1. High ISOs can have minimal noise.
Sure, back in the film days it was true that working at high ISOs and pushing film processing often translated into so much grain that it became hard to make out details (especially in the shadows) of negatives. But with the advances in noise reduction from today’s digital cameras and RAW converters, higher ISOs produce image quality unparalleled by high ISO films. Not only that, digital high ISOs are so low noise these days that many photographers are putting them to use in ways they never considered before—like nighttime exposures that are short enough to render starry skies without blurring, or photographing in dark interiors without the need for fill flash illumination. Photographers today know they can pull detail out of the darkest of shadows, and much of that is thanks to the same improved signal-to-noise ratio that makes high ISOs so darn attractive.
2. Digital ISOs aren’t contrasty like film.
When working with fast film speeds, photographers often found themselves pushing exposures by underexposing in camera and overdeveloping in the darkroom. While this process did provide higher-working ISOs than the film was rated for, it also introduced some major drags on the image quality—both the grain mentioned previously as well as a whole ton of contrast. So much contrast, in fact, that high-ISO film images could sometimes look like abstract art. Imagine a grainy image with very little detail in the shadows and highlights—and none available to be pulled from the negative. That just doesn’t hold true today when working with RAW files. RAW is especially capable of rendering detail in very dark shadows, and the only processing that eliminates shadow detail comes from underexposed files. If you can overexpose a digital image even slightly, you’ll be amazed at the detail that remains in the dark tones without any contrast gain at all.
You only shoot at ISO 400 or less, so why should you care about better performance at higher ISOs? Because those low-noise/high-detail qualities are carried across the entire ISO sensitivity range. In layman’s terms, ISO 400 from today’s D3 looks a lot like ISO 100 did in the D1 of yesterday. If you like image quality, you like to see advances in ISO capabilities due to better signal-to-noise ratio. Those gains will show up in your low-ISO images too. So when you hear of new cameras reaching new high-ISO low-noise heights, it’s good news for you too.
4. Sometimes it’s better to choose a higher ISO even if you don’t have to.
Generally speaking, this one appears as if it might be awful advice. But let’s say it’s a fairly low-light situation in which you’re working, and you need a fast shutter speed of 1/125th of a second to keep the subject from blurring. The lens only opens to ƒ/4 and you’ve maxed it out, and you’re at ISO 400. Still, you’re just barely getting the correct exposure. One way to ensure you’re getting an even better exposure is to boost the ISO to ISO 800. Why? Because if you capture RAW you’ve got the added benefit of being able to pull extra detail out of shadows from slightly overexposed image files. That can produce an image with more detail across the spectrum from shadows to highlights. As long as highlights aren’t blown out in the capture, an overexposed RAW file will actually produce a higher-quality file after image processing than a “correctly” exposed RAW file would.
Some might say boosting the ISO offsets any noise gains that come from overexposure, and that could very well be true. But in terms of shadow detail, there’s no doubt: the extra detail that can be pulled from brighter shadows is awesome. This holds especially true in low-key images full of dark tones where you’re more likely to keep from blowing out highlight detail.