Missing Pages: ISO Setting
The Missing Pages column is a collection of all of the information that should have been included in your camera’s Owner’s Manual—but somehow got left out. This is a hybrid assortment of short articles that delivers the know-how you need to derive the maximum enjoyment—and creative expression—from your equipment.
It’s sort of a juiced-up User Guide for creative people who are not necessarily technical. Each part will teach you how to use one of the camera features or functions that you previously ignored or left set on Auto. And each will include a Creative Project so that you can try some scripted experimentation.
We will explain complex technical subject matter a way that everyone can understand. And if you happen to be a technical expert yourself, we’re including “Nerds Only” sidebars just for you. That way you can dig in deep—or just straddle the edges—of the technological stuff. It’s your decision.
Installment I: ISO Setting
[also known as Sensitivity Setting or ISO Sensitivity]
Imaging sensors are designed to perform their best when light is at a certain level. For the convenience of photographers who are accustomed to understanding the different sensitivity levels of film, manufacturers (for the most part) build sensors that have roughly the same sensitivity as a roll of ISO 100 film. In an attempt to keep everything familiar and understandable, camera makers use a numbering system that’s closely equivalent to the old ISO ratings from film photography. Some even refer to the relative speeds as ISO, but that’s technically not correct. Nonetheless, for the sake of simplicity we’ll do the same.
This sensitivity can be adjusted to accommodate lower light levels. Increasing the sensitivity setting to ISO 200, for example, makes the sensor twice as sensitive. Changing to ISO 400 makes it 4X as sensitive as 100. Increasing the ISO will allow you to shoot in low light at a shutter speed that’s fast enough to prevent camera shake. That’s the primary reason why one would want to increase the ISO: to make it possible to shoot at a more desirable combination of f/stop and shutter speed.
Cameras that offer image stabilization lower the shake threshold, so you might be able to shoot at, say, 1/15th of a second instead of 1/60th. That’s a difference of two stops and the same as increasing the ISO from 100 to 400. This gives you an idea of how important image stabilization is.
Increasing the ISO and keeping the shutter speed the same will allow you to shoot at a smaller lens opening (f/stop). That will increase the depth-of-field and increase the zone of sharpness that extends behind and in front of the point of focus.
Then why not always shoot at ISO 1600?
Adjusting the ISO changes how the camera responds to a given quantity of light. These increases come at a price, however. As sensitivity increases, so does “noise.” Noise is the mottled, grainy-looking texture that appears when you shoot at a high ISO.
Nobody likes how noise looks, so camera makers use a countermeasure. They process the noisy digital image signals with noise filters and noise suppression algorithms. This operation occurs in the “signal processing engine.” Canon calls their engine DIGIC IV. Panasonic calls theirs VENUS. Camera makers brand their engines for marketing purposes, and also to track generational differences (DIGIC IV vs. DIGIC III, for example).
All camera makers handle noise suppression in their own way, so even if two brands of cameras use the same sensor (which is very common) the results can be quite different because the engines are different. The noise suppression process often reduces the image sharpness. This bears repeating: noise suppression is the enemy of sharpness. So we can say that every low-light image is a compromise between noise and sharpness.
But signal noise is only part of the equation (albeit the biggest part). Light level is also an important variable. Many folks don’t realize this, but shooting at a high ISO like 800 in bright light will yield better results than shooting at a high ISO in near darkness. Don’t misunderstand—it’s always better to use the lowest ISO that conditions allow. But it’s important to understand that noise is a product of high ISO and low light. In some cases it’s better to shoot at ISO 400 in bright light than ISO 100 in the dark. That’s because noise can increase as a function of time, also. And lower ISO settings require longer shutter speeds.
What about ISO 50?
Some cameras—early Canon PowerShots, for instance—have ISO 50 settings. Others have ISO 64 or ISO 80. Settings lower than ISO 100 generally produce the best images a camera can make—at least as far as noise is concerned.
Leaving your camera’s ISO setting on Auto is a mistake. The camera could use a higher setting than necessary and cause your image to appear grainy or noisy. You don’t want that. Besides, you cannot accurately control the aperture setting or shutter speed setting unless you also control the ISO sensitivity.
Digital SLR cameras generally perform better than compact cameras at high ISO settings. This is not always the case, and some compacts—the old Fujifilm Finepix F31 is a notable example—can hold their own even when photographing the legendary “black cat in a coal bin at midnight.” You can read more about the technical reasons behind this in the Nerd section below.
You don’t need any special equipment for this one, just your digital camera. If you happen to have a lightweight tripod you can expand the project, but that’s up to you.
Here’s the drill. In normal daylight, photograph a colorful subject at every ISO setting your camera offers starting at the lowest and working your way up to the highest. Shoot only one shot at each ISO so that it will be easier to compare the results.
Repeat the same exercise indoors under ambient light that is just sufficient for reading. Don’t worry if the images shot at the lower ISO settings are blurry—that’s normal. If you want to do the indoor portion on a tripod, that’s fine.
Depending on your camera, the actual light levels and the subject, you should find that noise increases as ISO increases. In the outdoor shots, the effect will be less pronounced because there is more light to saturate the photo sites. The indoor the images may get as shaggy looking as coconut pancakes—or look like a bad example of pointillism.
How does this help you creatively? You must learn to judge light levels and use the ISO setting that will produce acceptable results. Different camera brand behave differently so it’s important to understand how yours performs. Once you’ve mastered this sense for light levels you’ll know when to use flash or a tripod.
We’ll say this now and doubtlessly repeat it throughout the Missing Pages series: creative photography is about getting exactly what you want—it’s not about wanting whatever you get.
“ISO” is an abbreviation for International Organization for Standardization. Yes, I know that should make it “IOS” instead of “ISO” (but one must admire a standards organization that’s willing to be a bit unorthodox). Old timers may refer to the same specification as “ASA,” the more ethnocentric “American Standards Association.” And our European cousins still recall the German counterpart: DIN (Deutsches Institut für Normung).
The digital noise that appears when a high ISO sensitivity setting is used is partly the result of physical measurement between photo sites (pixels) that engineers call “pitch.” Pixels that are packed very densely together have a very small pitch and are noisier.
At a given megapixel level, small imagers have greater density, smaller pitch and therefore more noise. The image sensor (usually a CCD but sometimes CMOS) is measured diagonally and the dimension is expressed as a fraction. To confuse you, the fraction contains a decimal in the denominator. A CCD that is 1/2.5 inches is actually equal to 1 divided by 2.5 inches, or 0.40 inches. A CCD that’s 1/1.6 (which may seem smaller) is 0.625 and—all else being equal—will produce images with less noise.
To really confuse the issue, many people refer to the ISO setting as the “speed.” This is another throwback to 35mm photography where we often talked about “film speed.” It’s best to avoid this nomenclature since “speed” is already overused to describe large f/stops (“fast, high-speed f1.2 lenses”) and shutter speed. A better term for sensitivity adjustment, scientifically speaking, would be “gain.” Audiophiles and people who dig shortwave radios are already familiar with this expression as the technical term for an amplifier’s output/input magnitude ratio.
You can remove most of the noise from a digital image during post processing by using NIK Software’s Dfine or other product. Adobe Photoshop CS4 also has some potent noise-busting tools. But like poison ivy and hangovers, it’s always better to avoid noise than to cure it.
Text ©2009 Jon Sienkiewicz.
Text ©2009 Jon Sienkiewicz.