Breaking the Law

The Inverse Square Law

Some laws just don’t seem fair. Looking into the mirror these days, the Law of Gravity doesn’t seem fair; relentlessly pulling and sagging–I don’t like it. (As the Grant-Lee Phillips songs says, the “fountains of youth run dry”.) One time I was carrying three suitcases down a flight of stairs and my new shoes slipped part way down. The Law of Gravity kicked in and I had to dance the final dozen steps wildly to stay erect before I hit the floor. I did it, and without dropping a bag, no thanks to the long arm of the law.

Another law that’s hard to obey–this one relates to photography–is the Inverse Square Law. Everyone knows that light diminishes in intensity the further away you get from the source. But you might not realize how much. Actually, if you double the distance from from your desk lamp to the page of the book you are reading, the light isn’t just half as bright; it’s only one fourth as bright! That might not seem fair, but that’s the Law.

The Inverse Square Law States:
The intensity of Light Changes in
Inverse Proportion to the
Square of the Distance from the Source.

Nowhere is this more apparent than when attempting to add illumination for the photography of interior spaces. Complicating the photography of rooms is the adaptive quality of human vision. The pupils of our eyes respond so that when you look into a dark corner they enlarge and when you look at a bright window they contract. This happens so quickly that the sensation is a compression of the dynamic range. Unfortunately the camera doesn’t behave this way; the whole scene is viewed with the same fixed aperture.

Because of the Inverse Square Law, to create a natural-appearing interior photograph, it is hardly ever possible to use only the existing lighting. (I told you it wasn’t fair!)

In a brilliantly lit subject on a sunny day, the highlights may be 800 to 1,000 times brighter than the shadows. The ever responsive pupillary sensitivity of the eye allows detail in both areas to be seen. This dynamic response is also beyond the ability of film or digital sensor which both prefer the same amount of light in all circumstances as adjusted by the aperture and shutter duration. (Although with RAW (16 bit) files and High Dynamic Range processing it is now possible to compress more of this information into an image.)

The challenge of interior architectural photography is to balance lighting so the subject can be recorded like the eye sees it. According to the Inverse Square Law, a table lamp close to the camera will be many times brighter than its companion on the other side of the couch. The shadow areas on the opposite side of the room will usually not be bright enough to register at all and will be rendered as black, while light sources will be overexposed and rendered as blank white on the final print.

One photographic technique that will overcome these deficiencies is painting with light. Additional light sources are employed to 1) raise the shadow areas to a level that can be recorded; and 2) add enough light to the farthest recesses of the room to make those areas visible.

Architectural photogaphers compose their images on the ground glass of a view camera, usually 4” x 5” but sometimes larger. The camera is always on a tripod. In the original painting with light technique I learned from the Ansel Adams book, “The Negative”, Adams would set up the camera and open the shutter while, wearing dark clothing, he would walk through the room with a light in a reflector and “paint” the dark areas. Exposure to film is cumulative. Our eyes can only record an image for about 1/16th of a second before being erased from the retina. Film doesn’t forget the exposure and continues to record the actinic energy over time.

This fascinating background is leading up to an interesting challenge I once faced: photographing the neo-Baroque interior of the St. Louis Fox Theatre. This is where I really wanted to break the (Inverse Square) Law, but, like everyone else, had to obey.

The ceiling in the Lobby of the Theatre is 75 ft above the floor, and distances of elements from the camera were immense. In order to get the subject in focus from near to far, required a relatively small aperture: f/16. And the small aperture required a long exposure.

Here is how the image would look with only natural light coming through the windows from the front of the lobby.




My only light source at the time was a portable electronic flash unit. Like many photographers I utilized Polaroid instant film to check exposure and composition. This is one of Polaroid’s most valuable products, instant film technology in a 4” x 5” view camera format. Like the Polaroid camera products, an instant print is processed in 60 seconds, but from the view camera.

Through a process of trial and error, checking with Polaroid film, a lighting plan was created.




The numbers indicated how many flashes were required to achieve proper exposure in each area. 35 to 40 exposures illuminated the general subject area, but a column on the left could only accept 1 exposure. The ceiling required a total of 75 flashes!

Of course this plan of exposure was being made up on the spot, without prior experience in photography such a large space. And clearly the equipment was hopelessly under-powered for the job. But there was a deadline involved: the client needed final enlargements by Friday evening. And we couldn’t start on the project until Wednesday morning!

After exhaustively exposing Polaroid film for hours, it was finally time to repeat the whole lighting procedure on film. Color negative film was chosen, rather than transparency film, because of the greater latitude available. Two final compositions were attempted on Wednesday, both in the lobby. On Thursday the subject was the auditorium where the ceiling was 90 feet above the floor; a vast space with great exposure challenges. Bear in mind that the building had been closed for several years–most of the light bulbs had long burned out. Even getting the camera in focus was a major challenge under these conditions. Nonetheless, two compositions were attempted on Thursday also.

Today it’s possible to simply review an image in a digital camera by bringing it up on the LED screen in the back of the camera. With film the only indication is what shows up on the Polaroid tests. As handy as Polaroid was, it was subject to severe Reciprocity Departure. In this case the Polaroid required at least twice the amount of exposure as color film.

Imagine the uncertainty of working for two solid days on four pictures and not knowing how they would turn out until the day they were due to be delivered. I knew that if I got the film to the lab by Thursday evening, processing and proofing would be available by Friday morning, and the rest of Friday would be used for making final prints.

As it turned out, half the exposures were ruined. I attempted to lock down all aspects of the camera. I locked the shutter open and an assistant held a black card in front of the lens. We synchronized the removal of the card and the flash exposure over and over until the lighting plan was completed. However, no matter how carefully done, there was some accidental camera movement. One lobby picture and one auditorium picture could not be used. The remaining exposures, however, were brilliant. See for yourself:

Fox Theatre, Lobby, 1981
© John Wm Nagel

Fox Theatre, Auditorium, 1981
© John Wm Nagel