There are some things about photography that remain as true today as they were 100 years ago. Narrow apertures produce photos with broad depth of field, for example, while wide apertures produce images with shallow depth of field. Fast shutter speeds freeze action, slow shutter speeds create motion blur. And high ISOs create images with noise or grain, while low ISOs do not. Except that the last one isn't really true anymore—but people still think it is. Read on to learn more.
What is ISO?
Before we can have a real discussion about ISO and the effect it has on your images, it's helpful to understand what exactly ISO is. The term itself stands for "International Standards Organization," which at first seems to make about as much sense as calling a scale-less, tail-less, non-swimming echinoderm a starFISH. Basically, the term “ISO” harkens back to the days of film, when just about every film manufacturer had different standards for assigning light sensitivity ratings to its products. Because of this inconsistency, there was a recognized need for some kind of standard across the board, so photographers would know what that number meant when they purchased a pack of film. The International Standards Organization is the group that came together and developed the standard for that rating, which from then on was known as "ISO."
Astrid by Flickr user ChlouÃ©
With film photography, light sensitivity is built into the film itself. The ISO number is an expression of how sensitive the film is to light—the lower the number, the less sensitive the film is. Of course, digital cameras don't use film so that ISO number is actually an equivalent—the number tells you how sensitive your image sensor is to light, compared to the equivalent pack of film.
So with film, of course, you purchased a pack of film at the ISO rating that you thought you would need, and then you were stuck using that ISO for the entire roll. So, for example, if you were photographing a birthday party, you couldn't just shoot the first half of the roll indoors and then follow the kids outside for the second half, because the ISO would be all wrong for one or the other. If you anticipated shooting an event both indoors and outdoors, you would either need two camera bodies, one loaded with the lower ISO film and one with the higher ISO film, or you would need to make sure you finished off that first roll so you could switch to the lower ISO film before moving outdoors. That was a big inconvenience either way you look at it.
Today we can just dial in the ISO we want to use, depending on the lighting situation. The only real problem with the digital ISO system is forgetting to change the ISO when we move in and out of different lighting situations.
Now obviously, being able to change the light sensitivity of your camera's image sensor is infinitely useful. We don't always have the good fortune of getting to shoot events in good light. Every photographer is sometimes faced with a situation where the light is too low for low ISOs, and it's great to have the option to turn that setting up when needed. But some photographers are reluctant to do this, choosing instead to use a wider aperture or a slower shutter speed, or even to engage the flash. And there are certainly some situations where this is appropriate—scenes that don't have any movement in them, for example, can be shot at slow shutter speeds and low ISOs as long as the photographer has a tripod to stabilize the camera during that longer exposure. And wide apertures are a great alternative, too, provided depth of field is not important. Flash is another viable alternative provided it's used well—direct flash can be very ugly, causing washed out faces, red eye and black shadows, but if you have an off-camera flash it can be bounced or diffused, which means the light will look a lot more natural.
What causes noise in high ISO images?
But if you don't have an off camera flash, or if you do need broader depth of field, or when there is motion in a scene, it's better to choose the higher ISO. But some photographers still are reluctant to do that, choosing instead to live with camera shake or motion blur so that they can keep that ISO number down. Why is that? It comes from a centuries-old belief, once based on very real concerns, that high ISO photographs are poor quality.
With film, high ISOs were associated with what we call "film grain." Film is made from millions of light-sensitive silver halide crystals. In low ISO films, those crystals are very small and invisible to the naked eye, but in high ISO film the crystals are much larger—large enough to see, and large enough to be really obvious at very high ISOs.
Pusteblume by Flickr user masine
Now, with digital we get a similar visual effect with higher ISOs, except that the explanation for it is completely different. Instead of "grain," we get what we call “digital noise,” which looks similar in that both grain and digital noise have a sort of sandy or gritty appearance. Digital noise is not caused by silver particles, though (obviously)—it's caused by the signal-to-noise ratio, or a random distribution of photons, which are limited when the light is low. While film grain varies in size according to ISO, digital noise is always the same size—one pixel. You get an increase in the amount of noise in a digital image as the ISO increases, rather than an increase in the size of each individual manifestation of that noise. And the color of film grain is neutral, while the color of digital noise can vary, being the most visible in the blue channel.
Now this was all very true and relevant a decade ago: just like when you select a high ISO film, selecting a high ISO setting on your digital camera would often lead to an image with that sandy or gritty appearance, thus decreasing the quality of the photograph. And that very long-held idea about ISO has persisted even through this last decade, even though it's simply not true anymore.
Now, even in modern cameras, high ISOs can still cause problems with photo quality. But the level at which those quality problems become apparent is much, much higher with a modern digital camera than it was with the cameras available as recently as 10 years ago.
