Number of Silver Halide Crystals vs Film Speed.

Discussion in 'B&W: Film, Paper, Chemistry' started by had3l, Oct 19, 2007.

  1. had3l

    had3l Member

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    Recently in my photography class, the following question arose:

    Do fast films have more silver halide crystals than slow films?

    My first reaction was to say: "Well, no, what determines the speed of the film is not how many crystals are in it, but the size of the crystals"

    However, my teacher (which I think is wrong on this one) said that fast films are thicker, and therefore must not only have larger crystals, but also more crystals.

    I've searched all over the internet for some kind of proof either way, but I found nothing.

    Can you guys help me? (And if you have a link to a source that supports your claims, even better!)

    (I also earlier posted this on the "Silver Gelatin Based Emulsion Making & Coating" forum, but realised that this question might be better answered here)
     
  2. Photo Engineer

    Photo Engineer Subscriber

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    Most all camera films have about the same amount of silver. The variables for speed are iodide content and crystal size.

    In fact, some slow films have more silver than fast films for a number of reasons. But the answer remains that it varies and basically does not directly depend on speed.

    One problem is that as silver content goes up, so does turbidity and scatter in the film layer which reduces sharpness. So there are practical design limits to the amount used.

    Now, as to references, the problem there is that the actual amounts are closely guarded and the only way to determine it properly is by X-ray fluorescence analysis.

    So, fast is not always thicker, nor is it coated with more silver. The answer is "it depends".

    It is an engineering question based on materials at hand.

    When the film is color, it becomes even more complex, by nearly an order of magnitude.

    PE
     
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  3. had3l

    had3l Member

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    Thanks Photo Engineer, that was really helpful :smile:

    "Most all camera films have about the same amount of silver. The variables for speed are iodide content and crystal size."

    That's exactly what I wanted to know!

    Is there a link to a website or some other source that backs up your statement, just so that I don't look like a fool in class next time I confront him?
     
  4. Photo Engineer

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    As I said, there are no published refereces, but I have another example. The closest discussions on this are in Mees and James.

    Imagine 2 films with the same speed. One uses flat t-grains and another uses octahedral grains. Now, lets assume that they both are coated at the same speed.

    The t-grain allows light through the grain like mica. The octahedral grain acts like a diamond. You cannot see through it, and you get lots of surface reflections.

    So, grains lower down in the t-grain emulsion coating still see light, but the tubidity and reflectivity in the octahedral emulsion limit the penetration of light and sharpness of the coating. The coated quantity of silver at the same speed might be different.

    Coating more silver in the t-grain case won't help much as you started at the max, and coating more of the octahedral grain can cause a speed loss.

    If we compare a 10 micron 1% iodide crystal with a 5 micron 10% iodide crystal, the silver levels might be the same and the speeds might be the same, but grain would be less with the 5 micron crystal all else being equal. So there is a very wide window of design parameters that must be considered.

    This type of engineering consequence is similar to asking how big beams should be in a bridge, and how many cables should be used. There are tradeoffs only apparent to engineers. I would not try to design a bridge and your teacher should not try to design a film unless he has actually done it in practice.

    If you have a problem, put him on APUG and have him PM me. BTW, what school is this???? It sure does not sound like RIT.

    PE
     
  5. mark

    mark Member

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    Just tell the prof that some one smarter than him gave you the answer. ::smile::smile::smile::smile:

    It should go over well. Whatever source you get, do not use wikipedia.
     
  6. Photo Engineer

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    Mark;

    There is an excellent article on film written by Chuck Woodworth formerly of Kodak's production emulsion group which is on a site called howthingswork IIRC.

    It has lots of information on color mostly which is what Chuck worked on. He and I worked together for many years, and I sat next to him at one of my last meetings at EK before I retired.

    He is/was a superb engineer IMHO.

    PE
     
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  7. had3l

    had3l Member

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    It's a film school, the teacher isn't really an engineer and doesn't have any sort of degree really, he is just used to be a professional cinematographer. He is probably alright in an artistic sense, but when it comes to technical details, well...

    Anyway, the exact question was: "Faster films have more silver halide crystals than slower films?" ( ) True or ( ) False.

    It's not a very technical course, and I'd asume this question was only to check if the students knew that what affects the speed of the film is the size of the crystals and not how many there are.

    Still, he insisted that the correct answer was "True".

    According to what you said though, it's clear that the answer is "False".
     
  8. Photo Engineer

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    I would say "IT DEPENDS".

    But the answer is False all things being equal. You can tell him that this was the opinion of a graduate engineer who spent 32 years at Kodak designing films and emulsions.

    The simple answer is that the higher the speed, the larger the crystal, and the fewer the crystals if everything else is held constant. That is a fact!

    Refer him to this thread, or to me.

    PE
     
  9. Ian Grant

    Ian Grant Subscriber

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    If he doesn't believe you tell him to view this thread :D

    Agfa used to provide the silver content of all their colour films & papers in their sales literature, this was actually for silver recovery reasons, I did have data for Kodak films, but it will be in the UK.

