But how do you know that the instrumant you use to measure the step wedge is responding in such a way as to tell you the true relationship between the steps of the wedge and your individual exposures? If you time those exposures, you run into possible reciprocity problems that vary from frame to frame. If you control them by f-stop holding time consyant, you need a ratio of max to min diameter of about 40. Your best bet for accuracy would be Waterhouse stops.
Originally Posted by PJC
I know we are comparing thought experiments. My premise from the start of this thread was that the measurements we contemplate and sometimes do are more nitpickingly accurate than the accuracy of the materials and chemicals we test warrants. Sandy does measurements that let him get a pretty good idea how his customary materials will respond at various stages so that he knows if he will be able to get a good print in the end. If we were engaged in testing some theory of the photographic process, we would be using specially prepared emulsions and chemicals of the utmost purity, not to mention special processing equipment.
Considering what we need to know to be assured of being able to make a print that satisfies our wants, we are worrying too much about whether we are doing it exactly right. At every step of the way, we have controls that let us correct errors in the previous step. These steps are "closed loop" in the sense that they can be repeated with corrections. The initial exposure is often open loop, in the sense that it cannot be repeated because the subject is changing, but it has the greatest leeway for error. What are we woried about?
Kirk, I agree with you and Pete with respect to the H&D curves, but in the case of the BTZS plotter program, it is important that the units in the y axis be similar to those of the x axis, as we are using that same density to provide the exposures. Since the program produces film speeds by calculating the average Gamma or Contrast Index, if the units are not uniform, the slope of the curve can change greatly and thus the film speeds. Of course if one chooses the b+f option of calculating film speeds then the "error" is reduced, but using CI is a more accurate way to estimate film speeds and the relation with development.
Which brings us back to what I imagine is the original question. How should the step tablet be read for the density units (or what we are calling y axis)? IMO and in my work I use the readings given by the Vis mode since in effect these densities are what are providing the exposure to the film and are blocking or letting the light the film "sees" pass through. I suppose one could input into the program evenly divided numbers for the density and choose the b+f option to make curves that are more similar to the H&D curves.
Sandy, I hope you happen to make it back here and take another look, as I have another example for you to consider.
Let's go back and look at your chart on the unblinkingeye.com. Your chart IS prepared correctly even though you have convinced yourself that it is not. Please allow me to convince you that you did prepare them correctly.
Your chart shows what happens when we expose a piece of film to light, process it, and then measure the resulting densities on our densitometer. I know that you have been very diligent in the preparations for making this film test. You have adjusted the height of your enlarger to give an even exposure, you have picked an exposure time that is typical of those you use in the camera, you have adjusted the f/stop of your enlarger lens to give an exposure on the film that will not severely underexpose or overexpose your film, and you have adjusted the color temperature of your enlarger light source to be similar to daylight, which is what you will be using when out photographing in the real world. That's great, all very good things to do.
So now you place your unexposed test film under the enlarger, you overlay it with your step wedge, you push a button, and light pours onto your film. As the light is exposing the film, the light which starts out daylight balanced, is filtered only slightly as it goes through our step wedge. We know this because we have measured our step wedge with our densitometer which is in good operationg condition because we have calibrated it. If we are lucky, we actually have a step wedge that has been calibrated by the manufacturer with VRGB densities. That is not necessary, because we can make these measurements ourselves with our own densitometer, but it would add some extra confidence to our procedure.
So now our step wedge has slightly modified our daylight, it is perhaps a little more blue or red than it was before it hit the step wedge, but it is still very close to daylight as we know our step wedge is well made and fairly neutral because of our testing it.
The photons in the light strike the sensitized silver particles in our film. We now have a latent image on the film. We remove it from the enlarger baseboard and go off and process it. We can use any developer we want. It does not matter. We fix, wash and then dry our test film.
Ok, so now we have made a test film that has an image of our step wedge on it. I'm sure you'll agree with the methodology of all the steps above.
Here's the really important question -
At what point did the UV density of our step wedge come into effect in this process? No significant amount of UV light in our light source that we used to expose the film. We used panchromatic film in our test - that's why we chose to make our exposure with light that was similar to standard daylight.
