Digital Photography from 20,000 Feet
by Wesley Fink on September 25, 2006 12:05 AM EST- Posted in
- Digital Camera
Photo Basics: Painting with Light
It is very important to understand that whether using digital still photography, digital movies, film stills, or film movies the principals of capturing images is the same. In every case you are capturing images of light on some photosensitive medium. While many people want nothing more than the automatic point-and-shoot available on every digital camera these days, understanding what is happening behind the scenes can help you in selecting the right tools for what you want to do - even if you never plan to use those features.
ISO Speed: The Sensitivity of the Digital Sensor
With 35mm film cameras, an important decision before shooting any photos was what film to buy. Films varied not just in types (slide and print) but most importantly in speed - the ability of the film to respond to light. Film varied from some super-fine-grained ASA 25 to some faster, but grainer, ASA400. Late in film development the most commonly used films were ASA50 to 100 for quality shots when you had plenty of light, and ASA400 to 800 where light was low and you could live with the increased "graininess" of the images.
Several competing film speed systems existed - the US ASA, European DIN, and the Soviet GOST to name a few. All of these systems for measuring film speed were eventually replaced by the ISO system, which basically combined the ASA and DIN systems.
Digital photography has completely changed our ideas of film speed, since the sensor is built into the camera. We no longer have to worry about film speeds or changing film, but digital sensors still use ISO speed to measure their sensitivity. The great thing about digital sensors compared to film, though, is that digital sensors in SLR cameras have a wide range of sensitivity speeds, and you can just choose how sensitive you want the sensor to be. It helps in choosing ISO if you understand what it means.
ISO, or the International Organization for Standardization, eventually developed a standard which combined both the ASA (linear) and DIN (logarithmic) scales into one ISO standard, officially ASO5800:1987. The most common ISO film speed ratings are:
25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°
The important point to understanding ISO is that a doubling of film speed means the film (or sensor) is twice as sensitive to light. On the logarithmic scale a change of 3° is a doubling of sensitivity. So each number above represents twice the sensitivity of the previous number.
What this means is that at the same shutter speed and aperture (lens setting) you can take pictures in 1/8 the light by using ISO 800 instead of ISO 100. Early digital cameras and digital point-and-shoots often only support a few ISO sensitivities (50, 100, 200) and they rarely support sensitivities above ISO 400. The Digital SLR, on the other hand, normally supports ISO speeds to 1600 or even 3200 on some cameras. This gives the user great flexibility in low light.
There is a trade off, however. As speed increases some digital SLRs do a much poorer job of resolving details in the picture. All Digital SLR cameras seem to do fine from 100 to 400, most go to ISO 800 easily, and some do to ISO 1600 with minimal loss of image detail. In reviewing a camera look at the effects of ISO speed on image quality.
Every digital SLR allows the user to adjust ISO speed, but some make it easier than others. The new Sony A100, for example, has an "Auto" ISO setting that automatically adjusts ISO between 100 and 800 depending on the light in the scene. The new Nikon D80 allows the user to customize the Auto ISO range depending on what they are comfortable with.
In the end, you can adjust ISO, the lens aperture (f-stop), or shutter speed to control the amount of light that strikes the sensor. Any of the three can control the light, and they all have different effects and different limitations. The Auto programs in all digital SLR cameras make these decisions for you, but it is good to know what they are doing in the background, or how to manually make these decisions yourself. One of the great advantages for digital users is that the sensor can operate at many speeds, not just one, and the rest of the "film" is a flash memory that you can reuse over and over. Nothing is consumed in the digital photo process until you want hard images (prints).
It is very important to understand that whether using digital still photography, digital movies, film stills, or film movies the principals of capturing images is the same. In every case you are capturing images of light on some photosensitive medium. While many people want nothing more than the automatic point-and-shoot available on every digital camera these days, understanding what is happening behind the scenes can help you in selecting the right tools for what you want to do - even if you never plan to use those features.
ISO Speed: The Sensitivity of the Digital Sensor
With 35mm film cameras, an important decision before shooting any photos was what film to buy. Films varied not just in types (slide and print) but most importantly in speed - the ability of the film to respond to light. Film varied from some super-fine-grained ASA 25 to some faster, but grainer, ASA400. Late in film development the most commonly used films were ASA50 to 100 for quality shots when you had plenty of light, and ASA400 to 800 where light was low and you could live with the increased "graininess" of the images.
Several competing film speed systems existed - the US ASA, European DIN, and the Soviet GOST to name a few. All of these systems for measuring film speed were eventually replaced by the ISO system, which basically combined the ASA and DIN systems.
Digital photography has completely changed our ideas of film speed, since the sensor is built into the camera. We no longer have to worry about film speeds or changing film, but digital sensors still use ISO speed to measure their sensitivity. The great thing about digital sensors compared to film, though, is that digital sensors in SLR cameras have a wide range of sensitivity speeds, and you can just choose how sensitive you want the sensor to be. It helps in choosing ISO if you understand what it means.
ISO, or the International Organization for Standardization, eventually developed a standard which combined both the ASA (linear) and DIN (logarithmic) scales into one ISO standard, officially ASO5800:1987. The most common ISO film speed ratings are:
25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°
The important point to understanding ISO is that a doubling of film speed means the film (or sensor) is twice as sensitive to light. On the logarithmic scale a change of 3° is a doubling of sensitivity. So each number above represents twice the sensitivity of the previous number.
What this means is that at the same shutter speed and aperture (lens setting) you can take pictures in 1/8 the light by using ISO 800 instead of ISO 100. Early digital cameras and digital point-and-shoots often only support a few ISO sensitivities (50, 100, 200) and they rarely support sensitivities above ISO 400. The Digital SLR, on the other hand, normally supports ISO speeds to 1600 or even 3200 on some cameras. This gives the user great flexibility in low light.
There is a trade off, however. As speed increases some digital SLRs do a much poorer job of resolving details in the picture. All Digital SLR cameras seem to do fine from 100 to 400, most go to ISO 800 easily, and some do to ISO 1600 with minimal loss of image detail. In reviewing a camera look at the effects of ISO speed on image quality.
Every digital SLR allows the user to adjust ISO speed, but some make it easier than others. The new Sony A100, for example, has an "Auto" ISO setting that automatically adjusts ISO between 100 and 800 depending on the light in the scene. The new Nikon D80 allows the user to customize the Auto ISO range depending on what they are comfortable with.
In the end, you can adjust ISO, the lens aperture (f-stop), or shutter speed to control the amount of light that strikes the sensor. Any of the three can control the light, and they all have different effects and different limitations. The Auto programs in all digital SLR cameras make these decisions for you, but it is good to know what they are doing in the background, or how to manually make these decisions yourself. One of the great advantages for digital users is that the sensor can operate at many speeds, not just one, and the rest of the "film" is a flash memory that you can reuse over and over. Nothing is consumed in the digital photo process until you want hard images (prints).
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Zaitsev - Monday, September 25, 2006 - link
That was a very interesting article. I've always wanted to learn more about cameras and photography, in the hopes of making it a hobby once the time and money is available. So thanks for explaining the background info for noobs like me :) I'll be looking forward to the rest of the series.