GS-DC: ISO, Histograms, Image Stabilization, and Exposure Modes

Review:

1. Formatting memory
2. Meta Data
3. Lenses
4. Focal Length - 35-110mm, for example
5. Aperture

What Is... ISO:

• ISO sensitivity (Auto; ISO 100, 200, 400, 800) expresses the speed of photographic negative materials (formerly expressed as ASA).

• In digital photography,  ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations.

• And, where you would have needed to physically change to a different roll of film if you wanted a different ISO speed, digital technology allows you to simply dial one in. In this way, you can record images taken at different ISO speeds on the same memory card.

ISO Speed & Exposure

• ISO speed affects the shutter speed / aperture combination
• Suppose your digital camera's light meter warns you there is not enough light to correctly expose a scene. You could use the on-board flash, but let's suppose again it's not allowed (like in a concert or indoors recital).
• You would then need to use a higher ISO. Set on "ISO Auto" mode, your digital camera will automatically select a higher ISO. Otherwise, you can manually select the next higher ISO and see if the increased sensitivity allows you to obtain a correctly exposed picture. If it does, you can now take a correctly exposed picture.
• Similarly, if you find the camera is using a shutter speed that is too slow (1/60 sec. and slower) to handhold the camera steady and shake-free (thus resulting in blurred pictures), and you cannot open up the aperture anymore, and you do not have a tripod or other means to hold the camera steady, and you want to capture the action, etc. etc. -- then you might select the next higher ISO which will then allow you to select a faster shutter speed.

ISO Speed & Noise

• However, all this increase in sensitivity does not come free. There is a price to pay with your image appearing more noisy.
• When you boost the sensitivity of your image sensor by selecting a higher ISO, the image sensor is now able to record a fainter light signal.  The increased sensitivity allows the image sensor to record more light signal and more noise
• An image sensor is usually calibrated so that it gives the best image quality (greatest Signal/Noise ratio) at its lowest possible ISO speed. For most consumer digital cameras, this value will be expressed as ISO 50, ISO 64 or ISO 100. A few digital cameras use ISO 200 as their lowest ISO speed.
• Just as with its film counterpart, an image sensor will exhibit "noise" (comparable to "graininess" in film) at the higher ISO speeds. Unlike film, where graininess can sometimes contribute to the mood of the image, noise produced by an image sensor is undesirable and appears as a motley of distracting colored dots on your image

Histograms:

• Most serious photo-editing programs let you use a histogram as a guide when editing your images. However, since most image corrections can be diagnosed by looking at a histogram, it helps to look at it while still in a position to reshoot the image. It's for this reason that many cameras let you display histograms on the monitor in playback mode or while reviewing an image you have just taken. A few cameras even let you see a histogram as you are composing an image.
• Evaluating Histograms
• Each pixel in an image can be set to any of 256 levels of brightness from pure black (0) to pure white (255) and a histogram graphs which of those levels of brightness are in the image and how they are distributed. The horizontal axis of a histogram represents the range of brightness from 0 (shadows) on the left to 255 (highlights) on the right. Think of it as a line with 256 spaces on which to stack pixels of the same brightness. Since these are the only values that can be captured by the camera, the horizontal line also represents the image's maximum potential tonal range or contrast. The vertical axis represents the number of pixels that have each of the 256 brightness values. The higher the line coming up from the horizontal axis, the more pixels there are at that level of brightness.

To read the histogram, you look at the distribution of pixels. Here are some things to look for.

• Many photos look best when there are some pixels at every position because these images are using the entire tonal range.
• In many images, pixels are grouped together and occupy only a part of the available tonal range. These images lack contrast because the difference between the brightest and darkest areas isn't as great as it could be. However, this can be fixed in your photo-editing program by using commands that spread the pixels so they cover the entire available tonal range. These controls allow you to adjust the shadow, midtone, and highlight areas independently without affecting the other areas of the image. This lets you lighten or darken selected areas of your images without loosing detail. The only pixels that can't be fixed in this way are those that have been "clipped" to pure white or black.

