The effect of flare on film exposure and development systems

Discussion in 'B&W: Film, Paper, Chemistry' started by Claire Senft, Apr 11, 2005.

  1. Claire Senft

    Claire Senft Member

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    On a different thread Mr. Sandy King suggessted to another person (Kirk Keyes) that a thread on flare as it pertains to exposure and development. Mr. Keyes passed.

    I find this to be a topic of much interest. I believe that exclusive of the use of filters, it is the hardest variable to predict and to control. I already am aware of the BTZS technique of making a general allowance for flare.

    How do you allow for and control the flare variable in your personal system of exposure and development?
     
  2. noseoil

    noseoil Member

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    Claire, my "system" for film testing is to use the camera and lens combo I normally reach for, and do tests in camera not in the darkroom. While I know this is not as accurate as using an enlarger to control variables, my basic idea is that flare is inherent and part of my exposure inside the camera. Since film tests are the result of in-camera exposures, flare is included as a normal variable and dealt with in development.

    I know there are gaps of logic here, but it works for me. As far as flare from an actual shot, I use a lens hood whenever I think it may affect the image. tim
     
  3. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    Even though flare is something that is taken into account in film development times, I think it's better to control it at the source by shading the lens properly with a compendium hood when possible, or with the best lens shade available when it's not possible to use a compendium.
     
  4. gainer

    gainer Subscriber

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    I think that we have to get used to the idea that we cannot predict everything about any photograph we are about to make. In years gone by, many of them for me, the first question asked when you announced that the pictures came in was "How did they come out?" Things have change a lot, but not when it comes to flare. A camera with ground glass and a good dark cloth is a help, but not absolute. We all know that the general effects of flare are to reduce contrast and to make a short toe film act like a long toe film, but how much? It's a good thing we who do our own have the ability to adjust what we could not predict.
     
  5. Ornello

    Ornello Inactive

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    Primarily, there is the choice of film stock and the use of flare-prevention techniques in taking. In a photograph that includes an overcast sky, there will unavoidably be an increase in flare, which will affect shadow densities most of all.

    Films intended for outdoor/multipurpose use (e.g., old Super-XX) have a short toe, which tends to fight the flare to a certain extent. Film intended for studio use, where flare is much lower (e.g., old Portrait Pan, Ektapan) generally have a longer toe. Using such films outdoors under high-flare conditions tends to yield rather weak, soft shadows.

    The number of films available has been severely reduced over the last 15 years or so. The variation in curve types has also been reduced.

    See attached.
     

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  6. Stephen Benskin

    Stephen Benskin Member

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    It's always a good idea to control excessive flare, but 80% of flare comes from the subject itself. A white card against a black background will produce a very difference flare value than a black card against a white even though both scenes have the same luminance range.

    The only way to realistically deal with flare is to use averages. For an average scene, flare is considered to be 0.34 for larger format lenses and 0.40 for smaller format lenses. The difference is mostly from the smaller number of lens elements used in large format lenses. The general rule of thumb is for a 0.10 difference in flare for each stop difference in luminance range. (2.2 is considered average luminance range). Kodak's aim CI numbers confirm the average flare values. They used to use CI 0.56 for normal and changed it in the early 90s to CI 0.58 to (I believe) better represent the predominance of 135mm users.

    There are two different camps on the issue of how to calculate flare's influence when determining aim contrast indexes. One is to use the average flare value for all luminance ranges. This is known as the fixed flare method. The other approach is to use an approximate flare value for the various luminance ranges (see previous paragraph) known as the variable flare method. The equation Kodak uses is Neg DR / LSLR - Flare. The difference between the two methods becomes really obvious as you move to the more extreme luminance ranges.

    Flare isn't always unwanted. Film speeds would be approximately one stop slower without it. Film processing times would also be shorter (sometimes too short). It also helps keep processing times from being too long with scene's with high luminance ranges (pushes) and too short for scenes with short luminance ranges (pulls). But what about the influence of flare on the quality of the image? Tone Reproduction theory suggests that average flare really doesn't cause too many problems. It seems that we are O.K. with compression of tones in the highlights and shadows as long as the midtones are produced slightly more contrasty than the original subject. In fact, our eyes compress the shadows normally to begin with. We not only don't mind slight compress of the extreme tones, but if the values of the print perfect match the values of the original subject in distribution and density, the average person wouldn't consider it to be a good photograph.
     
  7. Claire Senft

    Claire Senft Member

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    Mr. Beskin, I concur with your analysis that flare can be as much a help as a problem. I did get lost when you said NEG DR/ LSLR which I understood to be the density range of the negative/ ?. What is meant by LSLR. I freely admit my ignorance of what may be understood by all others.

