpretation. My take on this is merely that Kodak is saying they believe they have an extra stop of latitude, and that you can underexpose and still have enough detail to bring it up without resorting to extended developing. Which, is different from pushing for effect.

Matt's explanation is flawless, just let me also explain that with BW film you have a 3.3 stops latitude in the shadows if operating standard, this is using ISO speed and developing to 0.62 contrast, time recommended by manufacturer.

If you underexposse TMY with EI 800 you simply will have 2.3 stops latitude in the shadows, but if your scene has no deep shadows then no problem.

Sometime "ISO" is hormonated in the box lettering and datasheet, saying an speed that is calculated with an special developer, of contrast index.

ISO speed is calculated to deliver a 0.1D density at 3.3 stops underexposure, but you also have some variation from the rounding of real speed to standard commercial numbers (100-125-160). Also toe can have different shapes, so shadow detail from -1.5 to -3.3 can be a bit more or less compressed.

 

[Bill Burk]

Ah pentaxuser...

You are too trusting of the documentation.

There is the idea you can shoot at 400 or 800 without changing development... That’s because it’s an awesome film.

But CI is a measure of the slope of the film’s characteristic curve between two specific points. You cannot get a different CI from same film developed the same.

You can get negatives of different contrast by exposing film to different subjects with different contrasts. But that’s not what contrast index measures. CI measures sensitometric (step wedge) exposures.


It is a typo.

 

[pentaxuser]

Thanks all for the explanations. Certainly I found TMY-400 very good at 800 but having said that, the occasion I used 800 was a day on which the SBR was quite low and frankly in the U.K. this tends to be the majority of the time, although as Ian Grant pointed out our high SBRs tend to be at "strange" times of the year such a strong sun (relatively speaking) in mid to late Autumn when intuitively you'd not expect probably less problems rather than more.

It may be that developing a film such as HP5+ with frames exposed at both 400 and 800 and developing for the 400 time would give me negs as good as the TMY negs but I don't know as I have never tried this out

Bill, what might the real CI be at 800 and the same development time as at 400. I assume that there is a relationship between underexposing and maintaining the same development time. What might that be?

Seems a pity that Kodak made a typo like this 11 years ago and still has not corrected it.

pentaxuser

 

[138S]

what might the real CI be at 800 and the same development time as at 400.

CI is governed by development, not by exposure. With same development you have the same CI, not mattering what EI do you use.

 

[MattKing]

CI is governed by development, not by exposure. With same development you have the same CI, not mattering what EI do you use.

Mostly yes.
But when it comes to measuring the Contrast Index, you need to make a decision about where on the characteristic curve you take the measurements.
A change in exposure can change where the measurements are taken - and therefore change the result.

 

[Anon Ymous]

Mostly yes.
But when it comes to measuring the Contrast Index, you need to make a decision about where on the characteristic curve you take the measurements.
A change in exposure can change where the measurements are taken - and therefore change the result.

Nope, there's nothing about exposure that would alter contrast index. The characteristic curve covers a wide range of exposure anyway. When calculating contrast index, you place one point of a straight line on the film base plus fog density level. The second point is 0,2 units away from the first, but must fall on the characteristic curve. The third point is on the aforementioned line, but also on the characteristic curve and 2 units away from the second point. You don't have a choice where to put these points, the rules dictate where they would fall. The slope of this straight line is the contrast index.

 

[138S]

Mostly yes.
But when it comes to measuring the Contrast Index, you need to make a decision about where on the characteristic curve you take the measurements.
A change in exposure can change where the measurements are taken - and therefore change the result.

Yes, it can chage the tonal compression in the deep shadows with films sporting toe, which can also be used creatively. In linear films there is less a change. But technically the sensitometric CI it's the same... we have the same curve, just we place our scene zones in different parts of that curve, perhaps this is would clarify the point to OP: Same development same curve, but exposure changes what we compress or what we clip in the scene.

 

[MattKing]

Nope, there's nothing about exposure that would alter contrast index. The characteristic curve covers a wide range of exposure anyway. When calculating contrast index, you place one point of a straight line on the film base plus fog density level. The second point is 0,2 units away from the first, but must fall on the characteristic curve. The third point is on the aforementioned line, but also on the characteristic curve and 2 units away from the second point. You don't have a choice where to put these points, the rules dictate where they would fall. The slope of this straight line is the contrast index.

This is all true - except when you give the film less exposure (EI of 800), the resulting curve moves and changes its shape (a bit). So the three points you take the measurements from are at different actual positions (not relative positions) than the three points used for the measurement at an EI of 400.

 

[138S]

This is all true - except when you give the film less exposure (EI of 800), the resulting curve moves and changes its shape (a bit). So the three points you take the measurements from are at different actual positions (not relative positions) than the three points used for the measurement at an EI of 400.