11/52 by Flickr user ThrottleUK
Improvements in technology
So what has changed? Technology has changed, as it always does. Sensor technology has improved to the point where many modern cameras use a faster and more powerful processor that can reduce noise considerably in JPEG images. And the sensors themselves are better than they were in recent years—cameras with full frame sensors produce a lot less noise at high ISOs than cameras with smaller sensors. And the software is better, too—when you post-process a high ISO image, especially one shot in the RAW format, the noise reduction function is light years ahead of where it once was.
Now that is not to say that the problem of noise has disappeared altogether, it's just that it isn't a problem to the same degree that it once was. But your personal experience with ISO is going to vary according to a number of factors. First, how much noise can you personally tolerate in your photos? If you don't like to see any noise at all, then you will be shooting at lower ISOs than somebody who doesn't mind a little bit of noise. And your camera model has a lot to do with that question as well. If you have a brand-new full frame DSLR, for example, you're probably not going to start to notice any noise until you get into those very high ISOs. But if your primary camera is a smartphone, which has a very small sensor, you're going to start to notice noise at ISOs in much lower ranges.
Do some tests
It is a good idea for you to document your camera’s ISO performance, just so that you'll have a benchmark from which to work with. That's going to help you avoid taking risks such as lowering your shutter speed because you're afraid to turn up your ISO to compensate for failing light. This is a very simple experiment, and all you need is a tripod and your camera. Find a subject that doesn't move, and place it in a relatively low light situation, but not extremely limited light—inside your home in an area well-lit by a window is a good place to do this. Put your camera on a tripod and start by taking a photo at ISO 100. You may need to use a slow shutter speed for this, so make sure you're also using a remote release or your camera’s self-timer function to avoid adding camera shake to the shot. Gradually increase your ISO and your shutter speed one ISO setting at a time until you have one photo shot at every ISO your camera is capable of. Note that it’s better to change just the shutter speed for this experiment, because when you change aperture you change the depth of field in the image, which makes it more difficult to compare the low ISO shot with the higher ISO shots.
Canon EOS 60D ISO Noise Comparison by Flickr user Roger Smith
When you are finished, open the images up in post-processing and look at each one at 100% magnification. The noise will be more obvious to you zoomed in than it will be if you're zoomed out, so make sure you are examining the details. Now decide where that line is between the amount of noise you find personally acceptable and the amount of noise you find unacceptable. Make a mental note of where that line is, and then you will have a benchmark from which to work from. So if you're in a low light situation and you find that you have to turn that ISO up higher than you're comfortable with, you may instead want to consider switching to a prime lens with a wider maximum available aperture, or adding some light from an external flash.
Tips for keeping noise out of your shot
Now there are ways to limit the amount of noise you're getting in your photos even at ISOs that you're not quite comfortable using, so it's worth knowing how to get the most out of those high ISO images. The first tip is to use your histogram. If your camera shows you the histogram before you take the photograph, you can use that information to shoot a photo that is less likely to have noise in it. Specifically, you’ll want to "expose to the right." That means that you want your histogram to be slightly skewed to the right, with the highest peaks on the right side, but without any clipping in the highlights. In other words, the chart should taper off before it gets to the edge, rather than cutting off abruptly. When you use this technique, you can limit the amount of noise you will capture, especially in low-contrast scenes.
Remember that you can also increase noise in post-processing inadvertently. Lightening underexposed parts of your image can increase noise, which is why it's best to use the “shoot to the right” technique. And this problem becomes more pronounced the higher the ISO is. Because you don't have the reverse problem (that is, you won’t create noise when you make shadows darker), that’s a good reason to lean towards slightly overexposing in low light situations. Remember that you can always darken a slightly overexposed image without creating noise, while the opposite may not be true.
underexposed by Flickr user Halfmoon Hiker
And remember that noise is more obvious in smooth surfaces than it is in textured ones. For example, if you shoot a landscape that includes both trees and water, the noise will be less visible in the trees than it is in the water. That is one good argument for trying to incorporate texture into those lowlight images if you know you're going to have to shoot with a higher ISO.
Now I know I've just instructed you to find your own personal benchmark for tolerance of noise, but I do want to add in closing that there are some scenarios for which you may want to reevaluate your own opinion. Noise can be used creatively, too—think photos with a gritty, photojournalistic mood, or images where the noise can add interesting texture to an already aesthetically beautiful shape. Even if you are intensely uncomfortable with noise, I challenge you to turn your ISO up past that benchmark point and try shooting some scenes in which you incorporate noise into the composition. You may surprise yourself with some of the beautiful effects you're able to achieve not just in spite of the noise but because of it.
- What is ISO?
- For film
- For digital
- What causes noise in high ISO images
- Silver crystals
- Signal to noise
- Improvements in technology
- Larger sensors
- Faster processors
- Better software
- Test your equipment
- Keep noise out of your photos
- Expose to the right
- Shoot textured subjects
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