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  1. #1
    andrew.roos's Avatar
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    Characteristic Curve for Ilford Delta 100 in ID-11 (1+1)

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Name:	Delta 100 Characteristic Curve.JPG 
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    I used my LunaPro SBC with Lab attachment to measure the HD curve of Ilford 100 developed in ID-11 (1+1) for 11 minutes at 20 C with intermittent agitation (5 inversions once a minute). This is the time recommended by Ilford for normal development. Temperature was controlled using a water bath and the developer came out at 20.2 C.

    The blue line shows the measured density. The red line is a best fit straight line to the measurements with exposures from -3.1 to 6.3 stops. The X axis (exposure) is shown in stops relative to the exposure indicated by my spot meter when set to ISO 100 (box speed). Densities are shown relative to film base and fog. I estimate the error margin to be about +/- 0.1 density units (1/3 stop).

    The graph is close to a straight line for exposures from 3 stops below the indicated exposure to 6.5 stops above the indicated exposure at ISO 100 (in other words, from Zone II to half a stop above Zone XI if shot at box speed using my meter). Outside this region it flattens quite rapidly.

    Method:

    I used an A3 sheet of paper on my enlarger baseboard as the test target. Both the enlarger and the camera lens (Bronica 75mm f/2.8 PE) were stopped down to f/5.6 to reduce light fall-off. Exposure times ranged from 1/16" to 25 minutes 36 seconds in roughly half-stop intervals. 15 frames of 120 film were exposed. Each frame was exposed twice. During the second exposure, half of the test target was covered with black card so that the resulting frame was exposed to two different light levels. Measurements were made from within designated circles equidistant from the center of the test target that had been tested to receive identical illumination from the enlarger. The exposure indicated by my spot meter was 3.2 seconds at ISO 100, which indicates that the light level was about 3 EV higher than the point at which reciprocity failure would start to have an effect. The darkroom was kept dark (with no safe-light) during the exposures and measurements.

  2. #2
    ic-racer's Avatar
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    15 frames of 120 film were exposed
    If you are going to be doing more of those curves, a sensitometer can speed up the exposure process. http://www.apug.org/forums/forum43/1...-shootout.html

  3. #3
    andrew.roos's Avatar
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    Thanks ic-racer, that's good to know - I spent a couple of hours in a hot (28C / 82F) darkroom making my test film...

    Andrew

  4. #4

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    The curve seems a little odd to me. For one thing, it should not flatten out so abruptly that early (~7 stops above metered).

    I'm not 100% clear on the exposure methodology but here are a few tips.

    I would suggest keeping the exposure time for the entire series <1s to avoid reciprocity problems. Also, with a brightly lit sheet of paper, unless you are carefully masking the lens/camera there can be a significant camera flare factor. Aside from that, if you are covering half the frame, flare can have a significant influence on the density in the darker area. In general, flare would tend show up as increased film speed and reduced shadow contrast on the characteristic curve.

    If you are shooting a brightly lit test target I'd suggest masking the lens to reduce camera flare. If done effectively you'd be basically left with some veiling lens flare. To avoid flare altogether you'd have to contact a step tablet/wedge.

    Hope this helps.

    Good start. But can you explain the methodology a little more? Why was each frame exposed twice? Were you exposing film in the enlarger at the film stage? Etc.
    Last edited by Michael R 1974; 12-30-2012 at 03:07 PM. Click to view previous post history.

  5. #5
    andrew.roos's Avatar
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    Hi Michael

    Thanks for your interest and suggestions, which I shall take into consideration for future attempts. Here is a more detailed description of my methodology.

    I created a "test target" consisting of an A3 sheet of white paper cut down to fit the maximum enlargement size that can be projected by my enlarger using the standard 80mm lens onto the baseboard. The target had a vertical line in the middle dividing it into left and right sides, and two circles about 50mm diameter, one on each side of the central line and equidistant from it, and was taped to the baseboard. I used the circles (on the exposed negatives) as an indication of where on the negative the density should be measured, so I would be measuring it in the same place on each frame. Before starting, I used the light meter with lab attachment to confirm that the two circles both received the same illumination with the enlarger lens set to f/5.6.

