A number of years ago, Nikon introduced the flagship D3 and changed the face of photography by allowing us to capture images in near darkness. I remember taking an image indoors late one evening in 2007 and marveling at how clean the image looked at ISO 6400.
Yet, these days when I photograph, I preferentially seek good light, and my ISO requirements with an f/1.4 aperture lens rarely exceed 1600. Anything more than this, and I’m basically turning nighttime into daytime — which is often undesirable, as it destroys the ambiance of nighttime scenes.
I’ve learned the obvious: good light is good, and no amount of ISO boosting overcomes the disadvantages of poor light or poor lighting.
Good light reveals nuances in colours and textures, and generally confers depth to those flat representations of the world we call photographs.
That same ISO 6400 image from six years ago, when viewed now with a more critical eye, falls a little short on an aesthetic level, not because the lighting was dim, but because the lighting was poor.
Poor light is not necessarily the same as weak light. For example, the scant light available after a sunset is “good light”, if one harnesses it properly (with the aid of a tripod and slow shutter speed). The same thing may be said of incandescent lighting, if it is arranged on your subject in a pleasing way.
Good light is not necessarily the same as bright or plentiful light. For example, midday sunlight is often too stark to be desirable for portraiture, but may be perfect if the goal is to capture the interplay of light and shadow.
Admittedly, high ISO capabilities are desirable for specific applications — for example, astrophotography, where the goal is to “see” the faint light emitted by distant stars but the exposure time is kept to a minimum to “freeze” the stars’ movement (so that stars remain “points” of light in the final photograph). Or, if the goal is to capture action in dim light, or to record a memory for posterity and lighting considerations are secondary or beyond one’s control, then yes, a high-ISO-capable camera is vital.
If the goal, however, is to create an aesthetically pleasing photograph — even at night — then seek good light.