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Focusing Your Flash for 'Freezing' Motion

Difficulty
Time Required Average (6-10 days)
Prerequisites You'll need a camera with a separate flash attachment to do this project.
Material Availability Readily available
Cost Very Low (under $20)
Safety Be careful in sunlight: the Fresnel lens used in this project can focus enough sunlight to cause burns, melt your flash attachment, or set fires.

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?

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

Cite This Page

MLA Style

Science Buddies Staff. "Focusing Your Flash for 'Freezing' Motion" Science Buddies. Science Buddies, 3 Oct. 2014. Web. 28 Nov. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Photo_p013.shtml>

APA Style

Science Buddies Staff. (2014, October 3). Focusing Your Flash for 'Freezing' Motion. Retrieved November 28, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Photo_p013.shtml

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Last edit date: 2014-10-03

Introduction

photo of flash focuser
Figure 1. A Fresnel lens flash focuser (Wayne Schmidt)

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).

diagram illustrating the inverse square law
Figure 2. The diagram illustrates the inverse square law. As light from a point source radiates out into space, the area illuminated increases as the square of the distance and the intensity of the light decreases as the square of the distance.

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?

Bibliography

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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.

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Experimental Procedure

Safety Note: Be careful in sunlight. The Fresnel lens used in this project can focus enough sunlight to cause burns, melt your flash attachment, or set fires.

Making the flash focuser is quite simple. Refer to the photograph (Schmidt, date unknown) and follow the instructions.

Fresnel lens flash focuser

  1. Cut strips of foam core material, about 2.5 cm wide, to make a frame to fit around your Fresnel lens.
  2. Tip: if you bevel cut the ends of these strips at a 45° angle, it will help position the strips when you assemble the frame.
  3. Tape the frame pieces together, and tape the Fresnel lens to the frame.
  4. The only tricky part is to determine the optimal distance from the flash for mounting the Fresnel lens. You want the beam from the flash to be focused almost to its tightest point, but not quite. (If it is focused perfectly, then the image of the diffuser lens on the front of the flash will be in focus, and the light from the flash may not be uniform.)
  5. For a Canon 580EX flash, a distance of 23 cm (9") worked perfectly (Schmidt, date unknown).
  6. Once you have figured out the correct distance, cut the two side pieces from the foam core material.
  7. Tape the wide end of the side pieces firmly to the Fresnel lens frame.
  8. Tape the narrow end of the side pieces firmly to the flash body.
Testing the Flash Focuser
  1. If you have a tripod for the camera, use it.
  2. Select a test object to focus. It should be about the size of your hand. In fact, if you have a helper, you can use their hand.
  3. Position the test object about 5–6 feet from the camera. Angle the object slightly away from the camera so that the image will have some depth (for testing depth of focus). Make measurements and take notes so that you can reproduce the setup in case you need to take more pictures later.
  4. You will take a series of pictures with different lens aperture/flash power combinations. In your lab notebook, record the shutter speed (same for all shots), lens aperture, and flash setting you use for each shot.
  5. Use your camera's manual mode to set the shutter speed and lens aperture.
    • Choose the highest shutter speed that will synchronize with the flash (usually 1/60 or 1/100 s). You will keep the shutter speed constant.
    • Start with the lens stopped down to the smallest aperture.
  6. With your flash in manual mode, set it to the lowest flash power (1/16 or 1/64). Take a picture.
  7. Increase the flash power in steps, taking a picture each time.
  8. When you have gone through all the flash power settings, open the lens aperture one stop, set the flash back to the lowest power, and take another set of pictures.
  9. Repeat the test without the flash focuser.
  10. Remember to record the settings used for each picture in your notebook!
  11. Use the exact same process to produce the images for comparison. If you use a digital camera, use the same software settings when preparing pictures for printing. If you use a film camera, have all of the pictures developed and printed with identical processing.
  12. Compare the images side-by-side for:
    1. exposure (does the main subject have sufficient lighting?),
    2. depth of focus (how much of the main subject is in sharp focus?),
    3. uniformity of lighting (how much of the entire frame is well-illuminated?).
  13. From your comparisons, determine which flash power settings produced the best pictures both with and without the flash focuser.

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Variations

  • In the experiment described above, the flash-to-subject distance was held constant, and the lens aperture was varied. Repeat the experiment at various flash-to-subject distances to determine the range over which the flash extender will work.
  • For another experiment on light intensity and the inverse square law, see the Science Buddies project Where Is 'Full Sun' No Brighter than Twilight?.
  • Since the materials for making a flash extender are inexpensive and readily available, you could make several, using Fresnel lenses of different sizes. How is the size of the lens related to the factor by which flash range is extended? How is the power of the lens related to the factor by which flash range is extended? How is the size of the lens related to the optimal position from the flash?
  • If you have or can borrow a flash meter you could make quantitative measurements of the flash intensity at various distances with and without the focuser.
  • Use a moving test object for your photographs. One idea would be to shoot pictures of a propeller attached to the shaft of a battery-powered DC motor. You could use a potentiometer to control the speed of rotation. Can you 'freeze' the propeller motion at higher speeds with the flash focuser? See the Science Buddies project Measuring Vibrational Frequency with Light for a method you could use to measure the rotational speed of the propeller.
  • What percentage of the field of view is adequately illuminated when using the flash focus extender? Design an experiment to find out.

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