Why ISO 400
If you don’t want to switch ISO settings all the time for general photography perhaps this is the most useful iso setting you can select. Why is it so good? Because at ISO 400 we have a faster shutter speed to make sharp images.
It needs to know that the higher iso values usually means less quality. At ISO
400 on a Dslr camera we have little loss of quality, if we don’t pixel peeping.
For landscapes I suggest to use ISO 100 tough, ISO 200 on older Canon cameras (the iso 200 is significantly better according to scientific test) to maintain the highest dynamic range. Dynamic range is less and less as we higher the ISO values. At the beginning the difference is not much, but at higher ISO settings we lose lots of dynamic range.
A Nikon D7000 has a 13.9 EV Dynamic range in NEF format, which is at IS 3200 becomes around 9-10 EV, and even less in the higher settings.
When ISO 400 is not appropriate ?
In very bright weather, where no shadows around (for example on a beach) better to switch to ISO 100, because we usually have enough shutter speed here as well, and we get better quality.
Other situation when in good light we want to use bright apertures (f/1.4 – f/2.8) for shallow depth of field. In these cases the shutter speed can be too high which results that the camera stop down the lens automatically, which results more depth of field. Professional photographers in very bright circumstances usually uses ND filters to avoid this effect.
In darker situations, like inside the room the ISO 400 setting is simply not enough for a proper shutter speed. If we want to make sharp images of moving objects we must use higher iso if the light is not enough. Inside the room it is very often we need ISO 3200 or even more for sharp images.
Other situation is with longer lenses in overcast weather we often need higher settings.