The true ISO (International Organization for Standardization) of black and white film refers to the film's actual sensitivity to light. It is important to determine the true ISO of a film because it allows you to correctly expose the film and achieve the desired results in your photographs.

Here are the steps to find the true ISO of black and white film:Set up a controlled lighting situation. Use a light meter to measure the intensity of the light and record the readings.

Load a roll of black and white film into your camera and set the camera to the desired aperture and shutter speed.

Take a series of test shots at different ISO settings, starting from the manufacturer's recommended ISO and increasing or decreasing the ISO by one stop for each shot.

Develop the film as usual and evaluate the results. Look for the shot that appears to be the best exposed and has the most detail in the shadows and highlights.

Compare the ISO of the best-exposed shot with the ISO used to take the shot. If the ISO used is the same as the ISO of the best-exposed shot, then that is the true ISO of the film. If the ISO used is different from the ISO of the best-exposed shot, then adjust your exposure accordingly.

Finding the true ISO of black and white film is important because it helps you achieve the best possible results in your photographs. If you use the wrong ISO setting, you may end up with underexposed or overexposed images that lack detail in the shadows or highlights. By finding the true ISO of your film, you can ensure that your images have the right exposure and the right amount of detail in all areas of the photograph.