Ektachrome 100ASA 4x faster than the 25ASA film shot in Bolex's B8SL. So, some adaptation is necessary. This particular Kodak film is rated at 100 ASA. Your camera is set up for 10-25 ASA film. The shutter is fixed. Because it's fixed, there is no "half-shutter" option.
Later Bolex 8mm cameras do have a variable shutter, and can do a half or even less shutter. Closing the shutter has the effect of cutting the amount of light entering the camera by reducing the shutter angle.
Shutter angle is the time as expressed in either:
1) degrees of angle and
2) fractions of a second when computed with the Frame Per Seconds (FPS) setting of the camera.
To use a 4x faster film (25 ASA compared to 100 ASA) you need to cut the light in half, two times, or 2 stops. 100 cut in half is 50 ASA, 50 ASA cut in half is 25 ASA.
You can do this by several ways:
1) Using a neutral filter (ND) to cut two stops. Referring to the ND CHART located here:
It says that a .6 ND filter will cut two stops. You will need a screw-in type filter holder compatible with the thread of the lens used. Camera stores sell these. Although, they made need it to order it for the Bolex lens'. You may or may not also need and adapter ring to adapter the thread sizes.
Sometimes the lens threads are not the right size and use of a step-up ring/adapter is common. Avoid using two or more filters. Although a .3 ND (1 stop) and another .3 ND (1 stop) do add up to a .6 ND (2 stops), the use of multiple filters can sometimes introduce artifacts. Especially if the camera points towards a strong light source.
2) Close the lens f-stop by 2 stops from what the manual tells you to do. This effectively compensates the 100 ASA film to act as 25 ASA as its now not getting enough light (2 stops less). This is what John Schwind recommends. Unfortunately not all Bolex 8mm lens are capable of stopping down to f-22.
3) Calculating the actual f-stop required with a light meter.
F-STOP CALCULATING PROCEDURE
To calculate the actual f-stop required for any film requires using 1) shutter angle, 2) film FPS (speed) and 3) the film's ASA number. Once the f-stop is calculated for that particular films ASA number, you need only dial it in on the lens.
From the tag located on the Bolex's body, and referring to the "American Cinematographer Manual", 4th edition, I calculated (estimated) the shutter angle for the camera running at 16 FPS, to be 170 degrees or 1/35th of a second. We'll use this "ballpark" setting of 135 degrees for filming at 16-18 FPS.
I'm indoors. Using a Sekonic Studio Deluxe L-398 light meter, I set the ASA to 100. Then I put dome of light meter under chin of subject after the indoor lighting is set. Then press and release the meter 'read' button. The meter shows the incident light reading falling on the subject face. I might check a couple of other places in the scene where there might be more light in the background than on the subject. If there is 2 people, I would check them both. The light reading of the subject is the reading I'll use.
This is one way to take a meter reading. Hollywood cinematographers use this method commonly. Then, I turn the meter's EV/F-stop "dial ring" until the pointer points the incident light value. In this case, it's about 40.
I look at the "cine scale" and read the f-stop opposite the shutter angle which is expressed in fractions of a second. I know the angle is 1/35 of a second. I see the numbers: 1/30th and 1/60. I figure the value is under the "0" of the 30 (slightly to right of the 30 mark).
Opposite that 1/35th value (guessed!) I see the f-stop scale. It says about F-2.0. I try to set f-2.0 on the camera lens. I see that the lens I'm using only goes down to f-2.5. Not enough light. Solution: add more light(s). A half stops worth. Since this is film, I could probably get away with using the f-2.0 setting anyhow. Picture will be slightly (1/2 stop darker) than "normal". Whatever "normal" is. Keep track of finished footage. If consistently darker than you like, adjust f-stop slightly more open than calculated (more light).
Let's say I'm outdoors. I press the button, the needles jumps off the scale. Oops. Too much light outside. I insert the "high slide" that comes with the meter. The high slide cuts the light to a corresponding scale on the meter. I do the reading as before. The reading is about 140 with the high slide inserted. So, I read opposite the RED colored pointer (high slide pointer).
It says to use f-19 (best guess) for the f-stop setting. This lens goes to f-16. I need to reduce the light by 3 more stops. Looking in my kit bag, I locate a .9ND. That's 3 f-stops reduction. I screw it in and set the f-stop on the lens to f-16 which is the top limit of that particular lens.
There are other considerations. Lenses themselves are usually "little liars". They might say "I'm a f-2.5" on the lens barrel but they might test out to be f-2.9. .4 of a stop darker. And at the other end, the F-16 might actually be something else.
You might put a #85 filter on top of the ND to convert type A film to outdoor. The #85 filter handout says "cuts light by 2/3 of a stop". 2/3 rds of a stop plus .4 equals about 1 full stop. So knowing this, I might add one stop to the lens opening it up to f-16. And another adjustment is I may have used this film type before and it actually looks the best when shot through the Bolex if I set it to 1/2 stop more closed than what is calculated.
When the film is developed, it usually done on batches. If done towards the "end" of the chemical run, the film may be underexposed. Sometime you can ask the developer to wait until he changes chemical and then run you stuff at that time. Or your could pay extra and have you stuff given preference immediately. Ask you friendly developer for advice. Work with them. Ask them what you can do to bright down the processing charges and make their life easier. This is a common practice with film students.
Film has a high latitude for light. 1/2 to 1 full stop either way is typically considered, "ballpark". Not so with inexpensive digital video cameras where you only have a 5 stop range. Go one stop over the limit and the picture is blown out. No detail in the light areas. Go one stop to low and no detail in the dark areas. With film, if you keep records of how a particular film and light meter/camera combination looks when finished and what you did to get it that way, you can alter the final finished product by adjusting the f-stops one way or the other.
Or you can just slap a .9 ND and use the 100 ASA film! Keep a written record of how everything turns out. Adjust f-stops accordingly. If you film is consistently too dark, opening up 1 stop on that lens.
THE LINKS BELOW ARE REFERENCE TO THE ABOVE FAQ
SEKONIC METER MANUAL
COMPUTING SHUTTER ANGLE
KODAK EKTACHROME PAGE:
COMPUTING SHUTTER ANGLE:
FILM SUPPLIERS LIST:
FILTER F-STOP ADJUSTMENTS:
ND FILTER FAQ
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