Main Page

Back     Contents     Glossary     Index     Forward

Chapter 2


The Aperture and Shutter Speed


The Aperture Or f-stop

The aperture had been known to strike terror in the hearts of all but the most stalwart shutterbug. All those little weird numbers, they must mean something to somebody.

Well, here is a surprise, because f-stops are really quite simple. In fact the f-stop isn't a thing at all. It's a relationship between the focal length of the lens and the lens diameter. As the lens diameter changes the
f-stop changes.

The f-stop, or aperture, controls the intensity of the light reaching the film. It does this by acting like a window blind.

Open the aperture and more light passes through the lens. Close the aperture and less light passes through. It is just that simple.

Let's talk about window blinds. Blinds admit more light when open of maximum than when open 1/4 of maximum, in this same way, f2, is larger than f4. Yes, f2 means that the aperture is one-half the focal length, f4 means that the aperture is one-fourth of the focal length.

In terms we all understand, dollars and cents, one-half dollar more than one-quarter of a dollar. So, f2 is larger than f4.

It really is just that simple. A half dollar is more than a quarter of a dollar, amazing how simple it is when we relate it to money, is it not?

The adjustment for the f-stop is located on the lens barrel. It will be marked with numbers such as:

1.2, 1.4, 1.7, 1.8, 2.0, 2.8, 3.5, 4, 5.6, 8, 11, 16, 22, 32, etceteras.


An f-stop Is Born

Back in the days when lenses were first being developed the technology to make a variable aperture did not exist. To provide a large, small, or in-between circular opening for light to pass through the lens makers did the only thing they could. They took a metal plate and drilled a hole in it, in fact, they made a series of these plates each with a different size hole.

The size of the drilled hole was calculated so that when changed it either let in twice the light, if the hole was larger, or one-half the light, if the hole was smaller.

The lens was built so these pieces of metal could be easily inserted and removed.

When photographers of the day wanted to admit more light they chose a plate with a large opening.

To reduce the light, a plate with a smaller hole was inserted. These plates became known as f-stops, because they stopped more and more light from entering the camera as smaller and smaller openings were chosen.

The name stuck, to this day we refer to "stopping down" or "opening up" our lenses.

Remember the aperture cuts the amount of light in half when closed one stop. The amount of light is doubled when the aperture is made larger by one stop.

This is very important, because the shutter is designed to do the same thing. Except that the shutter changes the time the light is allowed to be in contact with the film.

The Shutter, What Is It?

The shutter is the device that allows light to enter the camera and expose the film. The shutter is designed to remain open for a range of different amounts of time. The time that the shutter is open is referred to as shutter speed and is discussed below. Remember that the shutter speed is how long the shutter is open. Shutter speed does not refer to how fast the shutter is actually moving.

On the 35mm camera the shutter is composed of two curtains that travel across the film plane. In fact, some people refer to the shutter as the curtain. Because of this type of construction there are times when the shutter does not expose the entire negative or slide all at once. Rather, a moving slit travels across the film. This is why the camera has a flash synchronization speed.

A little digression about shutters might be a good idea. Think of watching a play performed on stage. Let's imagine the stage curtain is in made of two parts.

The left half and the right half. With the right half fully open and the left half fully closed only the left curtain is covering the entire stage. The right curtain is there but on the right side of the stage and covered by the left curtain.

Now open the left curtain to see the entire stage.

Now pull the right curtain across the stage and see the right curtain covering the entire stage.

This is what the focal plane shutter does in the camera.

At high speed the left curtain starts moving across the film-stage, but before it, the left curtain, reaches the left side of the stage the right curtain starts moving.

So, at high speed thee is a slit traveling across the stage-film?

. Getting back to flash sync speed. This can be seen by opening the camera back and shooting the flash against a wall while looking at the shutter. With shutter speeds faster than the flash synchronization speed the shutter window will plainly display only a partial image.

When taking pictures at the flash synchronization speed the shutter will uncover the entire film plane at the same time. When the entire film plane is uncovered the flash fires. This technique ensures the the film, then entire 24mm x 36mm film, receives the action of the flash.

How do determine the flash synchronization speed? It will be found on the shutter speed dial in a different color from the others.


Shutter Speed

This is somewhat of a misnomer. Shutter speed refers to the length of time that the shutter is open, not how fast the shutter is moving.

While the shutter is open light is exposing the film in the same way that light touching skin results in a tan.

For instance, a shutter speed of one second means that the shutter will be open for one full second. Shutter speeds are designed to double, or reduce by half, the light passing through the open shutter.

Time, in seconds, is represented by the number on the Shutter Speed dial.
Time (seconds) Number on shutter speed dial
B (Unlimited) Bulb keeps the shutter open as long as the shutter release is held open.
T (Unlimited)Keeps the shutter open until the shutter speed dial is turned off the Time setting.
88 (different color on dial)
44 (different color on dial)
22 (different color on dial)
11
1/2 2
1/4 4
1/8 8
1/15 15
1/30 30
1/60 60
1/125 125
1/250 250
1/500 500
1/1000 1000
1/2000 2000
1/4000 4000
1/8000 8000

BULB will keep the shutter open as long as the shutter release is held down. This term is a holdover from the days when a pneumatic air release was used to operate the shutter. It refers to a relatively brief exposure, but can be used for a time exposure as well.

TIME will keep the shutter open until the shutter speed dial is turned off the T setting. This is used when a long exposure time is needed. This setting is not common to all cameras.

Look at the shutter speed data in the chart, to see the length of time become shorter and shorter when one looks from top to bottom.

From bottom to top the time becomes longer.

Look at 1 second and second. One second is twice second. This same relationship exists through the entire range of shutter speeds. For example: 1/30 second is twice 1/60 and 1/60 is half of a 1/30.

This can be confusing. Lets talk dollars and cents. Starting at one dollar, next is one-half dollar, then 1/4 dollar, 1/8 dollar, 1/15 dollar. Just keep reducing by one-half.

When doubling (which is much more fun) 1/4 dollar (25 cents) to obtain a dollar (50 cents). Then double the dollar to obtain one dollar. See how easy it is when we talk money.

Using Shutter Speed

There was once an assignment given to a student in New York City: Photograph Grand Central Station without any people in the photograph. Well, Grand Central Station is never empty, so how was the assignment completed?

By using a very long exposure. In this way, as the people come and go, no one will be in any one location for a long enough period of time to register on the film.

And that is just what the student did. He put the camera on a tripod, blocked the legs, and opened the shutter. After eight or ten hours, he closed the shutter, collected his equipment, and developed the film.

What did our photographer have? A picture of Grand Central Station without people? Yes, that is just what he had and something more. A transparent suit case. Someone had put it down, left it for quite some time, and then picked it up and took it away.

To use the bulb setting for an extended length of time just lock the shutter open with a cable release.

Knowledge is power.

Knowledge is creativity.

Knowledge is never wasted.

Shutter speed and aperture control exposure. In fact, they are exposure.





Now go take great pictures.


Back     Contents     Glossary      Index     Forward

 Chapter 1              Main Page              Chapter 3