Just like "film speed," your camera's sensor has variable sensitivity to light. In automatic modes the camera manages this for you, and it tends to lean towards to conservative to get you the best picture. However, situations may arise such as dark rooms where you don't want to use a flash where you want to overrule the camera.
Terminology
ISO is a leftover term from film cameras, but it still refers to the sensor "speed." Typically cameras have a base ISO of 100 or 200, although some go as low as 50. Manufacturers are always trying to produce higher ISO. Most cameras have sensors that go to ISO 1600 or 3200 and then have "expanded" settings (High 1 and High 2) to 6400 and 12,800. The expanded setting is truly just software manipulation after the picture is taken. Very top end cameras may go as high as ISO 102,400 or ISO 204,800.
The higher the ISO the more "noise" in a shot and the less clarity, color depth, and dynamic range. Noise is apparent by patterns of dots. This can be somewhat pleasant like an old film grain, or it can be harsh like bands of color. Noise is most prevalent in the darker areas of high ISO shots. Cameras do have noise reduction which can mitigate most noise, but that it turn reduces the clarity or sharpness of a shot.
Color depth and dynamic range are also reduced with higher ISO. Color depth is the ability to produce a range of colors. At higher ISOs colors tend to become less true and clarity of the shot is also reduced as similar colors begin to blend together.
Dynamic range is the range of dark to bright that can be capture by a sensor. DSLRs typically have very good dynamic range and can capture a photo with shadow and highlight. However, as ISO increase shadows turn black more quickly and highlights turn white more quickly -- detail is lost.
Choosing ISO
As said above, the camera will typically choose ISO for you. You can also normally set the max ISO you find acceptable for automatic use through the menu system. However, you may find a time when you want to push the max ISO above your normal thresholds to capture dark scenes or so that you can use a higher shutter speed or smaller aperture.
While not really a situation I would use a high ISO, this first shot was taken at High 2 or 12,800 ISO equivalent. With very good lighting, the camera has done a good job of mitigating some of the negative effects. However, if you enlarge the image and compare it to others you'll note several points.
First, the color of the black sign isn't exactly true -- it's more yellow than it should be. Also, the plants in the bottom right corner are too brown. The mortar between the bricks isn't detailed and the plaque looks grainy.
The second shot is at ISO 1600. The differences are immediately apparent. The plague is a better black; the bush in the bottom right is the appropriate green, the mortar has more detail, and the text is less "noisy." There are still flaws in the color and clarity of the image, but they are much harder to find -- the camera has done a great job at minimizing them.
This was taken at ISO 400. While there are differences to ISO 1600, they are hard to determine. I would say the colors are a little more true -- more blue in them. Slightly less noise. But, in all it's hard to tell.
This shot is at ISO 100. The difference to 400 is minimal if apparent at all. However, that speaks to the quality of the sensor. You have to be very technical to find the difference between ISO 100 and ISO 400. However, this shot is clearly noise free, offers true color reproduction, and has lots of detail.
In Use
Determining your camera's ISO setting is important at first, then determine what your comfort level is for high ISO shots. In some cases, like an indoor sporting event, high ISO may be the only way to get the shutter speed you need to get a shot. So, it's high ISO or nothing. In other cases, you may be able to use a flash to compensate for a lack of light.
Once you've determined your tolerance for high ISO shots and know how to set it, use it appropriately. ISO 800 is a good trade-off between quality and range of exposure. However, that's too much on a bright day and not enough in a church.
ISO 100 - 400
- Great for portraits and landscapes. You get the absolute best color and quality
ISO 800 & 1600
- Good compromise ISOs. You're not giving up too much in quality, especially at 800. Good for indoors and not using a flash
ISO 3200
- This is an "if I must" setting. There is definitely a quality hit at this point, but the shots are still decent.
ISO 6400 & 12800
- Really, these are last resort settings. You'll get the shot and it might look good on a screen. But, you'll give up a lot, especially at the 12800 setting.
No comments:
Post a Comment