Just Like Film Speeds, But Different

By Mike Stensvold – Digital PhotoPro.com

Every photographer is familiar with ISO speeds, those numbers we dial into our exposure meters and cameras so they can lead us—albeit, sometimes somewhat circuitously—to properly exposed images. Most films have ISO speeds, and digital cameras have ISO equivalents. Actually, the digital figures are ISO speeds, too: Like ISO film speeds, they’re assigned based on standards issued by the International Organization for Standardization. ISO isn’t a photo term; it’s the global acronym for the body that publishes worldwide standards for everything from space-vehicle engineering and textile technology to business-to-business dealings. As you’ll see, ISO covers much more than just film speeds.

But there’s a fundamental difference between ISO film speed and digital ISO speed because there’s a fundamental difference between films and digital image sensors. When you put a faster film into a camera, the new film is more sensitive to light than the one you replaced. When you switch a digital SLR from one ISO setting to a higher one, the sensitivity of the digital image sensor doesn’t change; you’re just amplifying the data that it produces. More details on this in a moment, but first, what exactly is an ISO speed?

An ISO speed is a mathematical expression of a photosensitive material’s sensitivity to light. Originally, ISO speeds included both the arithmetic ASA value (used in the U.S.) and the logarithmic DIN value used in Europe (ISO 400/33º for Kodak Tri-X, for example), but now they just use the arithmetic value (ISO 400). In the arithmetic system, each doubling of the ISO number indicates a doubling of film sensitivity: An ISO 400 film is twice as “fast” as an ISO 200 film and half as “fast” as an ISO 800 film.

ISO film speeds and digital ISO speeds are assigned by the film and digital camera manufacturers based upon criteria set forth in the appropriate ISO standards. The purpose of ISO speeds is to provide a consistent standard for exposure: Theoretically, if you use any manufacturer’s ISO 100 film and set your camera’s meter to ISO 100, or set any digital camera’s meter to ISO 100, the resulting images should look about the same, exposure-wise. (This isn’t always the case in practice due to differences in exposure meters and their use among other things, but it’s the goal.)

ISO Film Speeds

ISO standards are copyrighted, and the ISO speed standards run four to 14 pages in length, so we can’t reproduce them in this magazine. If you want to read the actual standards in their entirety, you can purchase them on the ISO website, www.ISO.org. But basically, ISO film speeds are established by exposing the film in question to a range of exposure values, then giving it a specified degree of development in a specified developer. The point where a specified minimum density occurs on the resulting film curve determines the film speed.

Of course, laboratory-based film speeds don’t always apply perfectly in real-world photography. In fact, several alternative methods of determining working film speeds for black-and-white pictorial photography have been devised, most notably the Zone System made famous by Ansel Adams. And many color film shooters rate their emulsions at something other than their ISO speeds because they prefer the results they get that way.

Your ideal speed for a given film depends on such things as the subject matter you shoot, the meter you use (and how you use it) and your personal taste. But you have to start somewhere, and ISO speeds provide a good standardized starting point.

Digital ISO Speeds

Films are processed in developers and produce transparent negative or positive images that can be read with a densitometer. Digital images are processed by the camera (or by the user after shooting via special software, in the case of RAW images), and don’t exist in solid form like film images. Viewed digital images vary with the setup of the monitor on which they’re viewed or with the prints made from the images. And the fact that digital SLRs (and some consumer digital cameras) allow you to set different ISO speeds further complicates things. So digital ISO speeds must be based on different criteria than ISO film speeds.

There are actually two digital ISO speeds: the ISO speed and the ISO speed latitude. The former is analogous to an ISO film speed; the ISO speed latitude covers the range of settable ISO speeds that will produce acceptable (per the ISO standard’s criteria) images with a particular digital camera.

Digital ISO speeds are based on the amount of exposure required to produce an image of specified brightness while restricting image noise to a specified acceptable level. For the main digital ISO speed, that noise level is quite low. For the ISO speed latitude range, the upper speed limit is determined by a higher but still acceptable specified noise level (“noise limited”), while the lower limit is determined by highlight clipping (“saturation limited”).

Because exposure duration, temperature and humidity can affect digital image quality, the ISO standard cites specifics for each of these. In real life, we shoot at a wide range of shutter speeds, temperatures and humidities so, as is the case with film speeds, the controlled laboratory criteria don’t necessarily apply to the wide range of real-world photographic situations. But like ISO film speeds, digital ISO speeds provide us with a standard, a starting point.

A Big Difference

When you change the ISO setting on a film camera, the camera’s meter reduces exposure (if you changed to a higher speed) or increases exposure (if you set a lower speed)—but the sensitivity of the film in the camera remains the same. If you have ISO 200 film in the camera and set the meter to ISO 400, the images will be underexposed (unless you push-process the film, which presents its own problems). If you have ISO 200 film in the camera and set the meter to ISO 100, the images will be overexposed (unless you pull-process the film). With a film camera, if you want to shoot at a different ISO, you must set the meter to that ISO and then put a roll of film with that speed in the camera.

