film speed and curve

Discussion in 'B&W: Film, Paper, Chemistry' started by ElectricLadyland, Mar 25, 2006.

  1. ElectricLadyland

    ElectricLadyland Member

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    so i have read some about film speed and film curve, but i can't know how the two relate. for example, you may have two films with the same personal speed,but still to reach the same density they may need different exposures, as their curve could be different. so, what am i setting my film speed for in terms of exposure? should it be where the meter's reading is medium gray, or where a couple stops under gives good shadows, even if then the meters reading is not middle gray becasue of the curve? or for highlights? how does it all come together?
     
  2. Donald Miller

    Donald Miller Member

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    Film speed, is the response of the material to light exposure. Higher speed materials will be more sensitive to light then lower speed materials. Processing does have an effect on film speed. If a film is processed (developed) longer it will exhibit a higher film speed then if it is processed for a shorter period of time. This higher speed attributed to lengthened exposure is not on the order of full stops...more likely 1/3 stop adjustments may be warranted.

    The curve of the film is a graphic depiction of the measure of the response of the film to light exposure...in other words how quickly it will build contrast. How pronounced the "toe" (beginning of sensitivity to light) of the film is or how abrupt the shoulder (diminishing response to increased light exposure) of the film.

    Hope that this helps. Good luck.
     
  3. smieglitz

    smieglitz Subscriber

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    You want to base your exposure with negative films on the shadow portion of the curve (low density values) because this region of the exposure curve is not affected to much degree by variations in development.

    OTOH, if you were to base your exposure on the midtone or highlight areas where development also impacts density values you may be mixing underexposure with overdevelopment to get the same density value for middle gray as if you exposed at the correct film speed and developed normally, or also if you overexposed and then underdeveloped. Basing your exposure on the shadows and ensuring adequate detail and density in that region minimizes the impact of development changes on the low values. Shadow densities just are not affected by development the same way higher values are.

    A speed point density value of 0.10 to 0.15 above the film base+fog value for a metered target underexposed from the indicated meter reading by 4 stops is a typical target when testing for film speeds. That usually ensures that shadow values where detail and texture is needed receive adequate exposure. Once the proper film speed (Exposure Index) is determined, then development can be adjusted to properly reproduce the overall contrast and desired tonalities in the midtones and highlights.

    After the proper Exposure Index is determined most people would meter a subject they wished to be reproduced as a shadow value 2 stops darker than middle gray yet with full texture. (Stuff like dark clothing or hair, dark stone or freshly-tilled earth, etc.) That sort of subject would be metered and the camera set to expose 2 stops less than the indicated meter reading suggests. Using that method pretty much guarantees adequate shadow densities will be present in the negative.

    This is all gone over in Ansel Adams' Zone System (see "The Negative" by AA) as well as other systems of exposure/development. If you are new to film testing you may wish to start by reading Fred Picker's "Zone VI Workshop" or Henry Horenstein's basic photography text.

    Joe
     
  4. ElectricLadyland

    ElectricLadyland Member

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    thank you smieglitz, that was helpful. i do remember reading to "expose for shadows, develop for highlights". just a thought, could you manualy set you cameras film speed to 1 stop under what it is and overexpose by 1 stop to get the same overall exposure and density?
     
  5. smieglitz

    smieglitz Subscriber

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    If by "under what it is" you mean setting the ISO to HALF the number recommended by the manufacturer (or discovered in your own tests) and your meter is behaving correctly, the procedure you describe would result in an overexposure error of 2 stops. Do you mean set the cameras filmspeed to 1 stop under (i.e., half the normal ISO) and then UNDERexpose "by 1 stop to get the same overall exposure and density?" That should balance out as would doubling the ISO and then overexposing 1 stop if you meter an average subject. I don't know why you would want to do this though.