    My main question of course was not aimed at lenshoods but rather achieving with more precision and accuracy in controlling its effect its effect on the negative at he time of exposure and in the determination of the development..
     
  8. Stephen Benskin

    Stephen Benskin Member

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    Sorry I wasn't more clear. LSLR is short for Log Subject Luminance Range. BTW, the Negative Density Range can also be thought of as the paper's Log Exposure Range (LER) and it's value comes from determining the LER in a paper test. An example using the equation is:

    1.05 / 2.2 - 0.40 = 0.58

    From there, it's really easy to calculate the entire range of CIs for your conditions. Below is a table of CIs for different LSLRs using the various flare methods. The question of which is the best method depends on your perspective. No Flare is good for contacts (ie dupe negatives, etc) but not for much else. Variable flare is technically the most accurate, but because of the large difference of flare depending on the tonal distribution from any given scene, is there enough benefit using the variable flare method to justify the additional effort to calculate it? Is the fixed flare good enough for government work? Kodak seems to thing so because they use the fixed flare method. I've also added compromise method that I recommend called the practical flare method (PFM)

    Dev__LSLR___No Flare___Fixed Flare - 0.40___Variable Flare____PFM
    -2____2.80____0.38_______0.44______________0.48_________0.46
    -1____2.50____0.42_______0.50______________0.53_________0.52
    N_____2.20____0.48_______0.58______________0.58_________0.58
    +1____1.90____0.55_______0.70______________0.66_________0.68
    +2____1.60____0.66_______0.87______________0.75_________0.81
    +3____1.30____0.81_______1.17______________0.88_________1.00


    Hope this helps.
    Attached is a graphic representation of the above equation, and a complete tone reproduction cycle. Notice how different the resulting values are in the tone reproduction quadrant (upper right) comnpared to the original values. This print would be considered of good quality.
     
    Last edited by a moderator: Dec 2, 2007
  9. glbeas

    glbeas Member

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    Ornello I wish I could read your attachment, the pixellation has pretty much ruined the typefaces. Maybe break the two pages into two images? FWIW I'm told a gif file is superior to a jpeg for graphic images such as this, making a smaller file. Jpegs do better on variable tones for whatever reason.
     
  10. Ole

    Ole Moderator Staff Member

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    Gary, the attachments are fine if you view them full size. Shrunk to fit the window (by MS Exploder?) they are unreadable as you say.
     
  11. Ornello

    Ornello Inactive

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    The contents of the page are really not that important. The page is simply a listing of various Kodak films with the applications for which they are best-suited. The point was the wide variety of Kodak films available for specialized purposes in 1950. Among the traits these films had was differeing toe characteristics, which determines whether they should be used under low-flare or high-flare situations.
     
  12. Claire Senft

    Claire Senft Member

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    What a nice piece of data

    Thank you so much Stephen.
     
  13. smieglitz

    smieglitz Subscriber

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    Stephen,

    I've read your article in Photo Techniques, followed a few threads you've been involved in on photo.net and apug.org, and have read through this current thread. Everything I've read seems to make sense yet I'm having a hard time tying it all together. I think this is partially because I've never really concerned myself with the reflective density measurements from a paper preferring instead to visually judge tones and make adjustments to the process on that basis. As a result, I'm wondering if you could clarify something for me that an earlier poster also brought up. I quote "Noseoil" here:

    "...my "system" for film testing is to use the camera and lens combo I normally reach for, and do tests in camera not in the darkroom. While I know this is not as accurate as using an enlarger to control variables, my basic idea is that flare is inherent and part of my exposure inside the camera. Since film tests are the result of in-camera exposures, flare is included as a normal variable and dealt with in development."


    If, for example, I choose to do zone system testing using a specific lens and camera and an average target (gray card), isn't the flare factor automatically taken into account? I mean, as long as I don't change the equipment and I photograph the same types of scenes always (after doing a pictorial test following the gray card density and CI calibration trials), as long as everything comes out in a way I can predict, why should I be concerned with all the computations and graphs regarding flare? Isn't that already visually represented in my personal data?

    I don't mean to be dense here (no pun intended), but I see what you are discussing as highly theoretical, great laboratory science, but not of very practical use to me. As long as I look at my contacted zone ruler and can match certain desired tones with certain densities, why or when would I ever have to calculate the flare factor? I'm really missing that point. I see all this as great if you want to measure true ISO speeds in a laboratory or if you are a film manufacturer or perhaps an optical designer, but isn't it overkill for general pictorial applications especially if you are running more practical tests with standard equipment? In a nutshell, I'm after a personal EI, not an ISO measurement of film speed.