As was pointed out by a previous poster - the data that we need to collect to place on our x-axis is data about the exposure that was given to our test film. Well, the amount of exposure we have given our test film has been determined by the amount of daylight that has passed through our step wedge. Since we used daylight to make the exposure on panchromatic film, we MUST make our logH measurements using the visual channel on our densitometer.
The UV density of our step wedge does not come into play anywhere here. Sandy, this is where you are getting confused.
The only time the UV density of our step wedge comes into play is when we are placing the step wedge itself onto our platinum paper in order to characterize the sensitometric properties of our paper (because we are going to be exposing our paper with UV light that has passed through the step wedge). That's it. Not when we are making exposures to panchromatic films with daylight.
So taking this all back to Sandy's chart on unblinkingeye.com, it is correct to plot the Visual, Blue, and UV measurements of that test negative all on the same chart. This is because we have given the one piece of film a single exposure. The exposure was made with our enlarger, and with the light quality as close to daylight as we could make it. Remember only one exposure was made - one exposure using daylight. No exposure was made with UV light. This means that all the data that we can generate by measuring the test film density can be plotted together on the same graph. The UV, blue and visual density measurements can all co-exist on the same graph. They all share a common exposure. And since that exposure was made with daylight on a panchromatic film, that exposure is calculated by measuring our step wedge with the visual channel only, not the UV channel.
So I now ask everyone, what am I getting wrong? What scientific principle am I forgetting in this assessment? If I am missing something, I really would like to know. I do believe that I am not missing anything here though.
Sandy, as to your reply above my last post, here are my comments on them:
"Once again, the logE units on the x and y axis are relative, not absolute and the thing that links them for the purpose of graphing is the measuring instrument that is used to measure the step tablet and the tests strips produced from it. Nothing else."
It does not matter if we use relative or absolute values for our log values. We can use either one if we like. Does not matter.
"If you mix values (i.e. different measuring system) you will get apple to oranges units of measurement, assuming that the step tablet is not absolutely neutral in color."
We are graphing two different types of data against each other. We have exposure data on the x-axis, and density data on the y-axis. As I pointed out above, there is no problem with plotting different types of data, and data made using completely different measuring systems. I could plot the position of a snail using fortnights for my x-axis (time) and furlongs for my for my y-axis. We could then figure out the speed of our snail in the units furlongs per fortnight. I can tell you right now the values will be quite small, be we can do it if we like.
When we make and H&D graph, we are plotting Exposure vs. Density. I think you will agree that this are two different types of data. They are apples and oranges. You can do it. (We should remember that "H&D" used to stand for Hurter & Driffield, but now the "H" is the international symbol for Exposure and the "D" is the symbol for Density.)
"If you measure the step tablet in Blue mode and the test strips in Blue mode each unit of density will occupy the same physical distance on the graph. If you measure the step tablet in UV mode and the test strips in UV mode each unit of density will occupy the same physical space on the x and y axis. If you do anything else the units of distance will be of different length on the x and y axis."
This is based on a false premise. See above.
"About the charts, this is no conundrum. Since, as you correctly surmise, the same step tablet values were used for the Visual, Blue and UV curves, the UV curve is not correct. [...] When I prepared those curves I assumed that that the Stouffer step tablet used to expose the film was of neutral color and almost certainly measured it in Visual mode. I now realize that it is not."
It does not matter what the UV transmission of the step wedge was - no UV light was utilized when exposing the test film.
"I have considered the IR film/Pt/Pd scenario you describe."
Thank you for finally addressing it. I have put a lot of thought and research into my posts here and I appreciate it.
"It is not relevant to my point because you are not using IR light to expose the Pt/Pd. print."
Using UV measurements of the film in determining the film exposure is not relavent as you are making it by measuring your step wedge with UV light. My description of the film exposing process should prove to you why.
"You can expose the film with any light to which it has sensitivity and will get an exposure. You could do this with red, green, blue, ultraviolet or infrared, either by using light of that spectrum or using appropriate cut-off filters. This will no doubt result in negatives of different contrast and density but it will have little if any affect on graphing. In other words, whatever light you use to expose the film the densities could potentially read differently if the film is not neutral in color according to the spectral sensitivity of the measuring instrument."
