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Many modern digital cameras boast incredible ISO speeds. Where ISO 400 or 800 was the top speed in the film era, usually accompanied by grain the size of golf balls, today’s digital cameras can give you top ISOs of 6400, 12,800, 25,600 or even higher. Camera makers boast of these high speeds and use this information to increase camera sales.
Inexperienced photographers will be tempted to jack up the ISO on their new cameras, and keep it there. However, just because the feature is there doesn’t mean you should use it all the time. In fact, the best practice is to keep your camera set at its lowest ISO setting by default so you will capture the best possible image.
With experience you will learn that higher ISO settings are more appropriate when shooting sports, street photography, photojournalism, and low-light situations where you need to stop action. Low ISO images will be cleaner (no digital noise), have a wider dynamic range (more shadow and highlight details), and produce better color depth (smoother color transitions). This is a better choice for travel, landscape and portrait photography, where good detail and accurate skin tone are important.
There are two ways you can understand the effect of low versus high ISO on digital images: by reading lab test results, and by looking at sample images. DxOMark Labs is a great resource for sensor tests. It’s an independent lab that tests most current digital camera sensors and publishes the results on their web site. For sample images, you can make your own. Continue reading to learn more.
Run any number of digital cameras through a battery of image quality tests and you’ll see a clear pattern emerge. At the lowest ISO setting, the signal to noise ratio, expressed in decibels (dB), is highest. That’s good, it indicates the lowest digital noise. As you increase ISO, the dB level decreases. The lower the dB level, the higher the digital noise, which degrades the image.
Example: If a Canon EOS 70D has a 41dB result at ISO 100, 30dB at ISO 1600 and around 17dB at its top ISO of 25,600, the image with the higher dB will have less noise than an image with a 17dB measurement.
Note: keep in mind that most images when viewed at the recommended distance will appear clean as low down as 30dB.
Similarly, the dynamic range of a digital camera is widest at its lowest ISO. Depending on the camera, that range can be anywhere from 9 stops to around 14. Typically, with DSLRs, it is around 11-12 stops. In carefully exposed high-contrast scenes, both highlights and shadows will have more detail at low ISOs than at high speeds. In test result charts, dynamic range starts to decrease in more of a curve than a straight line; typically the first few ISO settings are acceptable, but once you pass ISO 800 or 1600 (depending on the camera) the range becomes more limited.
Dynamic range is acceptable at 10 stops or higher. Below that number shadows and/or highlights will become blocked up, especially in contrasty scenes.
Tip: RAW image capture is best for dynamic range. You can use the shadow and highlight sliders in your RAW image editor to reveal even more details in both the shadows and highlights, effectively stretching your camera’s dynamic range. The better your image’s dynamic range is, the more information you can reveal this way.
You may have noticed that some images that show the sky or other areas that should have smooth, subtle transitions, instead show subtle strips of color as the shading changes. This is called banding, and that’s what happens when the tonal range is limited. This is usually an artifact from compressing an image too much when preparing it for web use, but it happens in a more subtle form in photos that come straight out of the camera and, as above, this phenomenon is more apparent in high ISO images, but is imperceptible in low ISO photos.
A camera’s ability to capture subtle changes in tone and color (both expressed in bits) is best at its lowest ISO, and decreases steadily throughout the ISO range. Most DSLRs have a 21-24-bit maximum for color sensitivity, and it can degrade to half of that. Tonal range usually peaks at around 8-10 bit.
Enough of the geeky stuff, let’s look at some pictures to see how this plays out in the real world. While the images here were shot with a Canon 7D, you can run your own tests:
Here are two image comparisons at a range of ISO settings.
The effect of changing ISO is most obvious when you photograph people. In this example, the model was photographed on a sunny day while holding a reflector, against a low-key (darker) background. As the ISO setting is raised, the effect on her skin becomes obvious. Camera: Canon 7D with Canon 85mm f/1.8 lens.
At ISO 100 (above), skin tone is accurate, with good contrast. In the 100% detail shot, there’s good, natural falloff of light, showing a wide range of tonality and good detail in the darker areas. You can especially see this in the eyes.
At ISO 800 there is a subtle increase in contrast, an indicator that dynamic range has decreased slightly. Noise has not yet become apparent in the detail photo.
By ISO 1600 there is a little more contrast, but again, it is a subtle change. However, you can see the noise in the blow-up, and the smooth gradation from light to shadow is starting to become a bit rougher, indicating that the tone has degraded.
ISO 3200, at screen resolution it may still be hard to see the change when you compare this image to the one shot at ISO 100, but by ISO 3200 there is a pronounced decrease in highlight and shadow detail, and rougher color and light transitions.
By ISO 6400, the Canon 7D’s highest resolution setting, even at screen resolution a difference is visible. If you look in the details of the shirt and hair, you can see that shadows are more blocked up. A quick glance at the 100% detail is all you need to see the graininess and reduced range of color and tone.
You can increase an image’s dynamic range by using your RAW image editor’s shadow and highlights sliders, which reveals more information in the highlights, and especially in the shadows. Compare this version of the ISO image with the ones above and notice how much more detail is visible in the shadows, thanks to tweaks done in the RAW image editor.
A scenic lookout in western New Jersey, shot in the middle of the day, at ISO 100, 800, and 3200 with a Canon 7D and Canon 24-70mm f/2.8L II USM zoom lens. While the dynamic range, tone and color only show minor differences here, the level of noise deteriorates at the faster speeds. After looking at these examples, you may wonder why anyone in their right mind would want to shoot above ISO 100 on a bright, sunny day. All samples are straight out of the camera.
At ISO 100, this is the full scene.
In this detail of a 100% blow-up of the ISO 100 image, there is no apparent digital noise, as expected.
At ISO 800, digital noise has started to become apparent in this 100% blow-up detail.
By ISO 3200, digital noise is obvious, and the overall image quality has deteriorated. The graininess covers up the loss of color quality and tonality somewhat.
While the samples above are unadulterated JPEGs, what happens if you try to coax more detail out of a shot in RAW? You get more detail in the shadows and highlights. Compare this shot to the original and you’ll see more detail in the sky (highlights) and in the bark and branches of the tree (shadows) on the right.
The bottom line? There may be times when you must pump up your ISO settings, and it’s nice to know that you have that option. But when you are shooting in daylight under normal shooting conditions, you will bring back higher quality images when you shoot at, or near, your camera’s lowest ISO settings.
Thanks to Adorama to sponsoring this series of articles by Mason Resnick