Number of Silver Halide Crystals vs Film Speed.
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)
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.
Last edited by Photo Engineer; 10-19-2007 at 01:51 PM. Click to view previous post history.
Reason: changed wording a tad
Thanks Photo Engineer, that was really helpful
"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?
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.
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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.
Last edited by Photo Engineer; 10-19-2007 at 02:50 PM. Click to view previous post history.
Reason: I forgot, Chuck is retired! :D
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".
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.
If he doesn't believe you tell him to view this thread
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"
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'.)