Capturing Light: The Science of Photography

In this activity, you will:

• Explain how cameras focuses light to create reverse images.
• Describe the difference between negatives and positives in photography and learn why negatives are necessary.
• Draw, to experiment with inverting light and dark like a photographic negative.

• Multiple Parts

• Pencils
• Tracing paper
• Printed copy of Nagina Mosque, Agra Fort, India

Look Closely at the Image of Nagina Mosque, Agra Fort, India

Spend two minutes just looking at its details.

• What do you notice about the tones in this image?
• Does anything feel “wrong” to you about the tones in this image?
• Where do you expect to see light and dark, and where do you actually see them?
• Does this look like a “normal” photograph? Why or why not?

Read About How a Camera Captures Light

A camera is essentially a darkened box with a tiny covered opening for light. When the opening is uncovered for a few seconds, light reflected off objects in the world travels through the lens and strikes a light-sensitive surface inside the camera. In the era that this image was made, paper or glass coated with silver salts were put inside cameras to capture the image. The silver salts reacted chemically when struck by light: the more light that hits a spot, the darker that spot becomes after chemical development. Cameras today use a digital sensor to record the same information electronically.

The result is a negative. Every tone is reversed—brilliant whites become dense blacks; deep shadows become clear or pale areas. To create a photograph that looks the way our eyes perceive the world, you have to reverse it again. You do this by shining light through the negative onto another light-sensitive surface. The dense (dark) areas of the negative block light and produce white or light tones in the print. The clear (pale) areas let light through and produce dark tones. Two reversals cancel each other out, and the world appears as we recognize it. So when you look at this negative, the paper or glass coating darkened wherever light struck it most intensely. The sunny sky flooded the emulsion with light, turning it dark. The shadowed interior of the building barely exposed it, leaving it pale.

Experiment with Reversing Light

Make a drawing that plays with reversing light in the same way that a camera negative reverses lights and darks.

First, look at the image and find the parts that seem brightest and the parts that seem darkest.

Then, place your tracing paper over the printed image and trace the outlines of the major shapes. Include arches, columns, doorways, the sky, and patches of wall. Don’t worry about shading yet. You’re just mapping where things are. Press lightly so you can see through to the negative underneath while you work.

The Inversion Rule

Here is the only rule you need for the next step, and it applies everywhere without exception:

• Wherever the negative is dark → you shade lightly.
• Wherever the negative is pale or white → you shade darkly.
• Wherever the negative is a middle tone → you shade with a middle tone.

Every decision you make will follow this rule. You are doing by hand exactly what happens when light passes through a negative onto printing paper. Dark areas of the negative block light and produce light tones in the print. Pale areas let light through and produce dark tones.

Work Through the Image Section by Section

Start with the sky, since it covers a large area and will give you immediate feedback on whether the rule is working. In Murray’s negative the sky is very dark, nearly black. Following the Inversion Rule, you should fill it in very lightly, or leave it almost blank. When you’re done, step back and check: does the sky now look like a sky?

Next, move to the architectural elements (the walls, arches, and doorways) and apply the same rule to each area. Take your time. For each section, ask yourself: is this area in the negative closer to black, white, or somewhere in the middle? Then apply the opposite shade with your pencil. Don’t rush through middle-toned areas. If a region is a medium gray in the negative, it should be a medium gray in your drawing too. The inversion of a middle tone is still a middle tone.

Check Your Shading

When you’ve filled in most of the image, hold your tracing paper up and look at it as a whole. You may notice that some areas look correctly inverted while others still look like the negative. Go back to any patches that seem off and ask: did I accidentally shade this in the same way the negative does, instead of following the Inversion Rule?

This is harder than it sounds. Your eye naturally wants to shade dark things darkly. You have to resist that.

Compare Your Drawing to the Negative

Place your finished tracing next to the original printed negative. You should now have two images of the same scene that look almost like opposites of each other. One is a record of how light struck Murray’s paper in the camera. The other is what his audience would have seen in a finished print.

Try the Inversion Again!

Take a fresh sheet of tracing paper and place it over your finished drawing, not over the original negative, but over the drawing you just made.

Trace the outlines again and apply the same inversion rule to your drawing. When you’re done, compare this second tracing to Murray’s original negative.

Reflect on Your Experiment

Compare your two drawings with Murray’s negative. Identify similarities and differences.

Then, write or discuss your responses to the following questions.

• The sky in Murray’s negative is very dark, even though the sky when he took the photo was bright. What do you think caused this?
• The mosque’s interior doorway was dim and shadowy, but appears pale in the negative. How does this fit with your answer to the question above?
• Write an if/then rule describing the relationship between how much light hits a spot on the negative and how dark that spot becomes.
• One student says the negative is “a mistake—everything is backwards.” Another says it’s “perfectly accurate, just recording something different.” Which do you agree with, and why?
• Why did the white marble walls reflect more light toward the camera than the dark interior of a doorway?