Well, that’s largely up to your personal taste. For best image quality, you should always use the lowest ISO setting that will let you get the shot. The higher the ISO, the greater the noise in the image, the worse the color rendition, and the lower the dynamic range and image resolution. But in situations that require high ISOs, most photographers are willing to accept some loss in image quality in order to use the required shutter speed and aperture. With today’s better high-ISO DSLRs and mirrorless cameras, photographers regularly use ISO settings of 6400 and 12,800 and sometimes higher. We’ve had surprisingly good results up to ISO 51,200 in night street scenes with the best DSLRs we’ve tested. No, you’re not going to use ISO 409,600 or 204,800 for professional work, but you may well be surprised by how high an ISO you’ll find usable with today’s pro-oriented DSLRs and mirrorless cameras. They deliver amazing image quality at higher ISOs—quality that would have blown away photographers in the film era.
What Exactly Is ISO?
You all know that ISO is a number you dial into your camera or handheld exposure meter so it can give you good guidance as to the right exposure to give a subject or a scene. But what exactly is ISO?
ISO refers to a whole bunch of worldwide standards published by the International Organization for Standardization, a voluntary group of standardization bodies from more than 150 countries (the U.S. representative is the American National Standards Institute, or ANSI). These standards publications begin with the letters ISO; those for film are in ISO 6:1993 (for black-and-white negative film), ISO 5800:2001 (for color-negative film) and ISO 2240:2003 (for color-reversal film).
With film, ISO is actually a measure of the film’s sensitivity to light, based on standards set forth in the relevant ISO publications. An ISO 400 film is twice as sensitive as an ISO 200 film. An ISO 400 film really is four times as sensitive to light as an ISO 100 film.
There’s also an ISO standard for digital still cameras: ISO 12232:2006. Its purpose is to correlate digital imaging with film. If you set your digital camera to ISO 400 and expose per the meter, you should get an image of equal brightness to an image made on ISO 400 film given the same exposure.