Abstract

Objective

The objective of this project is to measure the quality of a photographic lens.

Introduction

Lens testing can be as simple or as complicated as you desire, resulting in science projects that suit the needs of a wide variety of students. It's a fun way to learn more about your camera and learn how to take better pictures.

Terms, Concepts, and Questions to Start Background Research

To do an experiment in this area, you should do research that enables you to understand the following terms and concepts:

• The parts of a lens (lens elements, diaphragm, lens coating, focusing mechanism)
• Aperture
• Focal length
• f/stop
• Barrel distortion, pincushion distortion
• Chromatic aberration or color fringing
• Coma
• Light falloff, vignetting
• Flare and ghosts
• Sharpness

Advanced students should also study and learn about:

• Field curvature
• Diffraction
• Modulation Transfer Function (MTF)
• Subjective Quality Factor (SQF)

Bibliography

To learn about lenses, basic optics, and photography, try these sources:

• Aperture:  http://en.wikipedia.org/wiki/Aperture
• F-number:  http://en.wikipedia.org/wiki/F-number
• Optical coatings:  http://en.wikipedia.org/wiki/Optical_coating
• Lens optics (includes some lens defects):  http://en.wikipedia.org/wiki/Lens_%28optics%29
• Photography:  http://en.wikipedia.org/wiki/Photography
• Basic info on lens defects:  http://bobatkins.com/photography/technical/optics.html
• Glossary of lens terminology:  http://kenrockwell.com/tech/lenstech.htm

The Brick Wall Test (and other simple tests)

• This is an excellent article on how to do a brick wall test:  http://www.vividlight.com/articles/1814.htm
• This article covers a variety of less technical ways of testing a lens:  http://www.prime-junta.net/pont/How_to/ha_Testing_lenses/a_How_to_test_a_lens.html

The Star Test

• Most articles on the Web are about star testing of telescopes. This one talks about star testing of camera lenses, but not in detail:  http://www.astropix.com/HTML/I_ASTROP/I08/I0808/I0808.HTM
• This article talks about how to shoot star trails, but does not discuss lens defects:  http://www.astropix.com/HTML/I_ASTROP/I06/I0601/I0601.HTM

The US Air Force Test

• Modern Photography, a magazine now out of business, published a kit for performing lens tests using the 1951 USAF target. At the very bottom of this source you'll find links to scanned copies of the instructions for performing the tests. Unfortunately, these are rather clumsy to read:  http://medfmt.8k.com/usaf/
• This is a less detailed set of instructions, but much easier to use than the one above:  http://www.jimdoty.org/Tips/Equipment/USAF_Test/usaf_test.html
• You can obtain the actual target here:  http://medfmt.8k.com/mf/USAF.pdf
• Norman Koren's site also provides relevant information:  http://www.normankoren.com/Tutorials/MTF5.html#whytest

Modulation Transfer Function (MTF)

• A good, basic description of the gold standard for measuring sharpness, the modulation transfer function or MTF:  http://bobatkins.com/photography/technical/mtf/mtf1.html
• How to actually measure MTF for a lens:  http://www.normankoren.com/Tutorials/MTF5.html#whytest

Materials and Equipment

• A camera. It is easier to thoroughly test a lens on a camera with a manual exposure control option than on a camera that is strictly automatic. However, you can do meaningful tests even on an automatic camera (a "point and shoot").
• A tripod is highly desirable for this project, but you can also place your camera on almost any steady object.
• Some of the advanced tests require a test target. In many cases you can download these targets and print them out. And, if you use a film camera with a target, you'll need a magnifier or a low-power microscope.

Shop for Supplies at Science Buddies Online Store

Science Buddies has compiled some suggestions for harder to find items in our Amazon store. The store does not include every item for every project, but it does include items that we feel work for the projects on our website. If you have comments or would like us to add items to the store, please contact us at scibuddy@sciencebuddies.org.

Experimental Procedure

Some lens tests are amazingly simple, yet give a good indication of lens quality. Others are very mathematical and more precise. The table below is a brief summary of the tests you might perform. Use your background research to learn about each one, then choose the test that's appropriate for your camera and your science background. The sources listed above are a good place to start, and they will give you detailed instructions about how to perform the tests.

Lens quality varies with the aperture, and for zoom lenses it also varies with the focal length. So, for different trials in your experiment you will want to change these settings. See the table below.

Tips for doing your experiment:

• One of the most important things when doing a lens test is to make sure that your camera is absolutely steady during the exposure. Use a tripod or rest the camera on a steady object.
• If your camera has the capability, use a cable release or the self-timer to trigger the shutter. Like a tripod, this will help to keep the camera steady.
• If the test you choose uses test targets, or if you do the brick wall test, make sure that the entire area in the picture has uniform lighting. You don't want part of the scene to be in the shade and part in the sun.
• Lens quality tends to be higher in the center of the image than at the corners, so be sure to examine both.
• Make sure that you carefully focus your lens if you have a manual focus camera.

Variations

There are many possible variations with a lens test experiment. Here are just a few:

• Vibration or camera movement while the shutter is open is one of the biggest causes of poor photos. Compare the quality of photographs from a lens test on a tripod versus a lens test when you hold the camera by hand. You'll want to take a number of photos by hand for each test. How much difference is there between handheld photos and tripod photos? How much difference is there among the handheld photos--are some better than others? You could also compare the ability of different people to take "steady" photos, or compare different techniques of holding the camera. Shutter speed will be an important variable when testing handheld photos. Can you measure the impact of shutter speed on photo quality?
• If you have a camera that uses interchangeable lenses, test a variety of different lens to compare their performance.
• You can compare the results from different types of lens test. For example, how does MTF compare to a simpler test such as a star test. To do this experiment properly, you should compare the different lens tests for several lens or cameras.

• For more science project ideas in this area of science, see Photography, Digital Photography & Video Project Ideas.

Credits

Last edit date: 2010-11-08 16:09:36