Forty-Five Unhypered Fuji, Ilford, and Kodak Films Compared for Astrophotography

The following pages contain the results of tests I recently performed on 45 popular 35 mm films to investigate their potential for long-exposure astrophotography. These tests were carried out during the preparation of the book "Wide-Field Astrophotography" to be published early in 2000 by Willmann-Bell. In this book, one of the premises I promote is that today's color films are so good they can be used for long-exposure astrophotography without the need for complex hypersensitizing procedures. I feel the technicalities and expense of film hypering is an intimidating obstacle which deters many newcomers from attempting astrophotography. I thus set out to test the popular films available today to see how they respond to long exposures without first being hypersensitized.

I used simple test procedures and equipment that can be constructed by anyone at little expense. The testing procedures used were admitedly crude and have a number of inherent flaws, but the purpose of these tests was to identify promising astro films, not to provide hard numerical data about the emulsions. The resulting data can only be used to compare a limited number of one film's parameters against another film tested in the same maner. The testing procedure was in two parts. The first checked each film's reciprocity law failure and color shift over a two minute exposure. The second test checked each film's response to certain blue, green, and red wavelengths over a two minute exposure.

A Note About Film Speed

In this film evaluation, I speak about how much film speed was lost during two-minute time exposures. I use the term "loss of film speed" in a very general sense in that this is perceived effect, not an actual fact. In reality, film speed remains the same throughout a time exposure. However, during a long exposure, the effects of reciprocity law failure makes some films less efficient at retaining the latent image in very dim light. The dimmer the target, the greater the amount of reciprocity law loss, and thus the greater the perceived loss of film speed. But remember, this is a perceived effect and in reality the film speed remains the same at all times.

Testing for Reciprocity Law Failure

The f/stop speed loss from reciprocity law failure was determined by photographing a Kodak 18% gray scale/color patch card under daylight ballanced blue flood lights for 1/8th second at around f/11 or f/16. The lighting was varried to keep the f/stop in this range regardless of film speed. The same card was then rephotographed through a neutral density 3.0 (1000X) filter at the same f/stop for 128 seconds. This is a 10 f/stop difference, requiring a factor of 1000 in additional exposure. Theoretically, the 1/8th second and the 128 second filtered exposures should be of equal density, but in reality they are not because of reciprocity law failure losses. The extended exposures were repeated at 1/3 wider f/stop apertures until a full 3 f/stops had been covered.

The resulting negative strips showed the original 1/8th second exposure at normal density while the time exposures typically started off lighter than the 1/8th second exposure, then got darker at the 3 extra f/stop point. The point where the additional extra aperture gave an image density EQUAL to the 1/8th second exposure showed how much speed in f/stops the film had lost due to reciprocity law failure during the 128 second exposure. To assign a numerical value to the reciprocity loss of each film, the Swartzchild formula of P = 1 - (s/7) was used, where P represents the reciprocity loss and s is the number of f/stops speed loss after two minutes.


Camera stand and flood lamp for testing film reciprocity law failure.	Sample results of low and high reciprocity law failure.

Testing for Spectral Response

The relative blue, green, and red sensitivity was tested by constructing a light-tight box in which the film could exposed to a white target illuminated by a 470 nanometer blue, a generic Radio Shack green, and both a 650 and 660 nanometer red LED. Each color was partitioned from the other by black foamboard partitions. The white target was ruled at one-inch intervals to allow measurement of the film's sensitivity in each color. Exposures were for 15, 30, 60, and 120 seconds at f/2.8. In retrospect, the sequence should have been extended through f/4 because the 660 nanometer red LED saturated the faster color negative films at f/2.8.

The goal in using the two red LEDs was to see which films continued to record beyond the H-Alpha wavelength. Unfortunately, the 650 nanometer LED turned out to be much dimmer than the others and barely showed up at all on many films. However, its strong appearance on many new-generation films demonstrates the high red sensitivity of newer color emulsions.


Light-tight film test box to allow long exposure tests in ambient light.	Foam board partition inside test box to separate four LED light sources.

Ruled target under foam board partition.	Top view of partitions. LED light sources installed on left panel.

Some Observations From the Tests

The reciprocity loss tests showed that color negative films fared far better than black and white or color slide films. Also as expected, the films with the faster inherent ISO speed were generally lower in contrast and possesed the greatest reciprocity loss. For example, Kodak Royal Gold 25 and Fuji Superia 100 color negative films posseses the best reciprocity charcteristics and contrast, but have such slow inherent speed that they are useless unless unless used in a wide-sperture Schmidt camera. Color slide films followed the same trend. Ektachrome Elite Chrome 200 possesed high contrast and little reciprocity loss while the ultraspeed films Ektachrome P1600 and Provia 1600 both displayed lower contrast and two f/stop speed losses in two minutes. Follow the links at the bottom of this page to access reciprocity loss details for all 45 films.

The spectral response tests showed that as a rule, black and white films were all sensitive to the 650 nanometer LED but were nearly blind to the 660 nanometer LED. This sensitivity collapse centered on the red Hydrogen-Alpha wavelength shows why nearly all black and white films perform poorly on deep sky objects unless hypersensitized. The only exceptions were Ilford's SFX and Kodak's Technical Pan, both of which advertise extended red sensitivity but have high reciprocity law failure.

The color films also showed some interesting spectral anomolies. As explained earlier, the 650 nanometer red LED turned out to be so feeble that it was invisible on a majority of films while the 660 nanometer LED was bright. But curiously, the dim 650 nanometer LED showed up on several color films and was especially bright on Ektachrome P1600 and Elite Chrome 200, and all of the Fuji Superia films. Follow the links at the bottom of this page to access the graphics showing the relative spectral response of all 45 films.

Note: the film tests shown in the links below were performed several years ago. Many of these films are no longer available and therefore these film comparisons are no longer valid. I leave these test results posted here merely as a sample of the kind of results you can obtain by using the above listed testing methods on films available today.

Go to next page ---- Black and White Films
Go to next page ---- Color Negative Films
Go to next page ---- Color Slide Films
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