The film curve it's exaclty the same, but we may deformate the scene tonal curve by placing a certain area in the toe or in the linear section, for example.

 

[Anon Ymous]

This is all true - except when you give the film less exposure (EI of 800), the resulting curve moves and changes its shape (a bit). So the three points you take the measurements from are at different actual positions (not relative positions) than the three points used for the measurement at an EI of 400.

Nope, the points remain at exactly the same place. What changes is which part of the characteristic curve the photograph occupies. It is shifted 0,3 units to the left (one stop) and that's all. But this has nothing to do with the contrast index. The way it is calculated is regardless of what way you meter and expose your film when taking the shot.

 

[MattKing]

Nope, the points remain at exactly the same place. What changes is which part of the characteristic curve the photograph occupies. It is shifted 0,3 units to the left (one stop) and that's all. But this has nothing to do with the contrast index. The way it is calculated is regardless of what way you meter and expose your film when taking the shot.

Even if the exposing illumination changes?
I think we are talking at cross purposes here. The CI doesn't change if the test conditions don't change. But if the test illumination changes - than the CI does change, because the film responds differently to the lowered light levels.

 

[Anon Ymous]

Even if the exposing illumination changes?
I think we are talking at cross purposes here. The CI doesn't change if the test conditions don't change. But if the test illumination changes - than the CI does change, because the film responds differently to the lowered light levels.

What changes when giving film different exposure levels is the contrast at a specific exposure level, i.e. at a specific point of the characteristic curve. Pick any point on this curve and draw it's tangent. The more upswept it is, the higher the contrast at this point and vice versa. Now, imagine the shadows of a scene and where they may fall on the characteristic curve. They may be at, or near the toe of the film's characteristic curve. The toe is a region of rather low, to low contrast. If you underexpose the film, these shadows will be shifted to the left of the characteristic curve, just like any other point. But going to the left means going to a region of progressively lower (local) contrast. Going too far to the left (by grossly underexposing) means that some parts of your image will land on a region of zero contrast. This simply means no image is recorded.

Contrast index on the other hand strictly depends on the development of the film. The developer, the time, dilution, temperature etc and that's all. For a given characteristic curve, there's only one contrast index.

Hope this helps.

 

[pentaxuser]

CI is governed by development, not by exposure. With same development you have the same CI, not mattering what EI do you use.

So Kodak's typo is that it should have repeated the CI of 0.56 against 800? In this case what changes in the negative exposed at 800? I am assuming something "is lost". So is that loss noticeable either theoretically(presumably) or practically(maybe not?)

Clearly if nothing changes in the negatives exposed at 400 compared to those exposed at 800 in a film developed for the same time then we are back to the question: Why doesn't Kodak call the film TMY-800 as it would appear to be just that - a 800 film?

I may not need to say this but knowing how quickly threads on Photrio can become unproductive arguments, I'll state that I am asking these questions to learn and not to generate antagonism between the parties contributing to the discussion.

Thanks all

pentaxuser

 

[138S]

So Kodak's typo is that it should have repeated the CI of 0.56 against 800? In this case what changes in the negative exposed at 800? I am assuming something "is lost". So is that loss noticeable either theoretically(presumably) or practically(maybe not?)

Clearly if nothing changes in the negatives exposed at 400 compared to those exposed at 800 in a film developed for the same time then we are back to the question: Why doesn't Kodak call the film TMY-800 as it would appear to be just that - a 800 film?

I may not need to say this but knowing how quickly threads on Photrio can become unproductive arguments, I'll state that I am asking these questions to learn and not to generate antagonism between the parties contributing to the discussion.

Thanks all

pentaxuser

See this graph from TMX datasheet, for each developer and development time it tells the exact Contrast Index you will obtain, independently of exposure, development directly determines CI:

 

[MattKing]

I'm not going to disagree, but I would point out that the graph doesn't say anything about how contrast index is determined - it just says that it changes with changes in development - which isn't the issue, because it does.
What else can change it?

 

[Anon Ymous]

@pentaxuser It seems there's something wrong with the times/CI for TMax 400 in the j-109 datasheet. Other films have the same time and CI for box speed and plus one stop, but not TMax 400. TMax 400 isn't a 800 ISO film. Had it been so, Kodak would be noisy about it and would proudly be labeled accordingly, as the world's finest grain, sharpest, etc 800 ISO film. If you underexpose this film (and any other for that matter), you will simply lose some shadow detail. You'll probably need a harder paper/filter grade, but you will be able to pull something fairly decent, or even fine from it. If a stop less exposure is crucial to get something usable and sharp, instead of blurry, then go for it. No need to alter development time and push process for a stop less exposure, it's just not worth the hassle and complexity. Just mix such exposures with proper ones and be done with it.