Highlight Warning

One thing you want to avoid is overexposing highlights so they become so bright, or "clipped", they loose details. To help you avoid this many cameras a highlight warning when you review your images. Overexposed areas of the image that have no detail blink or are outlined in color.

There are two kinds of histograms. Most cameras show one of the gray scale brightness levels. A few show display an RGB histogram showing the brightness of each of the three colors, red, green, and blue

Image Stabilization:


 When you move the camera during an exposure, especially at slow shutter speeds, when shooting close-ups, or using a long focal length lens, it causes blur in the image. To reduce this blur, some cameras have image stabilization systems. These systems use a sensor to recognize camera movement and then try to compensate using a variety of techniques. The process goes under a confusing variety of names including image stabilization, vibration reduction, and anti-shake. Manufacturers claim 2 to 3 stop increases before camera shake blurs an image. This means that if you can shoot safely at 1/60 without IS, you can shoot at 1/15 or even slower with it.

• Optical image stabilization works by moving a prism in the camera or an element in the lens to redirect the light path to compensate for the unintended movement. The camera moves one way, and the prism or lens element moves the other. This is the most effective method, but also the most expensive.
• Digital image stabilization shifts the image on the sensor to compensate for motion. When this technique is used, not all of the sensor's pixels can be used for the image. Some of those on the border have to be reserved for the shifting image projected by the lens. Another digital technique is to process the captured image to try to remove the blur.
• Pseudo image stabilization just increases the ISO so the camera can select a faster shutter speed.

When the camera has a fixed lens, it doesn't matter which approach is used. However, on cameras with interchangeable lenses it does matter. If the system resides within the camera body it will work with any lens, if it resides within the lens it only works with special lenses. When thinking about image stabilization, keep in mind that it's always been available in the form of a tripod, monopod, beanbag, or a flat surface on which to rest the camera. You can increase stability by using the self-timer or remote to reduce vibrations.


Exposure Modes - The Camera's Choices:


Digital cameras have various ways of controlling the aperture and shutter speed. All modes give equally good results in the vast majority of photographic situations. However, when you photograph in specific kinds of situations, each of these exposure modes may have certain advantages. Here are modes you may want to look for although it can be complicated by the way camera companies use different names for the same things.

• Automatic mode sets the shutter speed and aperture without your intervention. This mode allows you to shoot without paying attention to settings so you can concentrate on composition and focus.
• Scene modes, which go by a variety of names (Nikon calls them Multi auto programmed modes), have preselected settings for specific situations such as landscapes, portraits, night portraits, sports, and close-up photography. On some cameras the number of these settings has gotten a bit out of hand since there are so many you have to select them from a menu.
• Shutter-priority mode lets you choose the shutter speed you need to freeze or deliberately blur camera or subject movement and the camera automatically sets the aperture to give you a good exposure. You select this mode when the portrayal of motion is most important. For example, when photographing action scenes, such as those encountered by wildlife photographers, sports photographers, and photojournalists, shutter-priority mode might be best. It lets you be sure your shutter speed is fast enough to freeze the action or slow enough to blur it.
• Aperture-priority mode lets you select the aperture needed to obtain the depth of field you want and the exposure system automatically sets the shutter speed to give you a good exposure. You select this mode whenever depth of field is most important. To be sure everything is sharp, as in a landscape, select a small aperture. The same holds true for close-up photography where depth of field is a major concern. To throw the background out of focus so it's less distracting in a portrait, select a large aperture.
• Manual mode lets you select both the shutter speed and the aperture. You normally use this mode only when the other modes can't give you the results you want. Some cameras have a bulb setting in this mode that lets you capture time exposures such as light trails at night. In this mode the shutter remains open as long as you hold down the shutter button. If it's open for more than 1 second, noise in the form of randomly-spaced, brightly-colored pixels may appear in the photograph. To reduce noise at slow shutter speeds, turn on noise reduction if the camera has it.
• Custom settings mode on high-end cameras lets you store personal settings. If you use the same settings over and over again it may be worthwhile saving them for future use. Some cameras let you save one or more sets and then instantly access them at any time just by turning a mode dial. Storing your own settings is as simple as setting the camera the way you want it and then selecting the menu's command that assigns them to the custom setting.