    A more practical method would be to inflate the exposure index 2 stops (e.g., use a film you've determined to be ISO 400 and set your camera meter to ISO 1600) and then meter a textured dark shadow subject that you wish to reproduce dark but with full detail (i.e., a Zone III subject in Adams' Zone Sytem terminology). The meter would then recommend an exposure to make that subject middle gray (2 stops lighter than it actually is), but the inflated ISO setting would cause a 2 stop underexposure thus placing the exposure correctly for that dark subject. Metering this way would generally ensure proper exposure for shadow values, but you must remember to only meter a shadow target. If you were to set the ISO this way and meter an average subject (midtone), your picture would come out underexposed by 2 stops.

    Joe
     
  6. Stephen Benskin

    Stephen Benskin Member

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    ElectricLadyland writes:
    " you may have two films with the same personal speed,but still to reach the same density they may need different exposures, as their curve could be different."

    There are only points of density you can be relatively sure of are those specifically tested for that film. And those points are usually the speed point and highlight, but because of various factors, such as flare, even those two points are relative.

    Depending on the curve, the same exposure for two films with the same film speed can have very different midpoint densities while still producing identical shadow and highlight densities. For example, the old Tri-X and Tri-X Professional, when adjusted for differences in speed, required an entire stop difference in midtone exposure to produce the same density.

    smieglitz writes
    "A speed point density value of 0.10 to 0.15 above the film base+fog value for a metered target underexposed from the indicated meter reading by 4 stops is a typical target when testing for film speeds."

    Here's an interesting question / thought. The four stop difference from the indicated meter reading is standard methodology. It's been used for decades by tons of photographers. Isn't it interesting that it uses four stops when the ISO standard is 3 1/3 stops (1.0 log units) from the meter calibration point to the speed point? What makes this even more interesting is that the shadow statistically falls 1.28 log units (~4 1/3 stops) from the meter reading.
     
  7. j-fr

    j-fr Member

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    The importance of the developer

    One very important thing to know when talking about film speed and curve: Developers are different! Especially when using a long-toe ISO 400 film like Tri-X, T-MAX 400 and HP5+. Developers like HC 110 and T-MAX RS tend to give an upswept curve, and developers like T-MAX (the one-shot version), XTOL and, most pronounced, Tetenal Ultrafin, produces a much straigther curve. So when using a developer of the first type it's not at all possible to get all the tones/zones correct, no matter how you expose and develop. But when working with a "straigth-curve developer" everything suddenly works. This simple fact is just one of many reason to curse the zone-system as superstition and wishful thinking
     
  8. sanking

    sanking Member

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    Same logic applies to working with films that have very straight line curves such as TMAX-400 or TMAX-100. But you have to take your hat off to persons who are able to make good prints with films like TRI-X 320 that have long toes and flaring curves, though in some lighting situations and with certain processes these film characteristics may be desirable.

    Sandy
     
  9. ElectricLadyland

    ElectricLadyland Member

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    smiieglitz, yes i did mean to write underexpose, not overexpose. it seems to me though that if your toe is very long after the speed point, lowering the iso from what it has tested will just give you easier metering, so that if the toe is long enough that 1 stop under the meter is a zone 3, you might want to half the EI to have the meter read a zone 3 tone as 2 stops under the meter.

    basically i dont see why having the EI precisely what it is matters or is beneficial, becasue curves vary between films so even same EI films will have differnt tones for the same exposure... but i thik what is important is knowing that at any given EI for a particualr film, this meter reading and developer/time will develop to this tone. what are some thoughts on this?
     
  10. j-fr

    j-fr Member

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    Have you ever seen an official definition of the term EI? EI is just a number that can be used when you feel that ISO is not appropriate. Kodak is using EI in many places in their written material, but nowhere do you get any hint of what they mean. Expose like this and develop like that and you get a negative. Yes, certainly, but how does that negative look?

    My own personal definition of EI is: The value you set on the exposure meter to get a middle grey (for which I prefer a neg.den 0.70). And always balance the grey scale from the middle and read the high and low values from that midpoint. That also makes it so much easier to understand contrast: It is not one contrast for the whole of the scale, but two contrasts: one for the lower part of the scale and one for the upper part.
     
