What is often true of all cameras, both film and digital video, is that the image that you see through the viewfinder or LCD display may not be the image that is actually being recorded by the camera. There are times in which the display image that you are seeing is being distorted from the image that is being captured.
On a traditional film camera the viewfinder eyepiece can be adjusted directly to how well the camera operator can see. It can be adjusted for any eyesight problems or irregularities, and simply the differences between the way that people see. To change this you adjust the Diopter, which changes the perception of the image by the camera operator. Adjusting the Diopter on a camera does not actually change the image in the frame, just the way that it appears to the camera operator.
With the majority of sixteen and thirty five millimeter film you can start by removing the lens. Then find a brighter source and direct the camera that way. There will be a Diopter adjustment ring that you should rotate until you can clearly see the grain that will be on the ground glass.
Most sixteen and thirty five millimeter cameras are at least partial ground glass or fiber-optic screens so you can use this process. The process will be differently on different types of film cameras.
The digital video process of adjustment is not always referred to as Diopter adjustment. If you are using an eyepiece then it can be considered a Diopter adjustment. If you are using an LCD screen you will have to use a completely different criteria, but usually if you calibrate the zoom it is going to be as clear as it can possibly be. This is done by zooming in to an image as far as possible, focusing the image, and then zooming out to the depth that you want to be at. This will keep the image in focus even if the camera operator cannot see it correctly. Color changes may need to be made on the LCD display so that you will be able to recognize the image correctly.