    FWIW, in practice I've found a CI of about .62 worked better for me with TMY when I printed on Ektalure. (I really haven't made an enlargement or silverprint in a couple years since I moved to LF & ULF and Kodak stopped making every product I liked.) I liked my negative zone I to be about 0.12 above fbf and zone V to be about 0.70 above fbf. I called zone IX threshold white and zone X paper base so my interpretation of the ZS was different than most standard texts if that makes any difference to this discussion. IIRC, my paper range was around 1.45 for that scale. I've observed that my negatives tend to be more dense and contrasty than others' but they work well for me.

    Now, I'm not really sure how I would apply the knowledge today although I am probably getting the greatest levels of flare that I've ever encountered. I've started using long diffused focus Verito lenses on big view cameras and shooting nearly wide open in full sun or with studio lights to get the lens abberation effects to kick in. I'm looking for flare. The exposures are complicated by the necessary addition of using Packard Shutters or doing time exposures with a hat and generally employing either 2.0 or 3.0 wratten neutral density filters. I don't think I can practically control the exposures with any degree of precision and repeatibility although I would like too. Plus, I've gone to long exposure scale processes like van dyke brown and salted paper which add further control problems into the mix. I'll probably investigate some long-toe old style films this year since the choices by Kodak in ULF are unsatisfactory (and I'm unsure of Ilford's continued availability). My next venture is into wetplate collodion which I'm assuming will add an even further complication to the issue of process control.

    As an aside, I'm finding your refences to Gustav Fechner and Psychophysics interesting and plan to check the citations you've listed in that regard. Also, I'm not sure anyone actually gave you the answer you were looking for in your thread on the Sunny 16 rule, but I believe that is derived from the "Exposure Formula." An old version of that formula (via AA in "The Negative") takes the reciprocal of the luminance of an object in candles per square foot as the shutter speed while the "key stop" is the square root of the ASA speed. Somewhere Ansel gives the luminance of the clear north sky 45 degrees from the horizon as 256 foot-candles. If one was using ASA 64 film the exposure would be 1/256 second at f/8 for that "average" subject. This would equate to an exposure of 1/ASA @ f/16, here 1/64sec @ f/16 by the Reciprocity Law. I've never read that derivation anywhere, but I believe that is where the Sunny 16 Rule originates.

    Joe
     
  14. Stephen Benskin

    Stephen Benskin Member

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    Wow, Joe, I hope you don't mind if I just touch on some of the subjects you've raised in your rather long and interesting post. The quote you give was the subject of the recent article in PHOTO Techniques. In a nut shell, I found that while there is some flare involved in shooting a gray card, it is only a single tone and since 80% of veiling flare comes from the subject, the card isn't sufficient enough to approximate the influence of flare in general use. I even tried a using a card with a checker board of tones, but it was near impossible to find the right tonal distribution. Ultimately, I found it best to use a technique that eliminated as much flare from the testing as possible and add the flare in later. I actually had lots of data, but it had to be cut from the final draft because of length. The original manuscript was 7000 words and PT likes the average article to be no longer than 2000 words.

    The average shadow falls around 1.28 log units below the meter reading exposure. As you know, ISO film speed is measured at 1.0 log units below the meter reading exposure point, and the Zone System stops the lens down 1.2 log units. The point where the shadow falls is approximately the same point as the fractional gradient speed point. While I'm thinking about it. The speed point isn't necessarily the point where you desire the shadow exposure to fall. It is mostly just a place in which to determine speed. Anyway, the one to 1 1/4 stops of flare bring the shadow up to just over the speed point creating a small safety factor. Of course, this is with normal flare. Higher flare conditions will produce a higher film speeds and lower flare condition will produce a lower film speeds. So, those concerned with "nailing" their personal film speed are mostly chasing their tail. I'm not saying don't test. It's that some people don't know when to stop.

    Now the problem I had with the ZS method of film speed testing is that it doesn't account for the addition of flare on the film speed. So, stopping down four stops is almost the same as the 4 1/3 stop difference assumed for the shadow in the standards, but the ZS stops there. People usually end up considering their film speed to be 2/3 stop slower than it is. Not that there is any wrong with a slight safety factor. I just think it's better to know you have a slight safety factor and not mistakenly thinking it is the actual film speed.

    An interesting observation that I couldn't cover as well as I wanted to in the article starts with the ZS originating in the 40s. At the time, the factional gradient speed method had approximately a stop safety factor. Most of the time, people using the Zone System found their personal speeds closely matched the ASA speeds. In 1960, the safety factor was dropped. Film speeds effectively doubled, but the Zone System didn't change. What does this mean? It means the results from ZS testing give speeds more like the pre 1960 standards then the current standards. So, if you ever read anybody complaining that there was too big of a safety factor prior to 1960 and that it had a detrimental affect on film quality, think about all the high level of quality all the ZS people have achieve. I think this is another example where theory can be beneficial to practical photography. Actually tone reproduction and exposure are both considered theories, just like gravity is still a theory. Technically, you are always using theory.

