Film Test Procedures

Updated 8/27/2002

The results of any test can be influenced by how the test is conducted. This description will help you understand how and why the test was conducted the way it was. After having completed some of these tests, there are things I would change, but change would have introduced inconsistencies in the results. No lab test selection of film is better than the real experience of taking pictures through your own telescope of objects in the sky. These tests compare films without the variability of scopes and sky conditions.

The reason for these tests is to show what these films will do with long exposures. I modeled my test setup based on the test setup by Robert Reeves (his tests really inspired my effort). But his tests didn’t include long exposures nor hypering effects. Since I shoot with a 7”/f9 Astro Physics refractor, some of my shots go for over 3 hours.

SELECTION OF FILMS:

Since I shoot color negative film and have experience with Kodak films (PPF400, PJ400, Supra400, and Royal Gold400) they were automatically on my list. PPF400 is no longer available, but I had a roll stashed away. I don’t have any PPF400 left, so that was the best use I could choose for it. The other films were chosen based on discussions I’ve read on the Internet and reports from other astrophotographers of the films they have tried. I didn’t try to do all the films available (cost and time have something to do with that decision). But I wanted to cover other film brands to see if any were good for astrophotography.

TARGET SET UP

I built a box about 2 ft tall with a hole in the top large enough for the camera lens and neutral density filters to fit through. The bottom of the box has a white paper target. The first few rolls of film used white ink-jet photo paper, but the paper was too thin and wrinkled, causing a few shadows of the LED light. I soon replaced that with a white card stock. I printed black lines at intervals of 2^n inches (2, 4, and 8) to reference multiple stops of light intensity. I ended up using the reference lines for measuring the light intensity in Photoshop at the same relative locations for each film. The LEDs were from Radio Shack, each with a rated wavelength. Blue (276-316) is 468 nm. Green (276-304) is 565 nm. Red (276-309) is 660 nm. The LEDs were setup to illuminate a fan shaped area on the target paper. They were powered with a regulated power supply, with a variable resistor in series with each LED to control intensity. The blue and the red were much brighter than the green, so their power was reduced slightly. An additional set of 4 dim red LEDs were place behind the main target LEDs with external switches to turn on and off. These 4 LEDs were used to indicate the binary code of the exposure time to keep from mixing negatives (hey, I exposed over 500 individual pictures to get this series done).

The light spectrum of the LEDs in my light box were measured using an SBIG spectrometer with an ST-7. The setup was calibrated with Neon light for the red and green LEDs. Neon doesn’t have any measurable light bands at the shorter blue wavelength, so we had to extrapolate from the red & green LED settings. The results show that the red LED is almost right on the Halpha spectral line, which means the tests are very good for evaluating films for nebula photographs. The blue and green are not on the Hbeta and Oxygen III lines. I will have to work on ways to make the test more realistic.

EXPOSURES

My Nikon FTN with Nikkor 50mm/f1.4 lens was used for the tests. I fitted a filter adapter to use 58mm filters and bought a set of neutral density filters to give either 2 or 3 stops per filter. By using the larger diameter filters, I avoided vignetting when I stacked 4 filters for the longest exposure. The short exposure was 1 sec at f2.8. I exposed for longer times by compensating with increased f-ratio and/or the neutral density filters. That way all the exposures were the same total light acculumulation. The exposure times (seconds) increased in powers of 2 from 1 second to 16384 (2^14) seconds (4 hours-33 minutes).

PROCESSING & EVALUATION

All the color films were processed at a local 1-hour facility with C-41 chemistry, standard processing (not pushed). All films were scanned with an Epson 1640SU with Negative Scanner Attachment using the same manual scanner settings for exposure, gamma, and color balance. I found that the scanner is very consistent after it has warmed up for at least 20 minutes before scanning. Color intensities were measured using the Photoshop Info command at the same location (max near the "4" line) on each exposure. The individual RGB numbers at a single location were summed to give the total intensity of that color location. The unlighted (black areas) were sampled as a tare value and subtracted to give a measure of true intensity. These intensity values were used to assess the reciprocity losses. A sample of the measuring process is shown here (RG200H36 at 1 sec):

The measurements were placed in an Excel spreadsheet and processed to create graphs of the results. The Schwarzchild Exponent (P)=1+(n/m) where time=2^m was calculated for each color. This exponent has usually been expressed as a constant, but I found that it varies with exposure time and color for each film. Here are some examples (good to poor):

The line graphs of the reciprocity exponent are interesting, but they don’t convey how usable a film will be for long exposures. The color intensities were normalized PhotoShop saturated color (compared to 255 reading in PS) and plotted directly into bar graphs. These bar graphs are presented in the Film Test Data page, along with samples of the actual test pictures. Here are the bar graphs for the exponent films above:

HYPERING FILM

The films were hypered using a Lumicon Model 300 hyper kit. The films are placed in the chamber in their cassettes and the chamber evacuated to about 26 to 28 inches of Hg. The lower figure is achievable with the hand vacuum pump. I reach the higher vacuum using an electric vacuum pump (converted air-condition pump) hooked to the outlet of the hand pump with hand pump action. I hold the film in the vacuum for about ½ hour and heat it to 30°C. Then I purge with hyper gas to 10 inch Hg and re-evacuate. I purge 3 time for the lower vacuum and twice if I reach 28 inch Hg. Then I fill the tank to +3psig and seal it by closing the valve. Hypering time starts with the 3psig. I end the hypering time by evacuating the chamber with the hand pump to about 25 inch Hg. I store all my hypered film in the freezer after placing the film back in its plastic container with a few grains of water softener salt (for absorbing moisture) and sealed with plastic electrical tape. I mark each container with the date of hypering.

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