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  1. #1

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    Flare question(s)

    Consider the following setup for a 35mm roll film test for tungsten speed and development time, versus contacting:

    Camera with 85mm lens aimed at a brightly lit white card target. The focal plane is about 12" from the target. The lens is focused at infinity. The card is illuminated by flood lights such that metering the card at EI 100 gives a reading of f32@1/15s, so that exposures can be made from zone I (f32@1/250s) to zone XV (f4@1s). The white card is cut to size so that it fills the frame but does not go beyond. It is surrounded by black cardboard. Further, I made a lens insert (fits into the filter threads) made of black cardboard which masks everything outside the frame (picture a black lens cap with a rectangle cut in it).

    So, is this high or low flare? Presumably the masking work reduces camera flare substantially, but I'm still unclear on whether or not the absolute intensity of the target makes a difference. For example, suppose you can expose a frame for zone I in two ways - 1) high intensity as above, f32@1/250s or 2) a target 10 stops less brightly lit, f16@1s. Assuming this is a uniform target, is there a difference in flare between those two exposures? Camera flare? Lens flare?

    And what about the same questions but assuming instead of a uniform target, each exposure is of say a reflectance grey scale tablet, or a backlit transmission wedge (ie rather than a uniform tone in each frame there are multiple adjacent tones)? Does flare play a different role in that case?

  2. #2
    Bill Burk's Avatar
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    Your test is low flare... Think of it this way... The whole picture is same tone... Say a photon for flare reasons doesn't hit the point on the film where the lens theoretically aimed it. Another photon is just as likely to randomly miss where it was supposed to go... and is likely to take the place of the first one... So since the light bounced around at random it didn't change the intensity of the test target.

    Now put a backlit transmission step wedge in the place of the flat white target and you create a high flare situation. Lots of photons from the clear patch don't hit where the lens aimed them, they bounce and land in a dark patch where you were hoping to have nearly zero light.

  3. #3

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    Bill, for the first situation - what about camera flare (let's assume less masking)? For those two equivalent zone I exposures, wouldn't there be more flare in the high intensity exposure versus the low intensity exposure? If the intensity of light reaching the film plane is higher, isn't there more reflection off surfaces inside the camera? Actually now I've just confused myself even further, because I'm wondering if stopping down the lens is different than reducing the exposure time - from a flare perspective.

    Regarding the transmission wedge, so this means that people testing by photographing a wedge taped to a window are getting significant flare effects? Does the intensity of the backlighting make a difference?

    What about photographing a reflection target (say a zone grey scale)?

    And another question - is my lens cap mask redundant since the target in my test is surrounded by black paper? Or does it indeed reduce camera flare even further?

    Thanks for participating. Flare is tough for me to understand (not the impact on testing, but rather how to know when there is flare and when there is not).

    Hopefully Stephen will write in also.

  4. #4
    Bill Burk's Avatar
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    I guess you have reduced flare coming from outside subject with masking and compendium hood... But that only made a naturally low-flare test even lower in flare. Your masking means changing aperture affects flare "less" because masking already ruled out the "sun hitting the camera chamber wall" kind of flare. And that kind of flare would be reduced in a real camera scene with a smaller aperture.

    But the test wedge shot in camera is still high flare. The intensity doesn't matter because time would be reduced proportionally. This might be affected by aperture changes.

    A reflection target would include "normal" flare, depending on the design and layout. One shot of a stepped grayscale could tell you the speed. But a reflection target would require multiple shots to test higher values. But you already know a one-tone target requires multiple shots.

    Many in-camera test target users are satisfied with tests that include flare. Stephen would say it is harder to understand the film response to light when you add variables into a test that could be excluded with a contact test.

  5. #5

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    Interesting - so a reflection target is probably a reasonable way to do things for average conditions. Most of my actual shooting conditions at night are likely low flare so a low flare test is useful. I'd like to compare the curves I get under various testing conditions. Contacting could serve as a "baseline" (which is why I started the other thread about using an enlarger to expose film).

    It's also interesting to consider the size of the lens image circle. I forgot to add the 85mm lens I was using in this particular setup is a tilt/shift. When I initially tried testing with the white card - without masking the lens, there was considerable flare inside the camera leading to significantly higher densities toward the upper and lower edges of the film, compared to completely even densities with the masking in place. This is important for testing with view cameras, and also underlines the usefulness of an adjustable compendium hood in the field.

  6. #6
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Consider the following setup for a 35mm roll film test for tungsten speed and development time, versus contacting:

    Camera with 85mm lens aimed at a brightly lit white card target. The focal plane is about 12" from the target. The lens is focused at infinity. The card is illuminated by flood lights such that metering the card at EI 100 gives a reading of f32@1/15s, so that exposures can be made from zone I (f32@1/250s) to zone XV (f4@1s). The white card is cut to size so that it fills the frame but does not go beyond. It is surrounded by black cardboard. Further, I made a lens insert (fits into the filter threads) made of black cardboard which masks everything outside the frame (picture a black lens cap with a rectangle cut in it).

    So, is this high or low flare? Presumably the masking work reduces camera flare substantially, but I'm still unclear on whether or not the absolute intensity of the target makes a difference. For example, suppose you can expose a frame for zone I in two ways - 1) high intensity as above, f32@1/250s or 2) a target 10 stops less brightly lit, f16@1s. Assuming this is a uniform target, is there a difference in flare between those two exposures? Camera flare? Lens flare?
    Michael,

    Like Bill said, this is definitely a low flare condition. Not only because of the careful masking, but the subject itself. 80% of veiling flare comes from the subject. A single tonal target should produce negligible flare. Another important element in a test like you described is the exposure placement on the curve.

