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.
There are five methods listed for determining digital camera ISO in the standards, three from the previous standards and two newer ones from CIPA (Camera & Imaging Products Association) standard DC-004. Today, the Japanese camera companies use one of these last two: Relative Exposure Index (REI) or Standard Output Sensitivity (SOS). The takeaway is that SOS is based on the reproduction of a standard middle tone in the sRGB image, while REI is based on whatever the camera manufacturer considers good images. Note that digital ISO ratings are based on evaluating images, so they don’t apply to RAW files, which aren’t images until they’re processed. Unlike film ISO standards, digital ISO standards don’t specify processing procedures for the RAW data. Canon, Nikon and Sony use REI, and Fujifilm, Panasonic and Pentax use SOS; in practice, we haven’t noticed a difference. Olympus and Sigma don’t state which method they use.
With film, you set the meter’s ISO index for the ISO speed of the film you’re using (or your own tested exposure index; some film photographers find they get better results rating the film lower or higher than the official ISO speed) and expose according to the meter reading (or adjust accordingly per your experience with your gear and that film). Digital cameras provide a range of ISO settings, which makes things easier when you encounter dimmer or brighter light. With film, if you’re using ISO 100 film and want to shoot in a dim setting, you have to switch to a faster film. With digital, you can merely change the ISO setting on the camera to a higher number. Many photographers work this way quite successfully. But it’s important to realize that changing the camera’s ISO setting doesn’t actually change the sensitivity of the image sensor. It just changes the processing of the resulting image data. Changing the ISO changes the gain (either analog, which electronically amplifies the signal, or digital, which just multiplies the digital values after the signal is converted from analog to digital form), and in auto-exposure modes, the metered exposure. But increasing a digital camera’s ISO is more akin to push-processing a slower film to a higher rating than actually changing to a film with a higher ISO speed: Image quality suffers as you raise the ISO (see the next section for why).
In effect, digital ISO is a brightness control. Depending on how the camera applies ISO gain, you actually may be better off shooting at the desired shutter speed and aperture at base ISO (100 or so with most cameras) and brightening the results yourself when you process the image in your RAW converter. You have more control over the results and your computer is more powerful than the one built into the camera. Your computer can handle more powerful noise-reduction algorithms in the camera’s RAW converter. Also, each time you double the ISO setting in-camera, you lop a stop off the highlight end of the dynamic range.
Why High-ISO Digital Images Are Noisy
There are several sources of noise in digital images. The primary ones are photonic noise and read noise. Read noise is produced by the camera’s electronics and processing pipeline, and generally, is visible only in the darkest areas of an image. Photonic noise is the noise carried by the light itself. As the number of photons increases, photonic noise increases, too—but by the square root of the photon increase. If 100 photons reach an area of the sensor, there will be 10 photons of noise (the square root of 100 = 10), for a photonic signal-to-noise ratio of 10:1. If 10,000 photons reach another area of the sensor, there will be 100 photons of noise, for a photonic signal-to-noise ratio of 100:1. The greater the number of photons that reach the sensor—the greater the exposure—the higher the photonic signal-to-noise ratio. When you set a high ISO, you give less exposure; fewer photons reach the sensor, so the photonic signal-to-noise ratio goes down and the image gets noisier. It’s not the in-camera processing that makes high-ISO images noisy; it’s the reduced amount of light used for the image. For optimal digital image quality, you want to give the most exposure you can without blowing important bright areas in the scene.
The Benefits Of High ISO
In general, using higher ISO settings allows you to use faster shutter speeds to minimize the effects of handheld camera shake and subject motion, and/or smaller apertures to provide more depth of field. You can shoot in dimmer light with the same shutter speeds and apertures a slower ISO would require in brighter light. That’s why available-light and action photographers especially like higher ISO settings.
It behooves you to test your own gear to see how high an ISO you can use and still get results you find acceptable. With today’s DSLRs and mirrorless cameras, you may be pleasantly surprised at how high that ISO setting is.
The Best High-ISO Cameras
Even though the best digital image quality occurs when the exposure is maximized just short of blowing out the important bright areas in the scene, today’s digital SLRs and mirrorless cameras produce amazingly good quality at higher ISOs (i.e., with less light), especially when compared with film. And some deliver better results at higher ISO settings than others. That’s due to technological advances in sensors, image processors, image processing and noise reduction. Sensors can be tweaked toward low-ISO performance (as medium-format CCDs are, for example) or high-ISO performance (like Sony’s a7S, Nikon’s D4S and Canon’s EOS-1D X).
In general, larger sensors generate better high-ISO images than smaller sensors. And, as you’d expect, newer sensors are better than older models. Larger sensors do better because their larger area can collect more light than smaller sensors can and newer sensors have more up-to-date technology. The 28 highest-scoring cameras for Low-Light ISO performance in DxOMark.com’s sensor ratings are all full-frame models. The highest-rated APS-C camera ranks 29th; the highest-rated Micro Four Thirds model is 76th.
