Spot Meter Measurment of Transmission Densities by R, Eugene Smith
By R, Eugene Smith (Dr. Bob)
All serious photographers using classical materials have at some time found it desirable to produce a film response curve, to check film speed variance of their equipment, materials, and/or developing techniques. This usually dictates the use of a densitometer. A good densitometer can significantly impact the budget and Ansel Adams indicated that although most photographers do not possess a densitometer, “...it is well worth the effort to find a means of reading negative densities.”
After much frustration making exposure tests that produced more confusion than order, it occurred to me that data in four or more significant figures is not necessary to establish an information base suitable for my personal requirements. Considering this concept, it seemed logical to attempt careful measurements with my digital spot meter as a possible cheap substitute. The viability of my technique as presented below, is corroborated with commercial densitometer readings and may be confirmed by comparison with published data at a later time.
There is ample literature in publication addressing the subject of obtaining density data with means other than a densitometer. Taylor and Martin recommend both the use of a densitometer and a visual comparison using the Kodak Projection Wedge . Schaefer offers a more detailed method by inspection using the Stouffer 21 step calibrated density strip . Patrick Gainer presented an elegant method recently in the Mar/Apr-2001 issue of Photo Techniques, which employed the Stouffer 31-step calibrated wedge. All the methods above employ a visual comparison to match the densities of the film under investigation with a known standard.
Phil Davis presents a detailed and complicated mechanism utilizing a specific spot meter and gives a comprehensive explanation of its use for measuring both negative and print densities. As I was interested only in measuring negative density , I attempted to reduce the mechanical complications, and simplify the procedure and data reduction. Also there have been allusions to the use of a spot meter for estimating densities in various news “discussion” groups but, as far as I have been able to ascertain, no simplified method has been formally presented in detail.
I used a small fluorescent light box powered with a 6 VDC power supply to illuminate negatives. I masked the negative area to be measured with a black card with a 0.5 in. diameter hole to reduce possible flare. My one-degree digital spot meter was positioned normal to the light box plane on a tripod.
My test negatives were exposed with the camera and gray card set up in open shade on a sunny day. I filled each frame with the gray card and exposed around the setting for zone V (the average meter reading.) I processed the negatives used to produce the data presented herein in a “normal” manner in all aspects for the particular films.
To make the density measurements I reduced the room illumination to prevent reading errors due to reflections. Several times during the operation, I checked the output of the light box to assure that any light intensity variations were taken into account. These base or “zero” readings were recorded for later reference. I used the EV (exposure value) setting on the spot meter for all data from the negatives. I made several successive readings of the area of interest and recorded the average. A blank frame was made on every test roll in order to determine the base+fog for that negative set. The film speed setting of the spot meter can be set to a value yielding a convenient range in numeric data. Table 1 contains an example of data obtained using the above procedure for TMY film developed “normally” in D76 1:0. Experimental data based on Tri-X pan film is presented in Table 2.
(now view the Table 1 image attachment at the bottom of this page)
(now view the Figure 1 image attachment at the bottom of this page)
. Data Reduction
Negative density in black and white photography is defined as the log 10 of the inverse of the ratio of light transmitted to the total light available;
Dn = Log10 (1/(L0 - Ln)
Dn = density,
L0 = Light transmitted through the film base (f+b) or Zone 0,
Ln = Light transmitted through the frame exposed Zone “n."
It is not necessary to compute the light levels in lux-sec or other units to obtain relative usable densities for plotting a “characteristic” curve. Starting with the fact that each unit change in EV represents a doubling or halving in the light intensity, the following simple equation can be derived:
Dn = 0.301x(X0 - Xn )
Dn = density, frame “n”,
X0 = EV reading of the film base (f+b) or Zone 0 frame,
Xn = EV reading of the “n’th” frame.
Figures 1 and 2 show density values converted from the spot meter EV data using equation 2.
The raw data used to produce the curve shown in Figure 1 was obtained on TMY film rated at a speed of 200 using a Koni Omega Rapid with a 90mm lens. The roll was developed normally in D76 1:0 and 20° C. Examination of these data reveals a Dmin of 0.1 that relates to the value recommended for Zone I in references and . The speed point indicates that the selected exposure index of 200 is correct for this particular set of conditions of camera, film, and development. Using the visual method of Kodak , one should compare their projection wedge #8 (for condenser enlargers) with the test negative exposed as Zone VII. As this seemed a little inexact, I measured the density of the #8 wedge using the spot meter technique. The density obtained was 0.903, which is very close to the Zone VII exposure frame measuring 0.903.
The TMY data analyzed above is an example of a “normal” situation. As an example of a problem case, experimental data for TX-120 film exposed at ISO 400 with the Koni Omega and processed in HC-110 (dilution b) is shown in table 2 and plotted in figure 2.
(now view the Table 2 image attachment at the bottom of this page)
(now view the Figure 2 image attachment at the bottom of this page)
It is immediately evident that the Dmin, found to be 0.24, is more than twice the typically “proper” value of 0.12. However a toe has formed indicating that little if any decreases in density could be achieved by decrease in exposure. The observed density at the Zone VII exposure is 1.28, which is quite diverse from the #8 wedge value of 0.903. The observed density at the Zone VIII exposure is 1.49, which is more than one zone higher than the maximum recommended by Schaefer et al. of 1.25. These data seem to indicate an overall fog and possible over development equivalent to at least one and one half zones.
A closer match in experimental processing of TX-400 in D76 can be found in “The Negative." The Dmin data do not compare but the Zone II to VIII difference of the published data, 1.25, is sufficiently close to the test data, 1.20, to conclude that the processing technique used was about as good as could be expected.
It is tempting to deduce that a shorter development time, or a lower developer concentration might lower the slope and the Zone VIII density thus producing a characteristic curve closer to “normal." Decreasing the exposure, i.e., increasing the exposure index would only move the curve to the left along the x-axis. Neither would lower the Zone I value.
In the discussion the advantages of obtaining reliable information of negative density by a simple technique were pointed out. Application of a digital spot meter for achieving film exposure/develop data can be a useful tool in limited applications. This simplified technique will suffice in practically all my requirements for equipment and procedure calibration. Thus far, I can visualize no pit falls in the methodology for anyone desiring to employ a similar technique. The value of using this technique is demonstrated in the data of Table 2 plotted in figure 2. I believe the data indicates this test to be potentially flawed and that I should repeat it with increased attention to processing to prevent possible fogging, either from film mishandling, improper processing procedure or developer characteristics.
Acknowledgements: I wish to express special appreciation to Tim Hickey of Annapolis Md. for his aid and expertise in performing the density measurement of the test negatives with his commercial densitometer.