    A flare factor of 2 means the shadow exposure is doubled. For a 125 speed film, the shadow exposure for the statistically average scene will fall around 0.0032 mcs. A stop of veiling flare will bring that up to 0.0064 mcs. Now, if you are placing the exposure at the metered exposure point, then the 0.0032 mcs barely makes a dent in the midtone exposure of 0.064. In camera Zone System type tests meter the target and then stop down. This doesn't change the camera image. All it does is shift everything down the film curve.

    Here it is in graphic form. The camera image curve in quadrant 1 represents a full range subject and not a uniform target. The camera image would be more of a dot if that were the case. The flare at the metered exposure point can quite possibly equal almost zero.

    Click image for larger version. 

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    And what about the same questions but assuming instead of a uniform target, each exposure is of say a reflectance grey scale tablet, or a backlit transmission wedge (ie rather than a uniform tone in each frame there are multiple adjacent tones)? Does flare play a different role in that case?
    Absolutely. Both of those situations increase the potential flare , but perhaps more importantly, you also now have exposure values that are low enough to be influenced by flare.
    Last edited by Stephen Benskin; 07-31-2012 at 10:13 PM. Click to view previous post history.

  7. #7
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    what about camera flare (let's assume less masking)? For those two equivalent zone I exposures, wouldn't there be more flare in the high intensity exposure versus the low intensity exposure? If the intensity of light reaching the film plane is higher, isn't there more reflection off surfaces inside the camera? Actually now I've just confused myself even further, because I'm wondering if stopping down the lens is different than reducing the exposure time - from a flare perspective.

    Regarding the transmission wedge, so this means that people testing by photographing a wedge taped to a window are getting significant flare effects? Does the intensity of the backlighting make a difference?
    From Theory of the Photographic Process, "Jones and Condit found that the magnitude of the camera flare depends on the distribution of luminances in the scene and in the areas surrounding the part of the field of view being photographed. When a large part of the field of view consisted of bright clouds, sky, water, light colored sand, or white buildings, the camera flare was much greater than it was when the dark foliage, dark buildings, or other objects of low reflectance occupied most of the field."

    A high proportion of lighter values in the frame will increase flare as will the higher illuminance that comes with increased luminance ranges, but only if the shadow exposure remains at the same value. The back-lit wedge test will produce a certain degree of flare, but if the intensity of the light source is increased, all the values from the step tablet are also increased. There isn't a change in the the exposure distribution and there shouldn't be a change in the amount of flare. On the other hand, if the intensity of the background is increased when shooting a reflection gray scale while the intensity of the light illuminating the scale remains the same, flare will be increased. Just like any back-lit scene will have a higher degree of flare than a front-lit scene.

    What about photographing a reflection target (say a zone grey scale)?
    One of the difficulties would be in determining the amount of flare present. Is it average? Too low? Too high?
    Last edited by Stephen Benskin; 08-01-2012 at 01:08 AM. Click to view previous post history.

  8. #8

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    So there are really many variables here (possible support for the contacting scenario - as you have said before). But I've learnt something in your last post - ie in photographing a range of tones, either by transmission of reflection, the amount of veiling flare remains constant with changes in the absolute level of illumination (assuming appropriate masking). Do I have that right? I think both you and Bill have tried to get that across to me several times before, but I didn't get it until now. Quite an important point I was missing. I apologize for being slow on this stuff. To be honest I have a hard time following the graphical/quadrant representations.

  9. #9
    Bill Burk's Avatar
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    You're doing alright... And I learned from your real test with your flat target masked off -- that you improved matters by eliminating the part of "Jones and Condit ... camera flare" that "depends on the distribution of luminances in the ... areas surrounding the part of the field of view being photographed"

    The four-quadrant graphs are pretty cool really. The first is subject (lower right) perfect 45-degree ideal and superimposed flare (departs from perfect mostly in the shadows), second (going clockwise) is the film curve you are familiar with, third (not shown) is paper... flipped from normal so that you can continue the dotted lines connecting what you get and what you got... the last quadrant (also not shown) shows the ideal 45-degree again with the final result of everything superimposed.

  10. #10
    Stephen Benskin's Avatar
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    I don't know if this helps since I'm not sure what part of the graph is tripping you up. Maybe we can use this as a jumping off point?

    Click image for larger version. 

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    The advantage of the camera image / flare curve is that it enables you to evaluate exposure separate from but interconnected with the film curve. I believe many people tend to confuse sensitometric exposure, which defines the film curve, with the camera image, which I like to picture as being superimposed over the film curve. The multiple quadrant approach makes it easier to distinguish all the elements.

    For a given scene, the camera image doesn't change shape. It just moves up and down with changes in the camera settings. And for a given film test, the film curve is completely static.

    Pick one of the guidelines starting at the subject luminance at the top of Quadrant 1. This represents the luminance of the subject. Following the line down until it intersects with the camera image curve. The f/stop and shutter speed settings on the camera determines what the illuminance value will be. From there, based on the film test, we know where that particular point of exposure will fall on the film curve.
    Last edited by Stephen Benskin; 08-01-2012 at 11:29 PM. Click to view previous post history.

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