• PHOTO INDEX
• BASICS FIRST
• LIGHTING
• MOVEMENT
• VIDEO ART
• CHURCH COW 1
• CHURCH COW 2
• CHURCH COW 3
• CHURCH COW 4
• QUALITY PIX
• SAVE A TRIP
• 2008 JUDGING
• OVER FISHING
• PHOTO FORUMS
• PRESENT 1
• PRESENT 2
• PRESENT 3
• PRESENT 4
• PRESENT 5
• HEROES
• HURRICANES
• PLANET HUG

Videolights

Jim Church's Cows
Part II

by Roger Roth

For many years the late Jim Church shared his expertise with underwater photographers.  One of the best tools he created was using cows to illustrate his points to understanding photography.  These barnyard lessons have been shared on many internet forums so I thought it would be appropriate to pass them on in my Videolights articles for those of you who haven’t read them.  My thanks to Jim for his lessons, his mentorship, and his friendship.

PART III -- THE COWS MUNCH ON FILM SPEEDS

WHAT ARE FAST AND SLOW FILMS AND FILM SPEEDS?

Film speeds are numbers that indicate a particular film's sensitivity to light. The higher the film speed number, the "faster" the film; the lower the film speed number, the "slower" the film. Confused? Just relax; it will soon be easy as chewing your cud. It works like this:

1. A FAST FILM is highly sensitive to light, so less light must be "deposited" on the film for a proper exposure.
2. A SLOW FILM is less sensitive to light, so more light must be "deposited" on the film for a proper exposure.

Stop for a moment and try to guess how the cows are going to explain this one!

BOVINE EXPLANATION

• Suppose the farmer seeded his field with FAST GRASS (fast film).  Fast grass needs fewer "deposits" (of light) to make it grow (expose properly), so fewer cows (light rays) are needed to make the deposits.  To reduce the number of cows making deposits, the farmer can: (a) only open the gate a tiny bit, or (b) only hold it open for a short time. In either case, fewer cows enter the field and make deposits.
• Suppose the farmer seeded his field with SLOW GRASS (slow film). A greater number of deposits are needed to make slow grass grow. To increase the number of cows making deposits, the farmers can (a) open the gate wider, or (b) hold the gate open longer. In either case, more cows enter the field and make deposits.

BACK TO REAL LIFE

• Because a FAST FILM requires a smaller deposit of light for an exposure, we can use a FASTER SHUTTER SPEED (or smaller aperture) to reduce the amount of light deposited on the film.
• Because a SLOW FILM requires a larger deposit of light for an exposure, we can use a SLOWER SHUTTER SPEED (or a wider aperture) to increase the amount of light deposited on film.
• To sum up, a fast film (high ISO number) requires a faster shutter speed (at any given f-stop) for an exposure. A slow film (low ISO number) requires a slower shutter speed at any given f-stop for an exposure.

ASA AND ISO FILM SPEEDS (BOTH USE THE SAME NUMBERS)

A film's sensitivity to light (how much light is required for an exposure) in indicated by numbers included in the name of the film (such Ektachrome 200 or Kodachrome 64). In the following two examples, assume that the f-stop will be unchanged.

1. Each time the film speed number is doubled (such as from 100 to 200), half as much light is needed for an exposure. We say the film is "faster" because a faster (doubled) shutter speed must be used. (Such as 30 to 60, or 60 to 125.)
2. Each time the film speed number is halved ( such as from 200 to 100), twice as much light is needed. We say the film is "slower" because a slower (halved) shutter speed must be used, such as from 125 to 60, or 60 to 30.

FAST AND SLOW SPEEDS--U/W EXAMPLE

Imagine that you are beneath 60 feet of clear, tropical water. Your exposure meter indicates that f5.6 at 1/60 with ISO 100 film will let in enough light for a sunlight exposure. However, If you used ISO 200 film (which requires half as much light), you could use the faster shutter speed of 1/125 second--thus, ISO 200 is a "faster" speed film than ISO 100.  Conversely, ISO 100 film is a "slower" film than ISO 200 because you can slow the shutter speed back to 1/60 second.

