Part of Lesson Plan: The Giant Camera Obscura

Activity Overview / Details

Next, ask the question again, “What happens to an image if the aperture size is reduced or changed?”  Demonstrate by placing a smaller aperture  cut-out over the original larger aperture, then a smaller one and smaller one.  “Does the image get sharper?”  Does the intensity of light get brighter or darker as the aperture gets smaller?”    (Another option would be to have students work in small groups and discover the answers themselves)

Ask the students what would happen to the image if the focal point were closer to the aperture.  Using a piece of white poster board,   hold it  back from the aperture opening and then move it closer to the aperture.  (Again, students could work in small groups and one student move the poster board back and forth.  Another could also change the aperture sizes with focal points at different distances.) 

Explain that this is exactly what happens inside a camera.  Different lenses will effect the focal point distances from the aperture to the CCD (Charged Couple Device)  in a digital camera. The camera will actually capture the image upside down with the light intensity dependent upon the aperture opening and magnification dependent on the focal point distance.  Also note that a larger aperture opening will blur the image more than a smaller aperture opening.  (I have included a simple handout for aperture sizes below, you can pass this out, or just pull it up on an LCD projector for this discussion.)

If butcher paper were hung on a projecting wall, students could draw the image on to the paper and or fill in the color with the same projection.