    I set my Bronica ETRSi up on a tripod with the 75mm f/2.8 lens at close focus distance, pointing down at the target at approximately a 45 degree angle. Due to the foreshortening of the target, the entire width of the frame was filled by the target, but only about 3/4 of the vertical height, which I centered in the frame. The lens was focused so the two circles were in focus.

    I used one 15-exposure roll. Because I wanted a long enough exposure range to see the shoulder, as well as half stop resolution, I made two exposures on each frame. (This may have been a mistake - but was done because I have a limited supply of film and chemicals, and every film I use in this way is one I can't shoot pictures with!) So for example, for my first exposure I exposed the whole target at 1/15". I then placed a sheet of black card over the right hand half of the target and exposed again at 1/30". This gave me two exposures, one of 1/15" and one of 1/10". Not exactly a half stop apart, but close enough to provide an intermediate value. For the next frame I doubled these times to 1/8" for the entire target plus an additional 1/15" with the right hand side covered by black card. And so on, up to 1024" + 512" for the last frame. (I messed up one of the exposures, so there is a data point missing between -4 EV and -3 EV on the graph.)

    The camera lens was set to f/5.6 as was the enlarger lens since I felt this offered the best compromise between light fall-off (which could make the measured value overly dependent on where it was measured on the negative) and reciprocity failure. I had previously measured reciprocity failure on Delta 100 and found it to be inconsequential at Zone V density level for exposures of 16 seconds and less. Since reciprocity is dependent on the light level not the exposure time it made sense to me that, provided the light level was significantly more than required to give a Zone V exposure time of 16 seconds, reciprocity should not affect the result. The metered exposure (using a spot meter located where the camera would be and pointed at one of the measurement circles on the target) was 3.2" at f/5.6 (ISO 100). Given my previous measurement of reciprocity failure, I figured this was OK although it would have been nicer to have shorter exposures if possible (but then I would have struggled to create a uniformly illuminated target). Note that if reciprocity failure did affect the result, then it would do so by shifting the entire curve on the X axis since the same illumination level was used for all points on the curve (and hence the rate of latent image formation would have been the same even if reciprocity failure had reduced it somewhat).

    I then developed the film as carefully as possible to get the timing, temperature and agitation as accurate as possible. I used a plastic tank (which is all I have) and immersed it in a water bath at 18 C during development because I have found that this helps compensate for hand warmth etc when inverting. I measured the temp of the developer when it came out of the tank and it was 20.2 C (it went in at 20 C).

    Once the negs were dry, I used the enlarger to project them onto my printing easel at approx. 8 x 10" size. I then used the light meter's LAB attachment to measure the intensity of the projected image within the measuring circle on each side of the negatives. This measurement was made in EV, and were in the range +4.3 to -2.6 EV with the meter set for ISO 100. I also measured the illumination with a section of the leader that had not been exposed in the enlarger, to give me a FB&F reference. The illumination values were normalized to the FB&F measurement and then converted to log10 density values. The chart X values were calculated from the actual exposure times by normalizing to 3.2" (the metered exposure) and then finding the log (base 2).

    One possibility for the flattening of the top of the curve is it could be related to the response of the meter that I used for measurement. The meter is rated to -4 EV at ISO 100, but it is possible that it's response is not accurate for light levels as low as -2.6 EV or that the slight spill-over from the enlarger illumination source was affecting the reading. The enlarger is shrouded by a black cloth that is hung over it like a tent, but one side (where I stand) is open, and there is a bit of light leakage out of the color head. I can try making the measurements again with the projected image size reduced to say 4 x 5", which would increase the intensity by 2 EV and should show if the low light level is affecting the measurement.

    Cheers
    Andrew
    Last edited by andrew.roos; 12-31-2012 at 12:11 AM. Click to view previous post history.

  6. #6

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    Hi Andrew, it seems like a relatively complicated setup with many variables (ie potential sources of error) to control. Here are some additional things to consider:

    -Reciprocity failure: Assuming I'm following your methodology correctly, reciprocity failure will have a greater influence as exposure time increases (since you are doing separate exposures). This could help explain the flattening of the highlights. It is best to eliminate reciprocity failure from the test. Using an enlarger as the light source for an in-camera test is problematic because of the relatively low level of illumination.

    -Lens extension: For an in-camera test it would be best to focus the lens at infinity. Focusing on your close target means the lens is likely extended enough to require exposure compensation versus the handheld meter.