When you change the ISO setting on a digital camera, the camera adjusts the exposure accordingly, like a film camera. But it also adjusts the in-camera image processing to match—you don’t have to “change film” to shoot at a different ISO. In fact, that’s one of the big advantages of shooting with a digital SLR instead of a film camera: you can shoot every shot the ideal ISO speed.

Image sensors have an innate “native” sensitivity, generally in the ISO 100 to 200 range. When you set a higher ISO speed, amplifiers in the image sensor’s circuitry increase the gain before sending the image data to the A/D converter to be digitized. The sensor’s sensitivity doesn’t actually increase; the camera is just amplifying the data it produces. In the process, image noise is also increased, making the image “grainier”—sort of like what happens when you “push” film speed. But generally, digital SLRs produce better image quality at higher ISOs than film, especially pushed films.

If you set a lower ISO speed than the sensor’s native sensitivity, the camera’s image processor adjusts the image data after the A/D converter converts it to digital form. In the process, the dynamic range is reduced. So it’s best to shoot at the sensor’s native ISO whenever possible.

All Digital ISOs Are Not Equal

Just as some ISO 400 color slide films produce better image quality than others, some digital SLRs produce better image quality at a given ISO setting than others, particularly at the higher ISO settings. An even bigger difference exists between the digital SLRs, with their relatively large image sensors, and the compact consumer digital cameras, with their fingernail-sized sensors. Bigger sensors contain bigger pixels for a given megapixel count, and bigger pixels mean less image noise and a better dynamic range, all other things being equal. So if you buy a consumer digital camera as a take-anywhere tool or backup to your digital SLR, don’t be surprised when you find its image quality, especially at ISO 400 and higher, noticeably worse than that of your D-SLR.

A Plan

From a practical standpoint, it’s a good idea to test your digital camera to see how it performs at different ISO settings in a variety of conditions. Shoot the same scene at different ISO settings and see which results you prefer. Some D-SLRs have an ISO-bracketing feature, which makes this process much easier.

Shooting at higher ISO settings increases image noise, but so does shooting at longer exposure times. So is it better to shoot a low-light scene at a higher ISO with a faster shutter speed, or at a lower ISO with a longer shutter speed (assuming that action-freezing or -blurring shutter speeds aren’t a consideration)? Again, the best way to answer that question is to try it both ways with your camera and see which results you prefer. Because I like to work handheld, I tend to go with higher ISO settings and shorter shutter speeds, but if you use a tripod, you might prefer to do the opposite. Many digital SLRs have long-exposure and high-ISO noise-reduction features; see how well they work at different ISO settings and exposure times with your camera.

If you shoot RAW images, you might also test to see whether your camera gives better results exposing correctly at a higher ISO or using the same shutter speed/ƒ-stop combination at a lower ISO (underexposing) and then “pushing” the image using the RAW software. Note that you don’t want to underexpose at higher ISO settings with a digital camera. A number of current digital SLRs have given me superb results at ISO 1600 when I nailed the exposure (no manipulation in Photoshop required), but no digital camera will produce good image quality when high-ISO images are underexposed.

When you get a new digital camera, try it at all its ISO settings and a wide range of shutter speeds to see what it can do. And bracket exposures whenever possible while you learn the intricacies of the new camera—it’s not like you’re wasting film. Once you’re comfortable with the camera, use the ISO setting you need to get the shot, expose correctly and enjoy the results.

A digital image sensor has a native ISO. When you adjust the ISO setting, the sensor itself isn’t changing; rather, the amplification is changing. This is fundamentally different from film.

Alphabet Soup

The International Organization for Standardization (ISO) includes national standards bodies from 156 countries. The American National Standards Institute (ANSI) is the U.S. representative. ANSI replaced the American Standards Association (ASA) in the 1970s, for those of you who remember the old ASA speeds.

Like the CMOS sensor shown on page 88, this CCD sensor has a native ISO that can’t be changed. The amplifiers further along in the camera’s circuitry boost the signal to change the “speed.” Boosting the signal also boosts noise, however. Everything is a trade-off.

ISO Film & Digital Speed Standards

• ISO 2240: 2003,“Photography— Colour reversal camera films— Determination of ISO speed”

• ISO 5800: 1987, “Photography— Colour negative films for
still photography—Determination of ISO speed”

• ISO 6: 1993, “Photography— Black-and-white pictorial still camera negative film/process systems—Determination of ISO speed”

• ISO 12232: 1998, “Photography— Electronic still-picture cameras— Determination of ISO speed”

Film ISO is adjusted through the chemistry of a particular emulsion. Silver-halide crystals and other chemicals are reformulated to give the film a faster or slower ISO. Elements such as crystal size and inclusion of additives alter the film’s native ISO.

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