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by Jim Doty, Jr.


Photographically speaking one stop (or step) means either twice as much or half as much light.  Stops apply to film speed, apertures and shutter speeds.

Some typical film speeds in one stop increments (slowest to fastest):  25  50  100  200  400  800  1600

 A 200 speed film absorbs light twice as fast as a 100 speed film.  There are in between film speeds in 1/3 stop increments.  Example: Kodachrome 64 is 1/3 stop faster than a 50 speed film.

Cameras and lenses control the amount of light that reaches the film with shutter speeds and lens apertures.  Longer shutter speeds mean more light.  Wider lens apertures mean more light. 

Apertures in one stop increments from widest (most light) to smallest (least light):

f/1.4   f/2.0   f/2.8   f/4.0   f/5.6   f/8.0   f/11   f/16   f/22

Most lenses have half stop settings and a few can be set in 1/3 stop increments.

Shutter speeds in one stop increments from fastest (least light) to slowest (most light):

1/1000   1/500   1/250   1/125   1/60   1/30   1/15   1/8   ¼   ½   1 sec   2 sec   4 sec   8 sec

There are both faster and longer shutter speeds. Some cameras have half stop shutter speeds and a few can be set in 1/3 stop increments.


If too little light hits the film, the slide will be too dark; if too much light hits the film, the slide will be too light.  Light meters are a guide to the combination of apertures and shutter speeds to use to get the right amount of light on the film.  The exposure is right if the tones on the slide are as light or as dark as you want them to be.  This is limited by both film latitude and and subject tonality.

Film latitude is the extent of light and dark tones that a given film can record.  The human eye can see a variety of light and dark tones, in both sun and shade, all at the same time.    Film can't do this, it can't record everything from white to black in the same light, much less in different light levels at the same time.

Photographic subjects vary in tonality in the amount of light they reflect.  If we call something halfway between black and white "medium toned," then other subjects will be lighter or darker by as much as several stops. Some examples:

 Subject        Difference from "Medium Toned" in Stops

 White sand                      +2
 Birch bark                        +1½
 Yellow aspen leaves         +1
 Grass                                0
 Dark green evergreens       -1
 Buffalo mane                    -1½


Camera meters are designed to make everything they meter look medium toned.  If you meter white snow in bright sunlight, the camera meter will try to make the snow medium toned.  If you meter a gorilla, it will try and make the gorilla medium toned.  The photographer must compensate from camera meter readings for light and dark toned subjects.

If your subject is light toned, you must add light to the camera meter reading to keep the subject light toned on film.  Remember this phrase:  ADD LIGHT TO MAKE LIGHT.  For light toned subjects, you must use a wider aperture or longer shutter speed (or both) to keep your subject light.

For dark toned subjects, you must take away light to keep the subject dark, this means using a smaller aperture or a shorter shutter speed (or both).

How much compensation?  This comes with experience, but the following table should help.



                          Very Light             + 2 stops
                          Light                    + 1
                          Medium                     0
                          Dark                      - ½
                          Very Dark               - 1

When metering the subject, make sure you are metering only the subject.  If you have a tiny white flower against a background of dark green evergreen leaves, the camera may be metering the dark background more than the subject.

What if you can't get close enough to meter the subject?  Use substitute metering.  Meter something the same tone as the subject and in the same light as the subject.

What if you aren't sure of the subject tonality?  Buy a photographic gray card.  A gray card is medium toned.   You can also use the gray card for substitute metering.  Meter the gray card in the same light as the subject and set your camera accordingly.

Even with a gray card, you have to allow for differences in subject tonality.  With a gray card reading, a white subject may end up washed out and a very dark subject may lack tonality.  Try these recommendations:


   Subject Tonality  Compensation from Gray Card Reading

              Very Light                        - ½
              Light                                  0
              Medium                               0
              Dark                                + ½
              Very Dark                         + 1

Don't get confused, metering a gray card and metering the subject are different and are the opposite in compensation.  With slide film, practice with both kinds of metering until you become comfortable.

Bracket your exposures from one stop more to one stop less than the one you think is the right exposure.


Reciprocity means a number of different combinations of apertures and shutter speeds would give the same exposure.  By using one stop more light of aperture, you can use one stop less light of shutter speed and get the same exposure.  If you determined that f/22 at 1/4 second was the right exposure for your subject, all of these combinations would give the same exposure (assuming the amount of light didn't change):

  f/22     1/4
  f/16     1/8
  f/11     1/15
  f/8       1/30
  f/5.6    1/60
  f/4       1/125
  f/2.8    1/250
  f/2.0    1/500

Reciprocity works until you get to shutter speeds around 1 second and longer.  At this point, most films begin to lose their ability to respond to light and require more exposure.  This is called "reciprocity failure".  You can still take pictures (and should), just allow more time or a wide aperture than the meter indicates.  Films differ, but as a general guideline, add ½ stop at 1, 2 or 4 seconds, and add a full stop for  8 seconds or longer.


There are some conditions in which you can estimate exposure.  A subject in bright sunlight is one of these situations.


BDE, also called the FIG rule, or sunny f/16 rule, is as simple as this:

For a frontlit subject in bright sun, the exposure is f/16 and a shutter speed of 1/film speed.

With 100 speed slide film, the BDE is f/16 at 1/125 second.  With Kodachrome 25, BDE is f/16 at 1/30 second.

For a sidelit subject in full sun, add 1 stop of light, for a backlit subject in full sunlight, add two stops of light.

For a white subject, subtract ½ to 1 stop of light (to keep it from washing out).  For a very dark subject, add one stop of light (to maintain detail).

Equivalents can be used.  For a frontlit subject with 100 speed film, instead of f/16 at 1/125, you could use f/11 at 1/250 or f/8 at 1/500.


For conditions other than full sunlight, there are a variety of exposure guides available.  Most film boxes have one inside.  One very complete guide is the BLACK CAT EXPOSURE GUIDE.  It is available from several sources, including L. L. Rue (info: 908-362-6616, orders: 800-734-2568, web site: www.rue.com).

This text is copyrighted by the author. And may not be reproduced without permission.

June 14, 2000


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