This post explains how to compensate for Kodak Ektachrome 100D and the Bolex P1 Metering system.
Kodak's suggested neutral density filters are general suggestions for this type of film used in a "daylight" situation to help avoid overexposure.
From the Kodak page they suggest using a .3 (1 stop) or a .6 (2 stops) ND filter to avoid overexposure. You could also drop a stop by rotating the iris. For example if the iris is set to f/5.6, and the needle centered, you could move it to f/8. That will give you a 1 stop decrease in light. The needle will show underexposed - ignore it.
If you instead used a .3 (1 stop) filter in the lens holder of the P1, the metering system will just see 1 stop less light and you would still center the needle. This would put the camera exposure back to what it thinks is 'normal'. And it would be still be one stop overexposed for the Ektachrome 100D film you are using. Keep in mind, this compensation as recommended by Kodak is ONLY for Ektachrome 100D film. Other films may or may not need compensation.
Keep written records of what works best. ASA, film speeds, compensation, subject lighting, overall lighting conditions, etc, before and after shooting.
Another situation is a "back light" scene where a strong light behind the subject affects the overall meter's setting. It 'reads' all light in the scene which 'skews' the overall metering values towards underexposing the backlit subject. Although the needle in center, the meter is 'lying to you' about the correct setting.
To fix a back lit scene, you would adjust the needle for that excessive light and the resultant developed film would be exposed correctly. Back light in any 8mm cameras typically calls for decreasing the iris to a larger number (smaller iris opening) by about 1/2 to 2 stops depending on the strength of the back light.
Three ways to correct:
1) Add more light on the subject(s) then recompose the shot
2) Close the iris 1-2 stops (guess)
3) Get close to subject, fill the screen with the subject, and then set the (meter). Back up and
shoot the footage. Do NOT readjust metering once set.
For more information, search Google for these terms:
If the needle centered at F/5.6, for a typical backlight situation, one would move the P1 camera iris lever half way (or more - depending on conditions) from f/5.6 to f/8. The needle would read underexposed by one stop, however, the subject would be exposed more correctly. The background light would be a stop less, too. Remember, you always (usually) want to set the exposure for the subject. Backlit situations set the iris for what is behind the subject. What is really needed is spot metering.
Unfortunately spot meting is not used on the Bolex camera. Bolex diverts all the light from the picture as seen by the lens. Under most condition this gives a good average of the light in the scene that the camera is shooting. But unfortunately does not allow for backlight conditions.
One way around this is to meter the subject with a separate light meter such as a Sekonic L-398.
Here's a link to its manual:
Unfortunately, these also while doing a great job (and at a low cost) for how they are intended to be used, the L398 cannot do spot metering :(
You can use a spot meter to meter off the face value from a distance or just guess the iris compensation based on experience.
Sekonic L398 Meters on eBay:
Regardless of what meter you use, using a meter can get better footage than guessing. I like the L398 as it has FPS (cine), ISO , aperture, and EV settings. Just dial in numbers and take a reading. Easy to use, no batteries, inexpensive (used).