    The thing about theory is that it is only theory until you need to use it. The variance in film speed is theory, but with it, you are aware the degree of tolerance in exposure and the degree of accuracy. You, yourself are also using theory in your testing with a camera example. It does come in handy.

    Ultimately, all the testing comes down to understanding the materials, and if you have a negative that does what you want it to do, it doesn't matter what technique you used to test with or if you tested at all. Who cares if your negatives have a higher contrast then other people. It's a good negative if it works for you. Testing just tends to simplify, clarify, and to better define. It's a helpful tool - not an end.

    About psychophysics. I personally feel it is at the very foundation of photography. Without it there wouldn't be a standard of quality. After all, quality in a photographic image is subjective.

    One of the reasons why Lyod Jones' work has stood the test of time is because he was the first to use psychophysics in connection with photography. Remember at all those different film speed methods before he came along? Before Jones, a particular methodology was chosen based mostly on assumptions. Hurter and Driffield put a lot of emphasis on the linearity of the straight-line portion. The Germans thought that by developing the film to gamma infinity, they would establish a constant contrast from which to determine the film speed. And the reasons went on. Jones had people pick the best looking prints. He then looked at the exposure and development information from those prints, established a procedure from the information and then compared the new procedure along with all the other film speed methods, then compared the results derived from the different methods to the technical information from the judged quality prints. The method that came closest to achieving the same quality in the most number of situations represented the best film speed method. BTW, Jones has a couple of papers specifically on psychophysics and photography if you're interested.

    You remember the Sunny 16 mess? That was yet another example of me miscommunicating my intensions. You are really close with the 256, but it isn't footcandles (and you need to add K to the value). It is footlamberts (divide footcandles by pi), and it isn't exactly the 45 degree explanation and definitely doesn't have anything to do with Adams. This is a really interesting subject. What makes it special for me is that for any exposure, your exposure meter wants to give the film 8 meter candle seconds of exposure at one second. The value of 8 thus becomes a constant represented as P. P = 8. For Sunny 16 (exposure equation has f/stop as A^2, so 16^2 = 256, giving unity between the exposure and f/stop. All that is left is to divide P by the shutter speed and when the shutter speed and film speed equal unity, the resulting exposure equals the exposure at the meter calibration point or 8/film speed (8/Hg or P/Hg). Notice how P is 10x greater than the constant used to calculate B&W film speed (0.8/Hm). Sorry, this is a rather bad and brief explanation of Sunny 16. What I find really cool, though, is the interconnection of all the equations and values and how it can all be reduced down to two numbers. Something for another day and thread.

    I hope I covered most of your questions. Let me know if there are any gaps or omissions. Also, I've attached an attempt at an example of the exposure equation and its relation to P and Sunny 16 (somewhat).
     
    Last edited by a moderator: Dec 2, 2007
  15. gainer

    gainer Subscriber

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    You can see the effect of flare on your own vision by looking into a shadow area with some detail in it. Use your hands or a short cardboard tube or anything similar to cut out the light from the sides and note how much the visual contrast increases in the shadows.

    As to whether we call it scene luminance or scene brightness, Hardy and Perrin in their description of the 4 quadrant approach to the subject of "Rendition of Tone Values" call it scene brightness. ("Principles of Optics", 1932 edition.) I won't nit pick if you don't. I'm old enough to remember the song with words "You say tomato and I say tomahto" or something like that.
     
  16. Stephen Benskin

    Stephen Benskin Member

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    Jones called it "brightness" too. I believe the accepted term changed sometime in the 1950s.

    I didn't know the "Jones Curve" (4 quadrant reproduction curve) existed as far back as 1932. Thanks for that info.
     
  17. Kirk Keyes

    Kirk Keyes Member

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    Well put, Stephen - I wish I had said that in the CI thread a few days ago!
     
  18. glbeas

    glbeas Member

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    I'm not sure what the old exploder did to it but even bringing the image up more than full size it was still pretty much unreadable. My eyes suffer enough as it is.
     
  19. glbeas

    glbeas Member

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    Should have said so, I can go along with that. I have a few tech manuals I can refer to along similar lines now that I can see your point.
     
  20. gainer

    gainer Subscriber

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    I like the Hardy and Perrin book. I first used it at NASA in some simulation and human factors work. A good part of it is aimed expressly at photography in a quite practical way. It's worth having, but you'll have to get it used. Mayhap you will find it at a good library.
     
  21. Stephen Benskin

    Stephen Benskin Member

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    Just found it at UCLA Library. Thanks for the suggestion.