Ortho sensitve emulsions
There has been some discussion herabouts regarding the senstivity of films and papers. So, having access to a spectrosensitometer, I have put together a small demo for you all.
First off, a spectrosensitometer is a special sensitometer that uses a monochromator (that is a device like a prism that splits light up into its components) and presisely exposes film at equal energy in 10 nm (nanometer) increments from 400 - 700 nm, which is the visible spectrum.
The examples that I show are as follows:
1. Raw emulsion (AgBr)
2. Sensitized emulsion (I overdid it a bit so it is foggy)
3. 1x level of ortho (green) spectral sensitzing dye added to #2.
4. 2x level
5. 4x level
6. Ilford MGIV paper
Now, the dye I used is a little long, I would prefer something that does not go so far into the red region, and the dye isn't perfect for ortho sensitivity either, but this is a first approximation of an ortho sensitive material.
Anyhow, this is the type of test used to view the sensitivity of a film or paper. These coatings were on Strathmore Smooth at 500 mg/ft square of silver and 1000 mg/ft square of gelatin. Glyoxal was the hardener, TX100 was the spreading agent, and the process was Dektol, stop, hardener fix and wash.
This not only shows where the silver-gelatin experiments are, it gives you a benchmark for what can be achieved with silver-gelatin. This paper material has an in-camera speed of ISO 25 which is what the Ilford MGIV has under identical conditions.
So, if you cut up some MGIV and expose it in-camera, you have a mighty fine ortho paper negative material, however, the contrast varies as a function of color which is a bit odd when you see the results.
PE you are ef-fing AWESOME!
Care to repeat the test with graded paper?
Using film since before it was hip.
"One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal
, Vol. 1 (8 Jan. 1819): 8-29. p. 11
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I disagree. Ilford MG4 has very much lower than that speed, more like 2.5 or lower, if you measure it as a negative emulsion. Of course, one of the problems is to develop the emulsion to a g-bar of 0.62 but even if you accept 2 or so, the speed is still very low.
Originally Posted by Photo Engineer
But bromide emulsion should be easily capable of 25 speed.
In particular, your raw and digested emulsions look quite a bit lower contrast compared to MG4 with monochromatic green light, so it should help you in getting speed above 25.
Also, I don't know which dye you used (it doesn't show J-aggregate so it's not the ones used for color films), but it doesn't exhibit much desensitization with extra dye, so I suspect you added quite a bit of iodide, or made it core-shell?
Finally, do you have a color scan of the test prints?
My exposure was using a piece of MGIV cut to fit a Polaroid holder, then placed on the back of my Mamiya RZ67. The meter on the polaroid back was set at 25, and I used a hand held Sekonic spot meter to verify the exposure. My own emulsion was exposed in-camera using the same method. So, I have in-camera comparison exposures.
Originally Posted by Ryuji
All exposures were then processed and gave good images of the subject material which was a MacBeth color checker.
The experiment also included Polaroid film, Endura color paper and several other B&W papers. I have only included a small fraction of the data here.
Ron, that's not how ASA (or ISO) film speed is measured. We measure speed by the toe exposure at a fixed g-bar, you know that.
But if you are getting speed comparable to MG4, that'll tell me something about the speed of your emulsion.
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Ryuji, I know how to measure ISO, EI and ASA speeds as well as several other methods you may never have heard of.
I have posted some valid results and have ISO 25 pictures to back up the results. I also have comparisons from Polaroid exposures. In addition, the wedge spectrograms were done with a fellow engineer present and were verified by him. These exposures were done a few months ago, and were only posted after I had accumulated all of the various pieces of comparison data.
If you have objections in the form of concrete data, you are free post your actual results. Pictures, wedge spectrograms or step wedge exposures would be appreciated. My pictures with step wedges are posted elsewhere.
I wish you the best.
Ron, Im merely pointing out that Ilford MG4 has ISO film speed of far below 25 even if you accept g-bar of 2 instead of 0.62. The actual speed I get is one-tenth of that. That's all I'm saying. You say you have valid results, but anyone can confirm my claim by sticking in a piece of MG4 in their camera to expose it as if ISO 25 and get a very thin image.
I understand that your wedge spectrograms were properly obtained. I could estimate the speed increase from your digestion process (actually, the toe speed didn't seem to increase much, probably due to the fog, and lack of gold) from them, for example. I also see that your emulsion is far softer than MG4 to monochromatic green light, but still far harder than negative emulsions. I'm actually pretty familiar with wedge spectrogram.
I have data for test emulsions, but I'm way past the stage where I get excited every time I get something new. I use triple jet precipitation (one of them is high linear velocity mixer for controlled iodide injection) I can control key parameters independently, and I usually get effects predicted by existing knowledge and computer models of precipitation process.
The MGIV info is a neat bit of information, and I can use it today !
( being just a humble picture taker, I appreciate it. )
"One of the painful things about our time is that those who feel certainty are stupid,
and those with any imagination and understanding are filled with doubt and indecision"
Hmmm, very interesting piece of information.
Tomorrow I'm doing a film speed test of a new film I intend to use. But this posting has given me something else to do as well
I've just figured out a way to insert small pieces of MGIV to the back of my Nikon F3.
I'll do a few tests on a grey scale and colour chart as well as a pictorial print.
I don't assume a speed of 25 ISO will be great, but it shouldn't take much time to get a working ISO using your information as a start.
Nothing like a bit fiddling here and there to keep the grey matter ticking over, eh?
Many thanks, Mick.
The problem with evaluating speed in MGIV or any MC paper is multifold.
1. Which emulsion do you check for speed point? There are at least 2 in MGIV.
2. Since there are two contrasts, which emulsion do you develop to the ISO standard?
3. Since it a paper, how do you apply film rules.
4. If you develop to a gamma of 0.62, this is severe underdevelopment of a paper material therefore leading to low threshold speed estimation.
So, the practical problem as opposed to theoretical problem is "what do you do to use a paper as an in-camera negative material".
I chose to meter it as if it were a negative material, and therefore got an effective ISO which is in reality an EI of 25 for MGIV. So, if you take MGIV and put it in your camera you can expose it at EI 25 (ISO 25). It works. The pictures are not perfect, but they indicate that the ISO (EI) is close to 25. Endura paper, BTW, is about 25 also when filtered to daylight. I knew this for almost 30 years, having used it (type "C" paper then) many years ago, but the contrast is again high.
There is the balance between theoretical and practical knowledge. Theoretically, developed to a gamma of 0.62, MGIV is underdeveloped and has low speed, but deveoped to a gamma of 2.5 has a speed of about 25 and yields decent pictures in-camera when the negative is scanned and inverted. You can do this in the darkroom or using a digital scanner editor program. You may also wish to lower contrast by using a low contrast grade paper or by digital means. I have done both.
I have examples of most of this on-hand to post if desirable to members. I ask Ryuji to post some examples to further extend our knowledge of silver-gelatin. We have seen many of his comments, but practically none of his work. Seeing some of it would be most helpful to us all, I'm sure.