    -Half-stop resolution: If you have limited materials, and given the variables involved, I'd suggest doing the test in full stops to simplify things. You can interpolate between points. Alternatively - if you contact a step tablet you could plot the full curve in 1/2 stops or 1/3 stops using only 2 or three frames of film. It's not an in-camera test, but it eliminates many variables, including flare.

    Michael

  7. #7

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    I've also been making density-curves lately. See the thread about Progress on XTOL-Concentrate. I've made and corrected some mistakes.

    I agree with Michael R: The long exposures are likely to cause reciprocity failure. I suggest that you photograph a brighter light-source. I'm using my light-table: It's bright enough to keep speeds at/above 1/4 second, and dim enough to capture the film's toe. Also, consider putting some neutral density (ND) filters on the light table to give you multiple densities with each exposure. A step-wedge would be ideal, but ND's would help if you have some lying around.

    And as you realize, avoid the corners of the neg due to fall-off due to the lens and also due to the lens' viewing the light-table at an angle.

    Cheers,

    Mark Overton

  8. #8
    andrew.roos's Avatar
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    Thanks Michael and Mark for your excellent suggestions. I will look to incorporate them next time I attempt similar measurements.

    The only one I am not convinced of is the effect of reciprocity failure. I would like to explain my reasoning so that you can point out the flaw if it is incorrect. My understanding is that the physics behind reciprocity failure is that the formation of a latent image center requires multiple photons. The intermediate states (latent pre-image centers) are unstable and as the time between photons increases, the chance that an intermediate state may return to its initial completely unexposed state increases. Since both the decay time of the pre-image center and the time between photons are randomly distributed, as the light level (average rate of photons) is decreased, there is a gradual loss of sensitivity due to the increased number of latent pre-image centers that decompose before they become stable latent image centers.

    This means that for a given film illumination level, the rate of latent image center formation is constant irrespective of the shutter speed. If the illumination level is sufficiently high that the efficiency of latent image formation is close to optimum, then reciprocity failure will not be significant. If it does occur, then it effectively reduces the sensitivity of the film by a certain amount which is independent of the shutter speed. So provided the light intensity is the same for all exposures, the effect of any reciprocity failure would be simply to shift the curve on the (logarithmic) X axis.

    By comparison, if I had kept the shutter speed constant and changed the illumination level (by varying the strength of the light source or adjusting the aperture) then the varying light levels on the film plane could have changed the efficiency of latent image formation, distorting the curve (since latent image formation would be more efficient at higher light levels and less efficient at lower light levels).

    Because I was concerned about the possible effect of reciprocity failure, I measured it using similar technique prior to measuring the characteristic curve. I found that there is no effect (to within about 0.03 DU) at Zone V density for exposure times of up to (and including) 8 seconds. Of course this is really short-hand for saying there is no effect for light levels as low as that required to expose an ISO 100 film to Zone V level in 8 seconds because it is not the shutter speed, but the illumination level that determines the amount of reciprocity failure.

    Since the illumination level I used to measure the characteristic curve was more than double this (3.2 seconds for zone V exposure), I concluded that reciprocity failure would be insignificant within the limits of my measurement capability (0.1 stops on the enlarger baseboard, which is 0.03 DU). The choice of illumination level was a compromise between light fall-off (which could be improved by stopping down both the enlarger lens and the camera lens) and reciprocity failure (which would be improved by opening up both the enlarger lens and the camera lens).

    If you find a flaw in my reasoning, please let me know!

    Thanks again,
    Andrew

  9. #9
    andrew.roos's Avatar
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    Click image for larger version. 

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    I measured the densities of the negatives again using 1.4 stops more light (by reducing the size of the projected image) to see whether this reduced the abrupt flatting at the shoulder. It did have some effect, with density continuing to increase above normal + 8 stops exposure. This suggests that the reason for the flattening was measurement error at high negative densities.

    The new measurement also shows the straight line portion of the curve to extend from -3 to +7 EV referenced to normal exposure at ISO 100. Gamma of the straight line section is 0.61, which is consistent with "normal" development.
    Last edited by andrew.roos; 01-01-2013 at 08:54 AM. Click to view previous post history.



 

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