Non Expert Photography for Non Expert Photographers
Questioner: I understand what "speed" means if we're talking about lenses, but I also hear the term "ISO speed"?. What does that mean? What does "ISO" mean?
Dan: Just like the the term "speed" in regard to camera lenses it goes back to the days of film photography. I will use the same "introduction" here that I used to explain speed in regard to lenses. In order to produce an image on film, it has to be exposed to light. As long as film is kept in the dark (literally) it will not be exposed. As soon as light is allowed to touch it, it is exposed. The exposure registers as a lightness on the film. Lightness vs. dark. So unexposed film would be completely dark. Fully exposed film would be completely light or white. Because scenery is composed of areas of light and dark, (shadow, highlights and everything in between) an image will expose different parts of a film differently so the image will be visible.
Because the thing which allowed film to be exposed was a chemical process, the degree of exposure of a particular film would depend on how long it was exposed to light. If it is exposed to light for a very short time period, it will be only lightly exposed. Increase the time it is exposed to light and the greater will be the exposure on the film. In other words, if a film is exposed to the light of a scene for an insufficient amount of time, the image on the film is under exposed. If it is exposed to light for an excessive amount of time, the image will be too bright. It will be over exposed.
How quickly that exposure occurred depended on the film. Some film responded to light quicker. Other film required a longer time of exposure to develop a properly exposed image. Film which responded quickly was called "fast" while film which responded slower was called "slow." The ability of a film to respond quickly or slowly is dependent upon the sensitivity of the film. Film which is more sensitive will respond quicker to light stimulation. Of course the opposite is true as well. Film which is less sensitive, responds slower.
Originally the term was ASA. Over the last several decades it has changed from ASA to ISO. Now that we are more interested in digital photography than film photography, the ISO for our discussion refers to the sensitivity of the digital sensor. The more sensitive the sensor is, the "faster" it registers or "collects" light and thus the more the picture is exposed.
Over time the term was adopted by photographers and lens makers to refer to the ability of a lens to capture, or allow light to pass to the film. So a lens which allowed more light to reach the film was said to be a fast lens. Thus the term "speed".
ISO is rated by number. Generally, 100, 200, 400, 800, 1600, etc. Depending on the camera maker, there may be numbers in between. Though most cameras start at ISO 100, some start at ISO 80. The more advanced cameras can operate at even higher ISO numbers
With film photography, film was made to have only one sensitivity. Film ISO came in 64, 80, 100, 200, 400, etc. If you selected ISO 100 for a particular photography project, in order to change ISO, you would have to switch to a film of the desired ISO number. With digital, you switch ISO right on the camera and you can change the ISO number any time you need to.
Ok, now that you have asked, ISO stands for International Standards Organization. Since film and digital photography are not the same technologies, the ISO numbers used in digital photography are "equivalents" to those in film photography. If you are shooting a scene with two cameras, one digital and one film, and you set both to the same ISO settings, the same exposure settings on each camera will result in the pictures from each camera being properly exposed. Actually, it may not be 100% the same but close enough to consider the numbers to be equivalent.
We select the desired ISO setting based on the light conditions. If the light conditions (let's say a bright, sunny day outdoors) are light, we can use a low ISO number. If the day is gray and cloudy, we might find that we need to move to a higher ISO number. If we are shooting in a much lower light environment, we will need to go to an even higher ISO number.
Below are three graphs which show exposure settings at three different ISO settings. The subject was the same for all three charts and the lighting was the same. The only thing different was the ISO setting selected. To understand how the graph works remember that the point of any metering system in any camera is to determine what the lighting must be for proper exposure to result. I said before that lens opening size (aperture) and the length of time the shutter is allowed to be open (shutter speed) determine how much light reaches the sensor. Those two items are adjusted to allow the light collected by the sensor for proper exposure. If the Aperture is set to f/4 by the photographer, the metering will set the shutter speed in the ISO 100 graph below to 1/30 of a second. Follow the other colored plot lines for other aperture settings. Or, if the shutter speed is set by the user instead of the aperture, the metering system will set the aperture to the correct opening for proper exposure.
Notice the shutter speeds in the ISO 100 graph. They are rather on the long side. The average photographer can hand hold a camera steady enough to get unblured shots at about 1/30 of a second or faster. Anything slower (further down the shutter speed column on the graph) and it becomes difficult to hold the camera steady enough to avoid a blurred image. I have often gotten very good shots hand held at down to 1/8 of a second but these were with a wide angle lens (shorter focal length) and by being very careful to time the shutter release with my breathing rhythm and to be very gentle on the shutter squeeze. Those shots were under conditions where flash was either not allowed or would not have been effective and I did not have my tripod handy. The point is, though low shutter speed shots can be made and turn out ok, you do not want to be trying that on your average family photo outings or your grandson blowing out his birthday cake candles.
If we increase the ISO speed, we gain shutter speed and thus can do better in lower light conditions. Notice in the ISO 200 graph that we're practically doubling the speed of the shutter meaning that the shutter is only going to be open about half the time during the shot compared to the setting of ISO 100.
If we increase the ISO to 400, we double the speed again. This shows us that under lower light condition where it would be difficult to get a steady, motion free shot at ISO 100, we can get it quite easily at ISO 400.
We may also want to use a higher ISO number if we are shooting action shots such as at a sporting event and we want to use a higher shutter speed to "stop the action" or to minimize blur caused by subject movement.
Now the question will be, why not always use a higher ISO setting? That is a very good question and with modern digital SLR cameras, it is possible to produce high quality pictures at higher ISO settings. Not too high though. With an increase in ISO sensitivity comes an increase in picture noise. By "noise" I don't mean something that you hear by ear, but artifact and graininess within the picture itself. The higher the ISO setting, the more the noise will be visible. The better the camera, generally, the better it handles noise so the pictures from such cameras may not show as much noise.
Here is an example of a picture I shot one night at ISO 800. If I were to show you the whole picture, it would be hard to see the noise in it. I blew it up (in my photo editing program) so that it is "zoomed in" to a small part of the whole picture. That process magnifies the visibility of the noise making it more obvious to the viewer. It is true that many photographs which are not going to be zoomed in much or not blown up to larger sizes for printing will not show the noise much and it can be overlooked.
Not all of the graininess in this picture can be blamed on the value I selected for the ISO number. The picture was shot at night time and the shutter speed was a full 16 seconds. Yes, the camera was mounted on a tripod or it would have been impossibly blurred.
Now let me offer a caution. Though a good SLR camera can shoot pictures at higher ISO numbers and be fine, that is not the case with compact point and shoot cameras. I shoot more than half of my pictures at ISO 400 and no grain is visible. If a point and shoot camera were used for the same shots and it was set to ISO 400, the grain would be glaringly visible. So my suggestion is to avoid setting the ISO number for compact cameras. On any non-SLR camera for that matter.
One more word of caution. All SLR cameras have in addition to the selections of individual ISO number settings, a position called "Auto ISO". I really want to caution you to NEVER use that position. Though it seems like it would be a help in that if the light conditions are poor, the camera will automatically select a higher ISO setting for you. The problem is, you don't know what ISO setting it will select and it may select a value above what will produce a noise free picture. It is much better to learn how to use the controls your camera provides and keep control of the picture settings. Other pages on this site explain the use of those controls and how best to use them.
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