Yes, that is why the spectral sensitivity of our measuring instrument needs to match the spectral sensitivity and the quality of light that we are using to make the exposure. And that is why if we are exposing a piece of IR film we need to make sure our measurements measure the amount of IR light that exposure our film. Not the amount of UV light like you propose. It does not matter if we are printing on Pt/Pd, silver or some IR-sensitive paper, the data we are collecting to measure the exposure of the film needs to accurately portray the amount of light to which the film was exposured.
"This discussion has been interesting. I had been vaguely aware of the issue for some time but when the discussions began there were some questions in my own mind as to what would be the correct procedure when dealing with a step tablet that did not measure the same in Visual, Blue and UV mode. Having now worked through the situation more thoroughly, and having heard everything that has been said by many people whose opinions and knowledge I respect, I am convinced that I am right, not about everything of course but yes with regard to how the step tablet should be measured."
"I would not preclude revisiting the issue at some point in the future but for now I am satisfied that I have nothing more to add. Folks will just have to read the messages for themselves and come to their own conclusion. But my position is that if you donít agree with me we will just have to agree to disagree."
Well, I agree that this has been a very good discussion. And I am sorry if you do not see the principles that I have present you with.
If you have some more ideas on this subject later, I would certainly be interested in hearing them. As I said above, if you find any errors in my reasoning, ones that can be backed up with scientific principles, please let me know. And that request goes for anyone else...
Ed wrote, "Only ... The letters stand for (H)urter and (D)riffield, two guys who did a bunch of work in trying to determine a standard way for expressing film speed."
Ed, Jdef - check in some of your modern photography texts, and you will see that H&D did originally stand for Hurter & Driffield. Later, the letters we commandered to stand for H=Exposure and D=Density as I said.
Last edited by Kirk Keyes; 08-13-2004 at 02:16 AM. Click to view previous post history.
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Originally Posted by Kirk Keyes
I am going to address this issue one last time in a short message.
For one thing I do not want to give the impression that I have either ignored your reasoning or found any fundamental mistakes in it. That is not the case. I have read each of your messages carefully and largely agree with you as it involves typical H&D plotting.
However, all of my messages in this particular thread have been in relationship to using the WinPlotter program in BTZS plotting and with this system there is very little doubt in my mind but that the units of density on the two axis need to be alike in the way they are graphed. Yes, you can use other values but the program has a number of features that do not appear to give reliable results if the densities are not equivalent on the two axis. As you saw from the graphs I posted yesterday a change in step tablet densities results in a change in CI, EFS and SBR on the plot. In my case the difference in reading between Visual and UV values was not all that great and the results still had considerable validity. However, for some time I have noticed that my negatives when developed to a given CI appeared to be giving more than expected contrast in actual printing, which I can now understand in light of the fact that I was using mis-matched densities on the two axis and the step tablet values were higher than they should have been. Of course we have controls in alternative printing that can compensate for minor differences in CI but my goal is to make the best negative possible for the job.
So the bottom line is that I still firmly believe that when working with the WinPlotter program the step tablet densities and the test strip densities should be read with the same light to assure that we are working with equivalent densities on the two axis. And I believe Phil Davis had this in mind when he wrote on the btzs forum, "If you really want to optimize the conditions you probably should buy or make a stained step tablet and use it for all of your calibrations."
Last edited by sanking; 08-13-2004 at 08:28 PM. Click to view previous post history.
Patrick wrote, "But how do you know that the instrumant you use to measure the step wedge is responding in such a way as to tell you the true relationship between the steps of the wedge and your individual exposures? If you time those exposures, you run into possible reciprocity problems that vary from frame to frame. If you control them by f-stop holding time consyant, you need a ratio of max to min diameter of about 40. Your best bet for accuracy would be Waterhouse stops."
Interesting question - that's why we usually have to do a little pre-testing or actually measure the light that is exposing our materials before we can do a proper test. As to knowing the response of our densitometers, we have to trust that the manufactuer has done their job and come up with a design that follows some standards in design and used materials that are comparable from instrument to instrument, like using the appropriate kind of filters like Status A or M filters.
I think people have found that using a step wedge is a much more precise and desirable way than Waterhouse stops - Waterhouse stops will require multiple exposures whereas a step wedge requires only one.
