400 speed film used to be considered a grainy compromise for low-light shooting. But film quality improved, and digital sensors can exhale images at 400 ISO without any noticeable noise. You could theoretically have cleaner images at 100 ISO, but in practice there is very little benefit. So here are the reasons even my digital camera is always set to 400:
Consistency - I can always guess my exposure settings quickly because I fix my ISO at 400 and only have to think about shutter speed and aperture. The sunny 16 rule is useful, and after a while it all becomes second nature. See more tips on being an exposure expert here. Less fiddling with settings means more time concentrating on the picture.
Shooting film - 400 speed film is in the Goldilocks zone - just right. It works outside on sunny days and indoors with a moderately fast lens. There are also plenty of choices for 400 speed film. Ilford’s HP5 is a classic, and Delta 400 is available for those who want less noticeable grain at the expense of exposure error latitude.
Speedlites - Using a higher ISO means you can use your lights on a lower power (1/4 the power compared to 100 ISO). Quicker recycle times, quicker flash duration and longer battery life are the result. It also makes up for the light your modifier eats. Indoors, you can still block all ambient light with a 1/180th shutter and f8.
Faster shutter speeds. Compared to 100 ISO, 400 gives your shutter 2 stops of advantage. Especially with longer focal lengths, think of the difference between 1/30th vs 1/125th of a second. And between 1/125th and 1/500th! Sharper images with the cost of very little added noise.
Higher and lower speed options. As 400 is the middle of the road, you’ll only change your ISO dial or film for a good reason. Lower speeds will reduce noise/grain, but you’ll need a lot of sun or artificial light to make up for it. You can use 800 or 1600 speed to shoot in the dark or higher shutter speeds for fast moving subjects, as long as you don’t mind some smoothing in your post processing.
Speaking for myself, 400 does it all. It simplifies my decisions with compromises I can live with.
Tell me about your go-to film and ISO setting in the comments below.