"Spectral Sensitivity" "Characteristic Curve" Charts

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DF

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I wish I could understand/interpret what these charts mean in the technical data pages of various films. I've tried but not to much avail.
How might they help or influence your choice of films and do they help with your picture taking?
Any suggested readings that might help?
 

Slixtiesix

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The higher the curve the more sensitive the film reacts to the corresponding wavelength. If you take FP4+ for example, this film is less sensitive to the blue part of the spectrum (400-500nm) than it is to the red part (600-650nm). The light of the wavelength the film is more sensitive to appears brighter in the final print. Older films from the pioneer era of photography were very sensitive to blue light, thus blue things became disproportionately bright in the print. The sky, for example, often became just a bright and uniform area. A dark-yellow filter was needed during exposure to separate it from the clouds. With FP4+, such a filter is not really necessary and if it is used, the effect will be more dramatic. On the other hand, the spectral sensitivity of FP4+ ends abruptly around 650nm, meaning this film is not sensitive to infrared light. If you look at the charts of infrared film, you will see that their graphs go far beyond 650nm.
To sum it up, these charts are pretty useful to judge how a film will convert real colours into different shades of grey and how it will respond to colour filtration.

Edit: This only refers to the Spectral Sensitivity. For the Characteristic Curve, see the posts below.
 
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markbarendt

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IMO one of the biggest things that you need understand about the characteristic curve is that for negative film it is just one part of the puzzle. The paper has a curve too and it's the way the two curves interact that produces the positive.

The other thing to remember is that the printable range, the papers range, is much shorter than the negatives range. During the printing process it must be decided how much of the information on the negative gets printed as a positive. This is an inherent trait of the process that no one and no machine can skip. This is done by adjusting print exposure either globally or locally.

Understanding this is probably going to take a while, books like the encyclopedia of the photographic process can surely help. Books of that caliber will be important in keeping the BS at bay.
 

AgX

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Many posters here at Apug misinterprete these charts as they only see the curves, but not how they came about.
 

tedr1

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There's a book that may help, Exposure Manual by Dunn and Wakefield available for very little online used, it probably has some information on this. Reams could be written on the subject and probably have been, however there are many factors that influence the choice of film for a project, and some very good pictures can be made with no reference to these curves at all just by following the instructions for film speed and processing.
 

markbarendt

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I second ted's thoughts.

Understanding and using film curves isn't required for good work to be done. IMO building one's experience in printing, and manipulating the print result, is much more important to making good photos.
 

removed account4

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I wish I could understand/interpret what these charts mean in the technical data pages of various films. I've tried but not to much avail.
How might they help or influence your choice of films and do they help with your picture taking?
Any suggested readings that might help?
hi DF

to be honest i don't look at or ever think about the curve, ever.
people talk about the curves to me, or i read their posts
and my eyes just glaze over. to some i am sure it means
understanding the film, understanding how to use it
and in what light conditions and how exactly whatever
film they are using will make negatives for them. they talk
about compressing values the toe and shoulder and all sorts of other stuff.
not me. i don't really care what the curves are or how to interpret the charts, i don't.
for me at least that sort of techical "stuff" is the least of my worries...

good luck !
john
 
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OP

DF

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Yes, I mean, my understanding of films such as FP4, PanF+, Delta100 is simply my experience - results I get shooting with them,
non-technical, hands-on, enjoy-it-for-what-it-is approach.
But I will give the recommended readings above a try.
 

markbarendt

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So Kodak has curves that show more info. On the two pdf's I've attached find the characteristic curves, then look at the exposure axis along the bottom.

The difference between each whole number on that axis represents a change of 3 stops of film (camera) exposure.

Notice that the 400 speed film's toe falls roughly about -3.0, the 100 speed film is closer to -2.33. The difference is .6666666667ish or 2/3 or 2-stops.

This shows that the difference in sensitivity to light between a 400 film and a 100 film is 2-stops; make sense? It's showing you mathematically/graphically the real difference in speed between the two films.

Now notice that there are various slopes represented depending on development times. Does the toe move significantly right or left with changes in development time? (The answer is no.)

What is a change in development time called? (The answer is a "push" or "pull" or if you are a zonista "plus" or "minus".)

So, look at the curve again, does a push or pull change the speed of the film significantly? (The answer is no.)

What the curve does when used this way is mathematically take the BS out of a discussion.

Now look at the density axis. This is where it gets fun.

Grade 2 paper will roughly straight print "1 high" on this scale. So lets just pick numbers, by say "placing" print black at 0.4 (with enlarger/paper exposure settings) that means print white will fall at 1.4 yep, it really is about that simple.

Notice that there is more detail below 0.4 and above 1.4 on the curve. If you want to straight print more shadow detail you can "place" say 0.3 as the black point (by adjusting enlarger/paper exposure) but that means white "falls" at 1.3 and there's less highlight detail in the print. "1" all you get to straight print with grade 2 paper unless you are willing to mess with other controls and each control comes with it's own compromises.

Again the curve mathematically/graphically allows us to take the BS out of the conversation.
 

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