"I know we are comparing thought experiments. My premise from the start of this thread was that the measurements we contemplate and sometimes do are more nitpickingly accurate than the accuracy of the materials and chemicals we test warrants. Sandy does measurements that let him get a pretty good idea how his customary materials will respond at various stages so that he knows if he will be able to get a good print in the end. If we were engaged in testing some theory of the photographic process, we would be using specially prepared emulsions and chemicals of the utmost purity, not to mention special processing equipment."
We are not being nit-picky here. The questions asked here have been in regards to some basic and fundimental principles of sensitometry, not some abstract theory of the photographic process, and therefore these questions apply directly to our daily photography.
Sandy has clearly demonstrated that with the data he presented above with his two graphs, he can get the Average G of his test film to vary from 0.96 to 1.08. That's quite a big difference, and he is right in trying to determine which set of results are true. Using the wrong set of results could make all of his testing for naught - as well as make his prints more difficult to make.
"Considering what we need to know to be assured of being able to make a print that satisfies our wants, we are worrying too much about whether we are doing it exactly right. At every step of the way, we have controls that let us correct errors in the previous step. These steps are "closed loop" in the sense that they can be repeated with corrections. The initial exposure is often open loop, in the sense that it cannot be repeated because the subject is changing, but it has the greatest leeway for error. What are we woried about?"
Remember that some processes will have less controls than others, and we may need to use more difficult techniques to correct any errors that we have in processing. Processing our film to a significantly less than optimal Ave. G will have an undesireable effect that ripples all the way down the process.
Last edited by Kirk Keyes; 08-14-2004 at 03:14 PM. Click to view previous post history.
Sandy wrote, "For one thing I do not want to give the impression that I have either ignored your reasoning or found any fundamental mistakes in it. That is not the case. I have read each of your messages carefully and largely agree with you as it involves typical H&D plotting."
I appeciate that. Thanks.
"However, all of my messages in this particular thread have been in relationship to using the WinPlotter program in BTZS plotting... "
Well there's a disconnect between us right there! I thought we were discussing general principles of sensitometry/densitometry and photography. I thought you were trying to use the BZTS software as proof of your side of the discussion! After all, You did say on page 5, "I can agree with many of the things you are writing but not with the issue of how the step wedge should be measured." Anyway... let's go on.
Sandy wrote, "... and with this system [BZTS software] there is very little doubt in my mind but that the units of density on the two axis need to be alike in the way they are graphed. Yes, you can use other values but the program has a number of features that do not appear to give reliable results if the densities are not equivalent on the two axis."
Even in regards to the BZTS software (I've read Phil's book and looked at the examples of the software so I'm not completely unfamiliar with it), I'm still confused by what you mean by "the units of density on the two axis need to be alike" - The units of the x-axis can only be units of exposure, and the units of the y-axis can only be units of optical density. These units are "alike" in the sense that we have taken the log of each of these values, and since we generate them by measuring a step wedge and our test film, and we can measuring each of them by using the same densitometer, they appear to be the same units. But they are not the same units.
If you mean that we can't plot a two curves on the same graph if they have different sets of x-axis values, I understand that. It is a limitation of the software. But since we are talking about plotting sets of the same film which have been exposed through the one step wedge using the identical conditions, then we will not have two different sets of exposure data values. Only one set of data - one exposure, one set of data. It does not matter is we are reading the UV, B, or Vis of that exposed sheet of film, it only had one exposure given to it. So we can plot UV, B and Vis all on the same graph, even if using the BZTS software.
Sandy wrote, "As you saw from the graphs I posted yesterday a change in step tablet densities results in a change in CI, EFS and SBR on the plot. In my case the difference in reading between Visual and UV values was not all that great and the results still had considerable validity [variability?]."
Yes, it is simple mathematics that causes this as I've said before.
Phil Davis in response to your question posed on the BZTS site even says, "Incidentally, the only reason why the x and y axes have to be marked off in equal increments is because that's what the Plotter expects. This is not necessarily a problem if you hand-calibrate."
So he does admit that you have some limitations that are imposed on you as a user of his software. This is a special case and it is not really too unexpected, it does makes sense for a software designer to force you to enter your data in only one format as it makes their job of processing that data much easier. As he said, if you plot data by hand there is no conflict here.