Medium-format sensors historically haven’t been great at high-ISO photography because they have been CCD-type sensors, not CMOS, and they have been optimized for low-ISO shooting. The highest-scoring medium-format camera in DxOMark.com’s high-ISO ratings ranks 36th, and it does that by binning pixels, which reduces its 40-megapixel resolution to 10 megapixels. That’s changing, however. Within the last year, Hasselblad, Pentax and Phase One have introduced 50-megapixel medium-format cameras based on Sony’s first medium-format CMOS sensor, and this should give a big boost to high-ISO performance. (DxOMark.com hadn’t tested these sensors at press time.)
Note that many cameras have a "normal" ISO range and an "expanded" range. The expanded range pushes the processing more, thus image quality suffers. Also, you can generally set normal-range ISOs in 1⁄3-stop increments, while the expanded ISOs go in full stops.
The full-frame, 18.1-megapixel EOS-1D X pro DSLR provides a normal ISO range of 100-51,200, expandable to 50-204,800. We had a chance to do some ISO 51,200 shooting with the camera and the results were impressive. You don’t want to use ISO 51,200 as your normal setting, but it’s amazing to be able to shoot at that ISO at all. The full-frame, 22.3-megapixel EOS 5D Mark III and 20.2-megapixel EOS 6D aren’t far behind, with a normal ISO range of 100-25,600, expandable to ISO 50-204,800 (5D III) and 50-102,400 (6D). We haven’t tested the new
flagship APS-C EOS 7D Mark II, but this 20.2-megapixel model has a normal ISO range of 100-16,000, expandable to 100-51,200.
The pro-oriented models from Fujifilm are mirrorless, the FinePix X-T1 and X-Pro1 mini-DSLR-style designs and the rangefinder-style X-E2. All are made with Fujifilm’s 16.3-megapixel X-Trans APS-C CMOS sensors with their unique RGB filter array that does away with the need for an anti-aliasing filter. All provide a normal ISO range of 200-6400, expandable to 100-51,200. The X-T1 and X-E2 feature the X-Trans CMOS II sensor, with on-sensor phase-detection AF.
The Hasselblad H5D-50c, Pentax 645Z and Phase One IQ250 back all incorporate versions of Sony’s 50-megapixel 44x33mm medium-format CMOS sensor and should deliver much better high-ISO image quality than previous CCD medium-format backs. Normal ISO range for the H5D-50c and IQ250 is 100-6400, and for the 645Z, it’s 100-204,800 (although we don’t expect that much difference among the cameras). While the sensors are similar, each camera has different processing and other tweaks to produce its own "look." Leica‘s S Typ 007 features a 45x30mm 37.5-megapixel CMOS sensor and a normal ISO range of 200-6400.
Micro Four Thirds
With their smallish sensors (17.3×13.0mm vs. 23.6×15.6mm for APS-C and 36x24mm for full-frame), Micro Four Thirds cameras generally don’t do as well at high-ISO settings as the larger-sensor cameras, but both the Olympus OM-D E-M1 and Panasonic Lumix DMC-GH4 are pro-level cameras with impressive capabilities. Both have a normal ISO range of 200-25,600, expandable to 100-25,600.
Nikon’s top high-ISO cameras are the flagship D4S and retro Df, which share similar 16.2-megapixel, full-frame CMOS sensors. The D4S has a normal ISO range of 100-25,600, expandable to 50-409,600; the Df has a normal ISO range of 100-12,800, expandable to 204,800 (the Df edged the D4S in the DxOMark.com Low-Light ISO rankings). The full-frame, 24.3-megapixel D750 (normal ISO range of 100-12,800, expandable to 50-51,200) and D610 (normal ISO range of 100-6400, expandable to 50-51,200) also made DxOMark.com’s Low-Light ISO top 10, while the 36.3-megapixel D810 (normal ISO range of 64-12800, expandable to 32-51,200) wasn’t far behind despite its pixel density. Nikon’s top APS-C low-light camera is the 24.3-megapixel D7100, with a normal ISO range of 100-6400, expandable to 100-25,600.
Pentax’s pro-oriented K-3 features a 23.4-megapixel, APS-C CMOS sensor and a normal ISO range of 100-51,200. Pentax, like Sony, doesn’t list separate normal and expanded ISO ranges.
Samsung’s pro-oriented camera is the "mini-DSLR-style" mirrorless NX1, with a 28.2-megapixel APS-C CMOS sensor and an ISO range of 100-25,600, expandable to 100-51,200.
Sony’s top low-light camera is the a7S mirrorless model, with a 12.4-megapixel CMOS sensor geared for low-light and 4K video. The a7S has a staggering ISO range of 100-409,600. The full-frame, 24.3-megapixel SLT-A99 DSLR features Sony’s unique Translucent Mirror Technology, which provides phase-detection AF and eye-level viewing for stills and movie shooting. Its ISO range is 100-25,600. The a7 mirrorless camera features essentially the same sensor as the A99 and a 100-25,600 ISO range, but scores higher at DxOMark.com
because the A99 loses a little light to the semitranslucent mirror. The full-frame, 36.3-megapixel a7R mirrorless model has an ISO range of 100-25,600, and actually scored higher than the a7 in DxOMark.com’s Low-Light ISO ratings. In APS-C, Sony’s 24.3-megapixel a6000 mirrorless model is one of the top APS-C models in DxOMark.com’s ratings, and has an ISO range of 100-25,600. The SLT-A77 Mark II DSLR features a similar sensor and ISO range, but rates lower because it loses a little light to the semitranslucent mirror.