Abstract

Here's an interesting flash photography project. With an inexpensive Fresnel lens, you can concentrate the light from your flash. You'll be able to shoot with a smaller aperture and a shorter flash duration. This will give you greater depth of focus and will allow you to 'freeze' motion at higher speeds. The trade-off is that the light will be concentrated toward the center of the frame. This project shows you how you can investigate that trade-off and find out how you can best use your flash for 'freezing' motion.

Objective

The goal of this project is to build and test a flash extender for a camera's flash attachment using a Fresnel lens. How far will you be able to extend the range of your flash?

Introduction

Electronic flashes for cameras use a xenon flashlamp to create a brief, brilliant illumination of your subject. Because the duration of the flash is so brief (perhaps as short as 1/31,000 of a second at the lowest manual power setting), the flash can 'freeze' motion. Sometimes, though, using the lowest power setting may require you to open up the lens aperture too much, limiting the depth of focus of your shot more than you would like.

For example, consider the problem of trying to capture a hummingbird in flight on film: "Shooting hummingbirds is tough. To stop their wings takes a flash duration of 1/12,000 of a second or less. When you're very close to them and stopped down to F18 to get enough depth of field for the entire bird to be in focus, you need all the light you can get. Unfortunately, even zoomed out to their maximum focal length flashes have typical beam spreads of 45 degrees. That means the light you want focused on a one square foot area is being spread over 50 or 60 square feet. Your only options are to push the ISO up (which causes graininess and red artifacts), open the aperture (reducing the depth or field), or slow the flash down to get enough light for a proper exposure (which gives you blurred wings.) Fortunately, there's a cheap and easy solution: a beam focuser." (Schmidt, date unknown)

The inverse square law describes the geometrical fact that light intensity decreases as the square of the distance from the source (Figure 2).

By focusing the light into a smaller area, the flash extender will allow you to take photographs with smaller apertures and shorter flash durations. The drawback is that the light will be concentrated towards the center of the frame The focus extender is not a good idea for lighting group photos, for example.

In this project, you will build a flash focus extender to concentrate the light from your flash into a smaller area. You'll take photographs over a range of apertures and flash durations to test how well the focus extender illuminates an object at the center of the camera's field of view.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• inverse square law,
• Fresnel lens,
• lens aperture,
• depth of field,
• shutter speed.

Questions

• When you increase the shutter speed by 2-fold (for example, from 1/250 to 1/500 s), what adjustment do you need to make to the lens aperture to keep the exposure value constant?
• How will this adjustment affect the depth of focus for the shot?
• How much depth of field can you achieve with a short-duration flash when using the flash focus extender?

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• 35 mm SLR camera with separate flash attachment:
  • you must be able to set the lens aperture,
  • the flash must have manual settings for full power, 1/2, 1/4, and 1/16 (1/64 optional);
• a tripod for the camera is nice to have,
• test object to photograph (about the size of your hand),
• for making the flash extender you will need:
  • rectangular Fresnel lens,
    • can be found at office supply stores, ask for "page magnifiers,"
    • also available from many online sources, do a web search for "Fresnel lens magnifier."
  • ruler,
  • foam core board or cardboard,
  • utility knife,
  • metal straightedge,
  • masking tape.

Experimental Procedure

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Photography, Digital Photography & Video Project Ideas
• My Favorites

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