Davis also says, "First, I suspect you can get good results with any reasonable test methods if you use them consistently and refrain from comparing specific numbers with those from other test methods. In other words, it probably doesn''t matter a whole lot whether you arrive at a nominal SBR of 8.3 or 7.5 as long as you adapt your working habits to match the number of your choice..."
He is certainly right about this - you could use either set of CI, EFS, and SBR vaules you've calculated. One set of results may work with no modification while the other may need an offset added to the results or some other sort of correction factor. But that kind of takes away some of the value obtained by using a good film testing method. We both want our testing to give results that are directly usable.
Let's go back to the data in your example above:
Sandy wrote, "However, for some time I have noticed that my negatives when developed to a given CI appeared to be giving more than expected contrast in actual printing, which I can now understand in light of the fact that I was using mis-matched densities on the two axis and the step tablet values were higher than they should have been. Of course we have controls in alternative printing that can compensate for minor differences in CI but my goal is to make the best negative possible for the job."
Yes, that's why it would be good to get to the bottom of this - is it a procedural/conceptual error or a calibration error that is causing this discrepance?
(Continued in the following post.)
(Continued from the previous post.)
Since you are convinced that you have no conceptual error in your proposed procedure, lets look at your calibration procedure -
Back near the top of page 3 of this thread, Jorge and Helen both discuss the UV vs. Visible light readings that they get when reading their step wedges. As Jorge correctly points out, the acetate base of the wedge will should have a higher UV absorbance than Visual, you found this yourself - 0.05 Vis versus 0.10 UV. Helen finds this as well - that's 3 people in agreement! Helen replied, "I've also noticed that the base adds a uniform level of difference between the UV and visual readings (UV density being higher than visual, as Jorge's readings) - but this does not, of course, alter the relative exposure values."
Helen and Jorge are on to something here. Unless the steps in the commercially prepared step wedge have some sort of staining going on or we have something going on with the shape of the processed silver grains in our step wedge that is causing them to vary absorbance as the optical density changes, the UV measurements should be very directly proportional to the visual readings. We do know that there is some variation in measurements between UV, V, R, G, and B channels, but these are usually pretty small (<0.05D). Your step wedge measures low at the high end by more than 2/3 stop (when you take into account the +0.05 bias that the wedge has when measuring the base in the UV.) That's pretty big and you are right to be conerned about it. By comparison, Jorge's says, "... in my densitometer the readings are uniformly higher by about 0.09 units." Since Jorge's densitometer is uniform in it's measurements (a slightly higher bias) his step wedge mesurements will only slightly affect (1/3 stop) his EFS calculations. Your densitometer not only has a bias with the film base, which is expected and perfectly normal, but your densitometer appears to have a proplem with it's linearity, which will most certainly have an effect on Ave G, EFS, and SBR.
So this kind of indicates that your densitometer, despite being calibrated, may have issues with either the calibration or with it's linear response. I suggest that you look into this issue more, as it may resolve the problem with your aforementioned conclusions.
As I beleive Patrick pointed out above, most densitometers are calibrated (for transmission measurements) by measuring a "Zero Point" by taking a measurement with no film or standard in the densitometer and then a "High" calibration point is measured to set the slope of our densitometer's response. The zero point is the intercept and the High point sets the slope that Patrick mentions.
It is the linear response of your densitometer that may be changing as you change channels that may be causing errors in your step wedge measurements. This could be an isue with the response of your densitometer's response or an issue with your calibration standard.
As I mentioned earlier, a friend and I were comparing film density results and we realized that one of our densitometers had linearity issues. It probably means that the problem densitometer either needs some adjustment to its electronics or it needs to have its internal channel filters replaced. We determined this by comparing the linearity of the response of our two densitometers.
I don't believe that you mentioned what kind of standards you are using to calibrate your densitometers. Could you describe your calibration procedure and the type of standards you use? Do you have any standards that you can use to check the linearity of your calibration, i.e. one with other steps on it? If so, could you describe them as well? Also, are your standards new enough that we can be reasonably certain that they have not changed significantly with time?
You say you have a Gretage D-200 and the X-Rite 810. I have not used either of them but I have looked at some info on them found on their manufactuers web sites. The Xrite 810 is designed to do Status M transmission RGB filters. These are designed for use with internegatives that will be printed on color neg paper. This is not the right kind of densitometer for measuring the materials under discussion here. Ideally, you should find a densitometer that uses Status A filters, e.g. the Xrite 811, 820 or 310. Your 810 does not do UV measurements I see.
The Gretag D-200 looks like a very nice machine! It looks like you have to swap out measuring tubes to switch between UV and RGB, right? Looking at the manual for the D-200 II, it looks like to calibrate it you make a reading on the "zero field" of the calibration film and then make a second reading with a high point of about 3.0D and adjust that reading to a value that Gretag has specified for that standard. Do they give individual calibration values for RGB/UV, or just one?
They then say to perform a calibration check by reading the standard again to verify the results. This only confirms that your densitometer can accurately read those two values. But it does not prove that there are not any linearity issues - we can draw many different curves that both pass through two points and they can all hit completely different points both in between those two points as well as beyond that high point. So we need to verify the linearity of your densitmeter.
What I'm wondering is does Gretag give you a standard for each measuring tube, or do they give one standard with a calibration value to be used for each measuring tube. If you only have one value and are applying it to all measuring tubes, that may cause the error that you may be seeing in your calibration as your calibration film is probably not completely neutral across the range of R, G, B, and UV.
This method of calibration also does not allow us to check the linearity of our densitometer either. As I mentioned on page 4, I have a calibration wedge made by X-rite that has 4 calibrated steps on it, and each step has calibratin values in R,G, B, and Visual channels. Originally I had a single piece of what looked like litho film that someone had marked 3.19 onto a piece of tape and affixed it to the film. Now that I have the X-rite calibration standard, and I have demonstrated that my 3.19 film was off by quite a bit. I had to reread all my step wedges because of that error.
As I said, the X-rite film has 4 steps - you calibrate the densitometer with no film for zero, and then use step 3 on the X-rite film for a Hi Cal point, it's around 3.0D. So I calibrate in exactly the same way as you do. But then the film also has 3 other steps, about 0.25D, 1.50, and 3.75 with which I can check the linearity with. One reason I'm interested in how you are calibrating is that each step on the X-rite cal standard has a slightly different value for Visible, R, G, and B channels. For example, on the Hi Cal step:
I have no idea what the UV value of this step is, but I have no reason to beleive that it is equal to any one of the RGBV values (although I suspect that is would be close, but it may be off a fair bit - who can say?).
Note that the R and G channel readings are different by 0.05D. Not far off, but not the same. And these are not based on measurements that I have made with my densitometer, these are the calibration values that the calibration technician at X-rite has hand-written on my standard. So I would expect that unless someone got a really neutral step wedge (which is probably not very likely), we should see a difference between readings in different channels.
So did Gretag supply a standard specifically for the UV channel, and do they have any linearity check standards that you can get to verify the performance of your densitometer?
I strongly suggest that you get one of these calibration standards for your RGBV work. The X-rite transmission standard part nr. 810-68 costs around $40. The reflection plate they offer also has a mid-point spot with which you can check reflection linearity. Check out this page, it lists both reflection and transmission standards suitable for what we are talking about here: http://www.xrite.com/product_accessories.aspx?Line=17 )
I hope this discussion has given you more things to think about for this subject. Remember that if we have an issue with our calibration, then that will affect all of our results. And it may be key in solving the issue about what channel to use in calibration - BZTS or not.
I understand that you are a professor of language arts and you probably do not have much background in instrumentation. I am an analytical chemist and it have been studying, using, and trouble shooting many different kinds of analytical intrumentation (including densitometers and closely related spectrophotometer) for over 20 years, so I hope you'll take my suggestions seriously.
Kirk, I think the reason it matters is that the data is entered into the BTZS plotter program into pre-set numerical 'buckets' for each step on the test wedge. It makes the assumption in the program that these 'buckets' are 0.15 Dlog units apart. If they aren't, then the gradient calculations will not be correct. Fortunately there is a provision in the program for entering what is called a 'custom' step wedge and that feature will allow you to put the actual units from the particular step wedge you are using. Does this make sense? I believe that you and Sandy are in violent agreement, only the BTZS program has it's own peculiarities in that it will make assumptions about the X values on the plot unless you tell it differently.
Originally Posted by Kirk Keyes