  11. df cardwell

    df cardwell Subscriber

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    An ISO rating refers to how a film performs under specific laboratory conditions, designed solely for the comparison of film peformance.

    An EI rating is determined by the photographer to describe how a film performs in real world conditions. Since film makers have no idea how you are going to use their product, the best they can do is to test in a certain way and then tell you to only use the data a starting point.

    Use ISO data to compare films, use EI to make pictures.

    .
     
  12. Stephen Benskin

    Stephen Benskin Member

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    Excellent observation. The fixed density method of the ISO standard is really a short cut, and the Zone System fixed density method is conceptually flawed. Most people just want to know just enough to make something work. They usually aren't interested in the reason why.

    Without going into great detail, the most accurate speed method is based on the gradient of the curve and not a specific density. It's call the Fractional Gradient Method. Todays ISO standard is representative of this method except it uses a fix density short cut method. Lyod A. Jones determined, after extensive testing, that consistently high quality negatives are produced when the speed point is at a point on the curve where the gradient is 0.3x the average overall film gradient. In many cases, this isn't at 0.10 over fb+f.

    With this in mind, using any fixed exposure or density method of print determination is also counter indicated since there isn't a specific base density in the negative to key on.
     
  13. Donald Miller

    Donald Miller Member

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    Stephen,

    Thanks for your excellent explanation of the objective reality of this matter.
     
  14. Photo Engineer

    Photo Engineer Subscriber

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    Having been involved extensively in speed calculations for one of the first Gold 400 films back in the 80s, I can say that there is a lot of 'slop' in all of these figures.

    We tried to use the ANSI methodology in such a way that ISO, ASA and EI all gave just about the same result. This involved getting the curve shape just right so that all three of these methods aligned to give similar speeds. If they still adhere to this, I would guess that most Kodak films work the same whether you use any of those three to meter your film.

    However, I've found that an overexposure of about 1/3 stop is usually indicated for most B&W and color negative films. This moves the exposure up the scale and gives you less imaging on the softer portion of the toe. It therefore tends to eliminate differences between films and the measuring methods. The latitude of most negative films is long enough to allow for that little bit of overexposure.

    PE
     
  15. j-fr

    j-fr Member

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    Use ISO data to compare films, use EI to make pictures.


    The film speed, ISO or EI, is only half the story. A film speed is like a fish without water. You need to know both the speed and the contrast. Only then can you start making photographs.
     
  16. Photo Engineer

    Photo Engineer Subscriber

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    Although this is correct in theory regarding speed vs contrast, it is actually ony 'correct' for ISO and ASA which are rather constant regardless of mid-scale contrast, and EI which varies very much according to contrast. (this is a relative statement with relation to these 3 values)

    The thing that damps this out is that a good negative film must have a long straight line portion to the curve to have good latitude, and the ANSI standards define that the mid-scale contrast of the best negative films should be between 0.5X and 0.6X or about 0.55 - 0.65 (IIRC - I have forgotten the exact values unfortunately and have no reference here to them).

    Some ANSI definitions seem to do this in terms of latitude and Dmax while others seem to do this in terms of contrast. Since I have usually read much of this in the works of others on this matter, it may be their way of interpreting the standard for some degree of simplicity.

    Now, the thing is that Kodak B&W and color films are designed with a contrast of about 0.6 - 0.63. The professional products are near the lower value and the consumer products are at the higher value (due to lens flare expected in cheaper cameras or single-use cameras). In any event, this coupled with the toe shape allows the ASA, ISO and EI to be close in value.

    In theory, you could have a high contrast, soft toe product which had similar EI, ISO and ASA values, but I wouldn't give the product a long life time. No one would like the results. It would have short latitude and bad tone reproduction along with a number of other faults. By math models, it is possible to come up with an 'ideal' toe and mid scale that yields the best film curve for all, but also brings EI, ISO and ASA into close congruence.

    By that, I refer you to my comment above. I believe that these values are all within about 1/3 stop of being identical on most modern Kodak films.

    PE