    The Kodak data was in one of their silver recovery publications available on the net, it compared the average throughput of minilabs - different types of film & silver content, it used actual examples of Kodak Films and the 400 ISO contained more silver than the 100 ISO. It wasn't a huge amount more but it was significant, so in one way your teacher is right the answer is "true"

    Ian
     
  10. Photo Engineer

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    Ian;

    That is why I said "It Depends". This question is so fraught with engineering and design considerations that you might be comparing a T-grain with a K-grain film in your example, and Kodak had no desire to clarify the matter for obvious reasons.

    Another thing is that different generations of a given product may vary. I've seen color paper vary by 500 mg / square meter between two minor variations due to a coupler change. But the results to the customer were unchanged. The photofinisher saw a difference in silver recovery rate.

    This is not a yes, no answer.

    Thanks Ian for the additional input. The ISO 400 film might have 300 mg/square foot and the ISO 100 might be 200 mg / square foot, but we would have to know the crystal habit and iodide content as well to make a valid judgement. (sorry for the mixed units but that is what I used at EK and I'm not going to convert what I 'know'.)

    PE
     
  11. had3l

    had3l Member

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    I guess so, you can say that some faster films have more silver than slower ones, but that isn't what makes them faster.

    He said he phoned his friend in the laboratory and he said that: "Well, it is thicker, so it has more crystals"

    I think all I have to say probably is: "Ok, but is that what makes it faster? Can I manufacture a fast film that has less crystals than a slow film?"

    I believe that the question is more theoretical than practical, even though kodak might make 400 ISO films with more silver than 100 ISO, it doesn't mean that faster films will always have more silver than slower ones, since it is not one of the factors that determines the speed.
     
  12. Photo Engineer

    Photo Engineer Subscriber

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    Educators teaching film design

    This thread bothers me at a fundamental level.

    If a teacher purports to have the ability to teach film design, they must have a firm grounding in emulsion and photo engineering at the least to do so. This requires references to literature articles, degrees, and etc.

    Both Ian and I have made emulsions and coated them and have reasonable and quite predictable differences due to design considerations and past experiences. I can agree and disagree with Ian without the slightest qualm due to my work at Kodak.

    The teacher OTOH, has nothing to base his/her experience or comments on and that is the crux of the matter.

    If we are to pass on the heritage of analog photography, we MUST pass on the engineering guidelines that go into making a product and this thread points that out. There is no one answer. It is like building a bridge. What bridge is best to go over this span. You get input from several architects and pick the one that best suits the situation.

    We are losing a heritage here. Pretty soon, no one will be able to design a film. :wink:

    PE
     
  13. Photo Engineer

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    Yes. It must have bigger crystals (therefore fewer per unit mass) and higher iodide.

    PE
     
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  15. Ian Grant

    Ian Grant Subscriber

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    The truth is that books written for teaching photography usually state that higher speed films contain more silver. They also state that high speed films are thicker.

    That may well have been the case pre WWII, Ron knows how film emulsions have changed & evolved at EK since then.

    Ian
     
  16. Photo Engineer

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    Ian;

    A simple example to the question is given in my post #13. I would also say that the gelatin would go down in the faster film to pack the grains a bit more tightly, and that is my design consideration. A way to increase speed without increasing silver.

    But that assumes that gelatin was thick enough to accomodate the larger grain in the first place. If the grains make the gelatin layer appear like sandpaper under a microscope, then the gelatin is too low and thickness must increase. Lowering gelatin if it is already too low can cause fog via contact sensitization.

    All in all, this isn't simple and has to be tested. Ian is right, espicially from the historical aspect, due to the fact that early emulsions became less efficient as grain size increased. These were usually single run emulsions in which the iodide became increasingly buried (encapsulated and less efficient) as grain size went up. Ammonia digest only does so much before it causes fog.

    Iodide on the surface can only go so high before it too causes fog via renucleation.

    Therefore, modern double run emulsions have enabled the engineer to change the coating formula more freely as an additional option. This is evolution for you.

    If you take a given emulsion, say 0.2 microns (pure bromide cubes with no finish of sulfur) and coate 50, 100, 200, 300 and etc. mg/ft square, threshold speed (inflection speed in the toe) will increase monotonically up to a given point and level off. Contrast will increase monotonically as well.

    If you coat emulsions of 0.2, 0.4, 0.8, 1.0 and etc at the same level with all else being equal, again speed will go up but not level off. Usually, contrast will go down as developability of the emulsion and covering power of silver will decrease. Threshold speed will become more difficult to measure as contrast goes down. Silver will be adjusted to increase contrast, not speed!

    Blending of 3 emulsions will place you in a mid point of coating parameters (silver and gelatin) for all speeds as far as is possible.

    I have done all of these experiments BTW.

    In any event, one learns from this how to turn knobs in design of a film to get the optimum position for a given emulsion.

    Now, throw in grain type, sulfur and gold sensitization and all of the other factors such as development accelerators, hardness, antifoggants and you have the world of photo system engineering at its deadly finest.

    I appreciate your comments Ian.

    PE
     
  17. AgX

    AgX Member

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    Filmotec released these and other comprehensive data for two of their films. May be they are not actual...
     
