Labs from YSI 93 :
Camera Obscura and Filter.

Dr. Jim Sweitzer
CARA Yerkes Summer Institute, August 1993

This is meant to be given to the students.


You have done several labs about the nature of light on campus in Chicago. Two things that should be somewhat familiar to you now are simple optics (focal length, aperture, and F ratios) and that the intensity of light from a star is governed by the Inverse Square Law. Now, up at Yerkes Observatory, we are going to start putting the pieces together. One of our first tasks will be to understand how photography works. In this lab you will do that and then you will take a picture of the Sun that will be very handy when you shoot some pictures of stars.

To accomplish this, we'll do a three-part lab today. First, you will learn how light can be collected for a period of time to make an image. Then, you will actually get inside a camera of sorts, to see how the optics effect image size and brightness. Finally, we will use the simplest of cameras to take a picture of the Sun. This picture will be used to compare with a picture of the Milky Way you will take at night during the course of the week. We will also test a filter that will be used in an evening observing project.

Part 1: Exposure Time

To take a photograph, you must have some way of recording light on a flat surface. In a normal camera you do this on film, which is a special chemical mixture smeared on a piece of thin plastic. At the observatory this week you may see photographs in which the surface is a piece of glass.

The most important ingredient is the chemical mixture. Recall that the Sun's light can be measured as an intensity, which is simply the energy per second per square cm falling on the surface you are using. Now, these chemicals are sensitive to sunlight. This is because the energy in the light changes the structure of certain chemicals. It's the total energy that strikes the chemicals that counts. And since the intensity of light per square cm is in units of energy per second, that means that the total amount of energy deposited in a one-square- centimeter layer of chemicals is proportional to the time the layer is exposed.

[Total Energy]=[Energy/Second]X[Seconds Exposed]

To see how this works, take three pieces of Nature Print Paper and try the following experiments with a partner. (Nature Print Paper is a light sensitive paper that is very simple to use. Just expose it for a few minutes, then you can "develop" it under water. Though not the same chemical reaction, it works in a very similar way to normal film.)

1. Put a piece of white paper on the Nature Print Paper. Put it in direct sunlight. Then, slide the piece of white paper down to expose one inch of the Nature Print Paper. Wait exactly one minute, then move it again -- just one inch. Keep doing this for five minutes. By the end you should have five stripes across the paper, each exposed for progressively longer periods of time. When the five minutes are up, immediately cover up the paper to protect it from the Sun, then soak it in a container of water for one minute to fix the images.

a. Describe what the paper looks like:

b. Which length of time seems to give the most pleasing exposure?

2. Next, find some object -- a leaf or a key would work best -- something with a sharp outline. Lay it directly on the piece of paper and expose to the sunlight for 3 minutes. Cover the paper and develop it like you did before. Next, use the third piece of paper, but this time hold the object a couple of inches above the paper. Expose it for the same amount of time as before and develop it the same way. Then, answer these questions:

a. Which way gives the sharpest image?

b. Why isn't this a good way to do photography?

Part 2: Forming an Image

Now, to see how we can form an image that we can then record, let's go inside the observatory to the specially darkened room. Did you know that the word "camera" is simply the Latin (I think. I know it is in Italian) for room? A "camera obscura" is simply a dark room. That's what it is like inside your camera, even though you probably never thought of it that way before.

The first demonstration shows how we can form an image on the wall using only a small hole about 1.5 cm in diameter. In the picture below, trace the rays of light from a tree through the camera obscura's pinhole to show how the image is formed on the back of the camera obscura. (Remember light from the tree center, top, and bottom travels in a straight line through the pinhole to the wall. Draw all three line to the wall, then draw in the image of the tree.)

a. Is the image right-side up?

b. Can your eyes see color?

c. Would you say the images are very much in focus? _______

Now we will make the opening half as large, to 0.75 cm.

d. How does the image change?

Sharpness: ________________________________

Brightness: ____________________________

How do you suppose the image would change if we used an even smaller hole, say the size of a pin?

Sharpness: ________________________________

Brightness: ____________________________

Why do you think we used a 1.5 cm hole, instead of a pinhole?


Next, you will see how lenses can be used to brighten up the image. The lenses give such a bright image, because they let in so much light.

First we will put a lens that is 1.5 cm wide up to the hole.

Can you see an image on the wall as before?__________.

How many centimeters away from the lens is the image in focus? _______cm. (Hint: Move paper toward the lens.)

Is the image created by the lens sharp everywhere?__________.

Is the image larger or smaller than the 1.5 cm hole?_________.

Is the image brighter or dimmer than the 1.5 cm hole?________.

The lens actually acts like a whole bunch of pinholes across a large area.

Now we will put a lens that is 12.5 cm wide up to a larger hole.

Can you see an image on the wall?__________.

How many centimeters away from the lens is it in focus? _______cm.

Is the image created by the lens sharp everywhere?__________.

Is the image larger or smaller than the 1.5 cm hole alone?_________.

Is the image brighter or dimmer than the 1.5 cm hole alone?________.

Recall the first lab you did in Chicago this summer. What do you call the distance between the lens and the image when it is in focus?


The net result of the optics of a lens is that: 1) the size of the image is related to the focal length (and) 2) the brightness of an image is related to the F ratio, which is the focal length divided by the aperture. For example, the big 40" telescope in the dome has a focal length of some 63 feet(=760'). Its aperture is, of course, 40".

What is its F ratio? (Remember: the F ratio is focal length divided by aperture!)


What is the F ratio of the camera obscura's 1.5 cm hole?

(Hint: the sharpest image is 240 cm from the window) ____________

What is the F ratio of the 1.5 cm lens? ___________________

What is the F ratio of the 12.5 cm lens? ___________________

Which lenses, the 40" one,the 12.5 cm lens, the 1.5 cm lens, or the 1.5 cm hole gives the brightest image? _________________________.

Which requires the longest time to record an image on Nature Print Paper? ______

Turn the room light on. Hold both the 1.5 cm and the 12.5 cm lenses above the floor until you can see a focused image of the bare bulb.

Which is brighter? ___________________

Which is larger? ___________________

What is the relationship between the focal length of a lens and the size of the image?

Part 3: Photographing the Sun with a camera obscura.

Now, let's use a smaller camera obscura to photograph the Sun. This one is 1.4 meters long (1400 mm) and has a one-half millimeter aperture pinhole. We'll fit a camera body at the end and use regular film. Don't ever try this with a regular camera, because the large aperture lens will collect so much light that is will cook the camera. Using a pinhole is one of the only safe ways to take pictures of the Sun.

What's the F ratio of our Sun camera?___________________________

Now, take pictures of the Sun with the film at exposure times of:

Finally, place a neutral density number 2.00 filter in front of the aperture and take the following exposure lengths: Develop the film and answer the following questions:

a. What happens to the image of the Sun as the exposure times are increased in images 1 through 3?

b. Which exposure time with the N.D. 2.00 filter looks the same as an exposure without the filter?

c. How much longer did you have to expose with the N.D. 2.00 filter to get the same result?

d. By what factor does the N.D. 2.00 filter block light?

We'll use this last result when we go for one of our big goals of this week -- to measure the size of the Galaxy. We'll use this filter with the "standard candle" (really a little flashlight) that we measured in the lab. Both will be used to actually get the distance to a star.

Important Disclaimers and Caveats: