Join Date: Dec 2005
Location: CT, USA
I could have typed out my own explanation but easier to use sources from others. From multiple sources put together. Higher ISOs are typically used for night shots but with higher isos come increased graniness or noise. Flash is ok but it only works in a set range and can create harsh shadows. Bumping up the exposure time will help but will require a tripod.
In photography, the metering mode refers to the way in which a camera determines the correct exposure.
Cameras generally allow the user to select between spot, center-weighted average, or matrix metering modes.
An image taken with spot metering. Notice how the flame is correctly exposed, while the background has become completely black.
With spot metering the camera will only take a tiny spot of the scene into account when calculating the exposure. This will typically be the very center of the scene, but some cameras allow the user to select a different, off-center spot, or to recompose by moving the camera after metering. Some cameras, including the Olympus OM-4 and Canon T90, support a Multi-Spot mode, allowing multiple spot meter readings to be taken of a scene, which are averaged. Both of those cameras, and others, also support metering of highlight and shadow areas.
Spot metering is useful when the scene consists of objects with varying brightness (high contrast). For example, if your subject's back is being hit by the rising sun and the face is a lot darker than the bright halo around their back and hairline. This is called being backlit. Spot metering allows the photographer to measure the light bouncing off of the subject's face and expose properly for that instead of the much brighter hairline and a dark face. The area around the subject's back and hairline will then become over-exposed, but this is usually considered a decent natural highlighting effect, or if that part is to be removed afterwards anyway, then the overexposure is not a problem either way.
Spot metering is a method upon which the zone system depends.
Center-weighted average metering
In this metering mode the camera will use the light information coming from the entire scene, but will give the light in the center a higher priority. This basically does what one would expect: the camera will ensure that the entire scene is correctly exposed, while taking extra care of the exposure in the center, where one would expect the most interesting subjects to be.
Partial area metering
Some cameras support metering of just the central area of the frame. This mode meters a wider area than spot metering, and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly.
This mode is also called multi metering mode on some cameras. This metering mode was first introduced by the Nikon FA, where it was called Automatic Multi-Pattern metering. Here the camera measures the light intensity in several points in the scene (more expensive cameras generally measure in more points) and then combine the results to find the settings for the best exposure. How they are combined varies from camera to camera. Many cameras use focus distance from the lens (if that is available) or data from the autofocus system to help with this decision. More advanced cameras with many autofocus points will know what parts of the matrix of metering zones are in focus and thus likely to be part of the main subject. Such metering is sometimes called 3D Matrix Metering.
On cameras where one finds this setting, it is supposed to give the best general overall result, and is thus the default setting. However, some photographers, including some advanced photographers and some older photographers who are accustomed to more traditional metering methods, may be uncomfortable with matrix metering, as it is not always easy to predict how it meters a given scene.
Cameras depend on light to capture an image, and they use a built-in exposure meter to measure the average brightness of the light within the lens' field of view. The typical exposure meter "sees" the world as medium-gray. The assumption is that most subjects are of average tone and that they reflect an average amount of light. This is perfectly appropriate for most situations, but there will be times when you'll need to compensate for your subject, for the conditions you're shooting in (weather, time of day, etc.), or both.
Compensation is accomplished by adjusting the camera's exposure value (EV). Most digital cameras allow you to adjust EV up or down by two full f/stops, in one-third increments. If you were to take a photograph of a snow-filled meadow, for example, the middle-gray EV calibration would make the snow in the photo appear gray. If you increase the camera's EV setting before snapping the shot, the snow in the photo will appear more naturally white.
Another common situation is when your subject is lit from behind-a person standing in front a brightly lit window, for instance. By default, the camera will set its exposure value based on the bright light, leaving your subject in silhouette. Increasing the EV setting in this situation will bring your subject out of the shadows.
When there just isn't enough natural light to properly expose your shot, turn to your camera's flash.
If you're shooting a very dark subject, on the other hand, you'll want to decrease the camera's EV setting; otherwise, the medium-gray calibration will render the subject too dark.
Exposure Lock is another trick you can use to compensate for troublesome lighting situations. If the sun is in the shot you're composing, and shooting in auto-exposure mode will result in the subject being too dark, recompose the shot so that the sun is not in the frame, turn on the camera's exposure lock, recompose the shot a third time (with the sun back in the frame), and snap the picture. Your subject will be exposed appropriately even with the presence of the sun in the frame.
ISO sensitivity expresses the speed of photographic negative materials (formerly expressed as ASA).
Since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given.
What 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 combinations you can use to obtain correct exposure.
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.
See, 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. However, it is also true now that it will record fainter noise, where noise is any signal that is not attributed to the light from your subject. Remember that an image sensor is still an analog device and it generates its own noise, too! The increased sensitivity allows the image sensor to record more light signal and more noise. The ratio of light signal to noise (S/N ratio) determines the "noise" in your resultant image.
An image sensor is usually calibrated so that it gives the best image quality (greatest S/N 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 coloured dots on your image.
Last edited by Neil; 05-22-2006 at 04:12 PM.