  18. Mark Antony

    Mark Antony Member

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    This thread has been most informative for me.
    A couple of questions I have are that are monochrome emulsions multi layered? like say shingle on a beach? or is the emulsion very thin 1 or 2 layers deep?
    I have read people on certain forums say that each film frame has 12 million or so 'grains' and that those grains when developed are black.
    Their argument being that a 12mp digi-cam has the same resolution as film (doubtful) and that each photo site has 256 values when film can only be black or white.
    Also I'd like to know if there is more than one sensitivity speck per grain, my understanding is that AgX has several silver molecules, that are bonded to other elements like Bromide ions, when a photo of light hits that element the silver is free to roam and seeks the sensitivity speck (silver and sulphur)?, I have also heard they use gold to make that process more reliable.
    I hope there aren't too many questions, its just there is scant information on how film actually works.
    Mark
     
  19. Photo Engineer

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    Mark;

    Film emulsions are generally Ag/Br/I atoms combined into crystals from about 1 - 10 microns in size. They contain millions of atoms and many sensitivity specks which consist of sulfur and gold sites placed there along with dye.

    When film develops, it can form anywhere from 3 silver metal atoms minimum up to the entire grain, and grains can be stacked, and therefore the dynamic range of density is analog in nature and virtually infinite. For practical purposes, it ranges from 0.1 - 3.0 density units in a normal negative B&W film.

    Normal negative B&W films are single layers of light sensitive material sandwiched between an undercoate to help it stick to the film and prevent backscatter, and an overcoat to prevent scuff marks.

    The emulsion itself can be a single emulsion or sometimes up to 3 emulsions mixed together in a single light sensitive layer.

    For color film, this is far more complex. I have posted a diagram of the structure elsewhere.

    There is, as I said elsewhere, a good article on this by Chuck Woodworth on the "how things work" site on the internet.

    PE
     
  20. Photo Engineer

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    Do you have a URL?

    PE
     
  21. Ian Grant

    Ian Grant Subscriber

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    Ron, you could try FilmoTec GmbH the site is in English & German.

    Ian
     
  22. Photo Engineer

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    Ian;

    Thanks.

    PE
     
  23. Photo Engineer

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    I've been thinking this over and trying to come up with a better way to express things in answer to the OP which is "number of silvler halide crystals vs film speed".

    Lets look at it again.

    As grain size increases, speed goes up. In a given coating of 100 mg /square foot of silver halide then, as the grain size and speed goes up, at a fixed silver level, the actual number of grains goes DOWN. The larger grain size and decreased number of centers will decrease contrast and increase grain as described in another post.

    To fix this, the grains can be packed more tightly by decreasing gelatin, or the number of grains can be increased to equal the number present in the finer grained film. The latter approach here increases the amount of silver that is coated, but the number of halide crystals can remain the same.

    Therefore, we must distinguish between the number of crystals and the amount of silver coated per unit area. These do not necessarily correlate if the grain size and speed are changed.

    Think about 1 boulder lying on the ground. Now, crush it into many tiny pieces of gravel. The weight is the same, but the number of lumps is greater in the case of the gravel.

    So, I can describe to you a slow film and a fast film which contain the same number of crystals per unit area but where the fast film has more silver per unit area. Therefore, both Ian's answer and mine are correct and again I say that 'it depends'.

    It is entirely possible to have a slow and a fast film with the same number of crystals but with different silver levels. It is also possible to have a slow and a fast film with the same amount of silver but with the fast film having fewer crystals. In this case, the packing of the crystals would be denser (less gelatin) or the crystal habit might have to change, or the crystal content might change.

    Have I done any better this time?

    PE
     
  24. Mark Antony

    Mark Antony Member

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  25. Photo Engineer

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    This statement here:
    ---------------------------------------------------------------------
    "Not so fast! Here's the catch that many testers trip over. Grain particles are binary. An individual film grain can only be either black or not-black, on or off, exposed or not exposed. Sort of a binary device. A photo site (pixel), on the other hand, has a range of thousands of brightness levels, because it's an analog device. (Curious isn't it, that at this level film is binary and digital is analog?)"
    --------------------------------------------------------
    is totally incorrect! A grain has many sensitivity sites and can form from 3 - billions of atoms of Silver metal, but a digital sensor is either on or off. The number of levels (the number of bits - 8, 16, 32 etc determine the number of brightness levels and they are not analog, they are either on or off). The digital effect therefore leads to jagged edges and a stepped look whereas the analog image has smooth curves and a smooth curve of density vs exposure.

    Sharpness is enhanced in digital pictures via software and this can be demonstrated by making a huge enlargement of a knife edge in both digital and analog and then comparing the edges. The edge effects in digital are 'huge' to say the least. To me, they add to the artificial nature of the digital picture.

    This error, from the article quoted above, can show how bad the internet can be for passing along information.

    PE
     
  26. Mark Antony

    Mark Antony Member

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    Thanks for confirming my suspicions. Can I use some of your info in my blog, I'm going to re-dress the balance with a re-buff.
    Mark