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Exposure Controls, Part 3

Shutter Speeds and ISO

In part 2 the standard f-numbers were established based on the factor-of-2 principle.  In this section, we see that a similar thing was done for shutter speeds and the ISO scale for rating film and sensor sensitivity.  

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-of-2 phenomenon coming to the front again.

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 speed, f-number, or both may be changed.  

For example, to change the f-number setting from f/8 to f/11 decreases the exposure by one stop.  Going from f/8 to f/16 is a two-stop reduction.  Changing the shutter speed from 250 to 125 is a one stop increase in exposure.  

In the simplest terms, a one stop change in exposure is the same as one click of either the f-number or shutter speed setting.  Two stops equate to two clicks, and so forth.

Shutter speed / f-stop reciprocity.  If a light meter indicates that the proper exposure for a certain subject is f/8 at 125, you are not locked in to those particular choices for the f-number and shutter speed.  You can use an equivalent exposure, which is a different combination of f-number and shutter that lets the same amount of light into the camera.

You can change the f-number, for example, if you make an equal change in the shutter speed in the opposite direction.  Or you can change the shutter speed, but you must compensate by changing the f-number.  This is the “reciprocity” feature; it works both ways.

Such changes may be made to adjust the shutter speed or f-stop to one that better suits your preferences.  For example, shooting at a high shutter speed tends to stop motion.  A low shutter speed may produce a blurred image and convey the illusion of movement or speed.  Shooting at a high f-number increases the depth of field.  

To illustrate, using the f/8 at 125 combination given above, you can decrease the exposure by changing the f-number from f/8 to f/11, but then you must increase it by changing the shutter speed to 60.  Each click amounts to a one stop change, in opposite directions.  The result, f/11 at 60, is an equivalent of f/8 at 125.   

This photo is a closeup of the dials on the light meter.  The f-stop and shutter speed rings are movable, and the exposure f/8 at 125 is set on the dials (highlighted). The combination indicated by the arrow is an equivalent exposure.  Others can be read directly, such as f/5.6 at 250.

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 aperture (f-number) that will give the proper exposure.  

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-thousandth of a second), the camera may not have the capability to set an f-number low enough to achieve an acceptable exposure. In this case, the camera will warn you in some manner that you are out of bounds, so to speak.

What is “aperture priority?”  With aperture priority, you set the f-number you want the camera to use to make the shot.  It then measures the light and selects a shutter speed to give the proper exposure.

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-slow” shutter speed is called for.

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-timer feature that almost all cameras have. With the timer set, the camera delays making the exposure for about 10 seconds or so after the button is pressed.  So what you do is set up the shot, press the button, and then don’t touch anything.  When it times out, it will take the picture with no movement at all.

Another way is to use a cable release, or its equivalent, an electronic, remotely-operated trigger for the shutter.  The advantage of this method is that you will know, and can control, exactly when the exposure will be made.

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-stop and shutter speed, of course.

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-in light meters, the ISO number for the film being used is set by turning a knob to the appropriate position.  This is how the camera knows how much light is required for the film.  It can then determine the f-stop and shutter speed to give the proper exposure.  

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-of-two phenomenon mentioned earlier.  That is, if you double the ISO number, the required exposure is one f-stop less. If you cut the ISO number in half, one f-stop greater exposure is required.  

Here is a graphic representation of (part of) the ISO scale.  It is constructed so that a one stop difference in exposure represents the same distance anywhere on the scale.

Here’s an example, from years ago.  A popular color film had an ASA rating of 100.  A  black and white film (Tri-X pan) had an ASA of 400.  The question is, how does the required exposure for the Tri-X compare with that of the color film?  

OK.  Color film, ASA 100.  Double this, and you get 200.  That’s one f-stop difference.  Double it again and you get 400.  That’s another stop.  Therefore, the Tri-X is two stops faster than the color film.  

In practice, on a blue-sky sunny day, the exposure for the color film might be f/11 at 250.  The exposure required for the Tri-X under the same conditions would then be f/16 at 500, or an equivalent.  That is, we changed the aperture by one stop and also the shutter speed by one stop, reducing the exposure in both cases.  

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-number, say f/11, for maximum depth of field.  Use the self timer to trip the shutter to avoid camera motion.  And that’s it as far as the exposure is concerned.  With my lighting setup (cheap halogen work lights), a typical exposure time is a low fraction of a second, like 1/2, or 1/4 second.  

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-stops, you will be a bit closer to the final objective.   

Here are a couple of links that have a large number of tutorials on just about every aspect of digital photography:


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