FILM SPEEDS AND THE AGE OF THE COW

ASA (American Standards Association) is an older organization; ISO (International Standards Organization) is a newer organization.  Different cows use these film speed ratings (numbers) as follows:

• Old cows often say "ASA" because that's the term they learned in the older books.
• Young cows usually say "ISO" because that's the latest terminology, and they wanna be "hep" (or whatever young cows say).
• Insecure cows use "ASA" and "ISO" interchangeably so they have a 50/50 chance of being correct.
• Smart-ass cows combine the terms as "ASA/ISO," so they are correct all the time. (I learned that from a cow named Dilbert.)

COWS PART IV--PUSHING AND PULLING FILM

After Part III, "The Cows Munch on Film Speeds," some cow-lovers asked them to comment of pushing and pulling film speeds. As this is a completely new topic for the cows, they had to reach to the bottom of their haystack for explanation.

REAL LIFE VERSION (Let's start with sanity):

An ISO 100 film is designed for exposure at ISO, its rated film speed. But what if you wanted to shoot at ISO 200, but only had ISO 100 film?

YOU CAN "PUSH" THE ISO 100 FILM TO ISO 200.

Pushing film is easy:

1. Load the camera with ISO 100 film.
2. Pretend the film is ISO 200 film and expose it at ISO 200.
3. The film will be "pushed up " to ISO 200 by a longer development time during processing.

YOU CAN "PULL" THE FILM BACK TO ISO 50.

Pulling film is easy:

1. You accidentally shot ISO 100 film at ISO 50, but all is not lost!
2. The film can be "pulled back" to ISO 50 by a shorter development time during processing.

REASONS FOR PUSHING AND PULLING FILM

NOTES:

• When pushing or pulling film, the new film speed is called an Exposure Index (EI). It replaces the normal ISO film speed, but who really cares to know this.
• I'm not a darkroom person, but I assume that a custom film lab can make the push/pull adjustment.
• You must tell the film developer what speed (EI) the film was shot at.
• I used ISO 100 film as the base for this example; the same ideas apply to films with other film speeds.

BOVINE VERSION--FIRST THE VOCABULARY:

• Speed = Elmer's normal gait (normal ISO rating).
• Pushed = Elmer speeds up (ISO rating is increased).
• Pulled = Elmer slows down (ISO rating is reduced).
• Gate = the camera shutter (the duration of its opening varies with Elmer's speed, assuming that the width of the opening stays the same.

Elmer, an anxious young bull, always walks at the same speed: 100 steps a minute.  But when he saw Elsie prancing around in the next field, his speed increased to 200 steps a minutes.  He was "PUSHED" by his love for Elsie.  He can now reach her in half the time because he is a "faster" bull.  The gate between the fields can now be opened for only half the normal time to let him pass through.

Suddenly--without warning--Elmer sees Elsie's mother (a big, mean cow from Wanker County)! His pace immediately slows to 50 steps a minute.  He is "PULLED BACK" by his fear of Elsie's mother.  It will now take him twice as much time to reach Elsie because he is now a "slower" bull.  The gate must now be held open for twice as long as normal.

In each of the above bovine situations, the gatekeeper needs to know if Elmer is being "pushed" or "pulled," so he or she knows how long to hold the gate open.

Critter corner: Cowfish bodies are supported by fused plates rather than a skeleton.

copyright © Roger Roth, 2002 - 2011

Roger Roth is a roofer by trade and lives in Cincinnati, Ohio. But his passion is underwater videography and after several decades of learning how to shoot and edit he has evolved into a teacher and a photographic philanthropist. Roger is the founder of the annual international Underwater Images Photo and Video Competition. You may contact Roger at rroth2@cinci.rr.com.