Pinhole camera and lens camera
Constructing a pinhole camera and investigating the action of a lens.
Apparatus and materials
For each student or group of students
Pinhole camera box (15 cm x 10 cm x 10 cm)
Frosted glass or greaseproof paper
Spherical lens, +7D or 150 mm focal length. (The focal length should be equal to the length of the box)
Carbon filament lamps 200 W (up to 8 students may share each lamp)
Mounted lamp holders (safety pattern)
Pin for making holes (dress-making rather than optical)
Health & Safety and Technical notes
The mounted lamp holders should have:
- a double-insulated two-core flex with a 13 A mains plug fused at 3 A;
- a cable anchorage within the mounting;
- a safety-pattern lamp holder which disconnects the pins when the bulb is removed.
1 The pinhole camera kits available from equipment suppliers will contain either dismantled card boxes painted black on the inside, or square-section black plastic tubes, with some, or all, of the other items listed above. There are also a number of web pages describing how to make pinhole cameras, although these assume that you will want to use photographic film for a permanent record rather than a translucent screen.
2 The lenses suggested are designed for a camera approximately 150 mm from front to back. If other sized boxes are used, the lens provided should have a focal length equal to the length of the box.
3 If the box has a removable lid, students can hold a lens inside, as well as outside the box when investigating the lens camera. Each box should have a square hole cut in it at one end, covered with frosted glass or greaseproof paper, which scatters light over a very small angle. There should be a round hole at the opposite end. This is a very cheap, simple pinhole camera - students can construct their own.
4 Carbon filament lamps are rated at 200 V. They will last longer if a 200 V supply is available.
5 Cut a piece of black paper to size. Fasten it over the 5 cm open hole at the end of the box. Glue or Sellotape can be used for this. You may prefer to wrap paper round the end and secure it with an elastic band.
a Place the lamps and lamp holders around the laboratory so that up to eight students can work with their cameras about 1.5 metres from a lamp. The laboratory can get congested. Placing the lamps high can alleviate this.
b Pull down the blinds, or otherwise shade the room. Ask each student to do the following.
i) Make a small pinhole in the black paper, then point the pinhole end of the camera toward the lamp. Look at the screen while moving the box closer to or farther from the lamp.
ii) Enlarge the pinhole and repeat the observation.
iii) Add several more small pinholes and repeat the observation.
iv) Pepper the whole sheet with pinholes and repeat the observation.
c Give each student a lens and ask them to slide it in front of the pepper of pinholes whilst the box is pointed at the lamp. You may need to tell students to move nearer to the lamp and farther from it and see what happens. They will soon find the position for a single brilliant image.
d Push a pencil, then a finger, through the pepper of pinholes. At each stage, experiment with using the lens in front of the pinholes.
e Try the effect of moving the lens a little away from the camera. Examine the effect when the box is further away from, and when nearer to, the light source.
You can also have the lens nearer the screen, by holding it inside the box. This is easily done if the box has a removable lid and is held upside down with one hand, while the lens is held inside it from below with the other hand.
f Paste a new piece of black paper on the front of the camera. Make a large pinhole in this paper and then repeat step e. You are now using a lens camera with a small aperture and students can observe the greater range of focus.
g Finally pull up the blinds so that students can use their 'lens cameras' to look at the view through the window.
1 Try this out first yourself in the laboratory in which you are going to do it. You need to judge:
- how dark the laboratory needs to be
- where to put the lamps
- how far apart the lamp and the camera with lens need to be to collect the multiple pictures together
- how to get students to navigate around the benches in semi-darkness.
Good planning will bring out the 'WOW' factor when students see their first picture and experience the delight when the lens collects the multiple pictures together.
2 It is helpful to get students to look at the same effects together. Work through each stage of the experiment, and discuss what they have seen before moving on to the next effect. Doing this helps to avoid the continuous replacing of the black paper in which the pinholes are made. Once they have worked through it together, students should be free to repeat the experiment on their own.
Students might need to be warned to hold the screen at 'book length' rather than up to their eye. Those used to the LCD screens of modern digital cameras will find this familiar.
3 Using one pinhole, students should notice that the picture is inverted, and that however much the distance between the lamp and the camera varies, the picture is always in focus; the depth of field is infinite. A slightly bigger pinhole will produce a brighter picture but it may be fuzzier. Eventually students will want to make bigger and bigger holes, but introduce them to the lens effect first. Otherwise, a new piece of paper will need to be fixed to the box.
4 If possible, find out beforehand the distance from the lamp to the camera which will make the lens bring all the pinhole pictures in stage d together in one bright sharp image. Then arrange students round the lamps in circles of that radius so that the experiment succeeds at once and they enjoy the startling effect.
The lens placed in front of the pepper-of-pinholes obviously collects up a fan of 'rays' (each of which was proceeding to a separate picture on the screen) and bends them all to travel to a single image.
Assuming that these 'rays' (really thick 'pinhole-passing' bundles of rays) travel straight in air before and after the lens, you are forced to think that this is what a lens does. It takes a fan of rays from an object point (a source of light) and bends all of them, so that after passing through the lens they all go straight to an image point.
The drawback to using a lens in place of a pinhole is that the camera will only collect the rays together for one particular distance of the camera from the lamp. The depth of field is less for a lens camera than for a pinhole. With one big hole, the depth of field decreases, but the intensity, of what is now called an image, increases. To alleviate this, the lens has to be moved away from the camera itself, to produce a focused image of objects at different distances.
A wavefronts explanation: an alternative way of thinking about lens action is to consider what happens to wavefronts. A lens changes their curvature.
5 Here are some points to emphasize in using the pinhole camera.
- A camera 'sees' things in much the same way as the eye does.
- Objects emit light in all directions, only some of which enter the camera.
- A small pinhole transmits what can be thought of as a ray of light from the object onto a screen.
- Large apertures produce brighter images but shorter depths of field.
6 Students can easily make a pinhole camera at home. It is useful for observing solar eclipses. (Usual warnings about not observing the Sun directly.)
7 How Science Works Extension: A pinhole camera can be used to determine the height of an object which it is difficult to measure directly – for example, a flagpole or a tree. Place the camera on the ground, facing the object. Measure the height of the image on the screen, and the distances from the pinhole to the screen and to the foot of the object. The calculation of height is a simple matter of ratios.
Students should consider how to improve this experiment. Is it better to be close to the object, or farther away? Multiple measurements from different distances will help to reveal how precise the technique is. How long should the camera be? A longer camera will give a bigger image, and this will reduce the error in the result. Have some large cardboard boxes available, together with black cloth drapes which the students can work beneath.
This experiment was safety-checked in January 2007