How to shoot thinner light trails using an iphone app

Question

I have taken several photos using an iPhone8 plus, tripod and using a procam app that allows control of the shutter speed in order to shoot light trails which I have successfully done, however the resulting light trails are thicker than I would like and not thin trails of light. I am no master of photography so looking for some advice. Is this a result of too much light and over-exposure? In which case should I be really lowering the ISO (it is not possible to control the aperture on a phone but I can control the ISO when taking a long exposure)

Thanks

So here is an example screenshot (has been post edited in lightroom but the trail thickness hasnt changed). I have seen a lot of trails that are like a thin single line of either white or red rather than almost blurred like this)

Answer 1

Your hunch is probably on the right track. Assuming that the thickness of the light trails is due to overexposure of the moving light sources, then reducing the ISO will reduce the apparent intensity of the light trails.

Note that, depending on the light sources creating the trails, the actual size might not decrease much. Headlights and tail lights on vehicles are not point sources of light, so they won't actually produce smaller light trails if you reduce the ISO. However, because their brightness would be reduced, the trails will certainly "feel" smaller.

Note also that if you decrease ISO, the brightness of the entire scene will be reduced, so if you want to keep the background the same brightness, you'll have to increase the exposure time accordingly. E.g.: if you halve the ISO, you should double the exposure time.