Exposure Controls, Part 3
Shutter Speeds and ISO
In part 2 the standard f-
Standard Shutter Speeds
First of all, the term shutter “speed” is not quite appropriate because what is actually being measured is a period of time rather than how fast something is moving. Nevertheless, the terminology is in place; there’s no use trying to change it now.
The shutter speed is the length of time the shutter remains open during an exposure. It is expressed as a fraction of a second.
Typically, the top part of the fraction, which is always 1, is not mentioned. So when a person says, “I shot that with a shutter speed of 125,” the actual exposure time was 1/125th of a second. With this in mind, the fractions of a second that have become standard are:
1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, 2000, 4000 . . .
To be sure you know what these numbers mean, the first is an exposure of a full second. The next is 1/2 second. Then 1/4 second, 1/8 second, and so on. That is, put a 1 over the top of each number and you will have it.
Note that as you go up the scale, the fraction corresponding to each number is roughly
one half the one that precedes it. For example, 1/500 is only one half the time
interval of 1/250. This is the factor-
Exposure variations: What’s a stop? It is customary to speak of changes or differences
in exposure in terms of “stops,” where one stop refers to changing the exposure by
a factor of 2. (That means either double it, or cut it in half.) Either the shutter
For example, to change the f-
In the simplest terms, a one stop change in exposure is the same as one click of
either the f-
Shutter speed / f-
You can change the f-
Such changes may be made to adjust the shutter speed or f-
To illustrate, using the f/8 at 125 combination given above, you can decrease the
exposure by changing the f-
This photo is a closeup of the dials on the light meter. The f-
OK, I know that nobody carries a light meter any more, but if you switch your digital camera to either shutter or aperture priority, you can make similar adjustments. This gives you control over your final image.
What is “shutter priority?” In this mode, you set the shutter speed to the value
you want to use. The camera then measures the light from the object and sets an
There are limits. If the object you are attempting to photograph is dimly lit and
you set a shutter speed of 1,000 (meaning, one-
What is “aperture priority?” With aperture priority, you set the f-
The limitation associated with this mode arises from camera motion during an exposure
when the shutter speed called for is rather slow. If the camera moves while the
shutter is open, the image will likely be blurred. Your camera is probably programmed
to issue a warning if a “too-
The way to avoid camera motion is to mount the camera on a tripod. However, unless the tripod is heavy (and expensive), this alone may not solve the problem. Chances are you will wiggle the camera as you press the button to take the picture. There are at least two ways to avoid this.
One is to use the self-
Another way is to use a cable release, or its equivalent, an electronic, remotely-
Film and Image Sensor Sensitivity – The ISO Setting
This refers to how much light is required to produce an image on film or to produce
usable data on a digital image sensor. Some films are far more sensitive to light
than others, and not all digital image sensors are equally sensitive. This sensitivity
is the third parameter that must be considered when setting the exposure. The other
two are the f-
Photographers refer to the sensitivity of a film to light as the speed of the film. Films that are very sensitive to light are referred to as being fast films, probably because fast shutter speeds (i.e., short exposure times) can be used with them. Slower films require more light and require longer exposure times. Similar terminology is applied to digital image sensors.
A numerical scale has been devised for specifying film speed, and a particular film, after extensive testing, is assigned a number on the scale. The more sensitive the film, the higher the number assigned. The scale was referred to as the ASA rating a decade or so ago, but it is now known as the ISO rating. ASA stands for American Standards Association; ISO is International Standards Organization.
In theory, the scale ranges from 0 to infinity, but in practical photography, the range is from 6 to 3200. The number 6 was assigned to the original Kodachrome, one of the early films for making color slides. Most films and sensors are assigned a number in the range from 80 to 400.
For film cameras with built-
With digital, the sensor is built into the camera which is programmed to correspond to that sensor. You don’t have to tell it anything; it already knows. However, you can change the ISO rating of many digital cameras, but there may be negative consequences as far as the image quality is concerned.
The scale is not uniform; it has a feature that derives from the factor-
Here’s an example, from years ago. A popular color film had an ASA rating of 100.
A black and white film (Tri-
OK. Color film, ASA 100. Double this, and you get 200. That’s one f-
In practice, on a blue-
Realistically, with digital photography, you can forget about the sensitivity. It’s built into the camera; the camera knows what it has, and it sets the exposures to match.
Putting all this into practice – my method. Even though the preceding explanation is rather long and a bit involved, the practical application is quite simple. When I photograph a completed turning or an object on the lathe, which I do frequently, what I do is ...
Use a tripod. Set the camera to aperture priority and use a high f-
But, unfortunately, there is much more involved in getting a decent picture. The lighting setup is a major consideration and is a big subject in its own right. Also, the color balance (or white balance) on your camera must be set appropriately or your photos will have an ugly color cast. And finally, there is the matter of focusing. Sometimes the auto focusing feature will not work properly, and you will have to set the focus manually.
Taking a decent picture is not a simple proposition, but if you understand the shutter
speeds and f-
Here are a couple of links that have a large number of tutorials on just about every aspect of digital photography: