Film isn't linear the way digital is. 30% under exposure doesn't give an image that is exactly 30% brighter, but may show up more as less shadow detail, compres

Film isn't linear the way digital is. 30% under exposure doesn't give an image that is exactly 30% brighter, but may show up more as less shadow detail, compressed mid tones, etc. Also, negative film can handle a huge amount of over exposure and still look good. You can get a better idea with positive/slide film, which is much more sensitive to exposure, but that is very expensive. Slower shutter speeds (1/30 second and lower) ar usually the speeds that are the most incorrect with old shutters. That can be tested more or less reliably by filming it with a high speed camera (I use a Fuji XT3 at 120 fps). You can also measure the speed by recording the sound of the shutter. Filmomat have a free app for measuring shutter speeds using audio, as well as a light sensitive doodad that you plug into a smartphone, which is accurate to up to 500th of a second. https://www.filmomat.eu/shop/photoplug https://www.filmomat.eu/shop/photoplug