Podcast
Questions and Answers
What is the primary factor that affects the focal length of a lens?
What is the primary factor that affects the focal length of a lens?
What is the term for the phenomenon where the focal length of a lens depends on the wavelength of light?
What is the term for the phenomenon where the focal length of a lens depends on the wavelength of light?
What is the result of using a compound lens to correct for chromatic aberration?
What is the result of using a compound lens to correct for chromatic aberration?
Who captured the first permanent(-ish) image in 1826?
Who captured the first permanent(-ish) image in 1826?
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What is the process by which bromine atoms absorb visible light and release an electron into the crystal?
What is the process by which bromine atoms absorb visible light and release an electron into the crystal?
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What is the size of the silver bromide crystals used in film?
What is the size of the silver bromide crystals used in film?
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What is the purpose of the developer solution in film processing?
What is the purpose of the developer solution in film processing?
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What is the time required for an exposure to produce an image using Daguerre's process?
What is the time required for an exposure to produce an image using Daguerre's process?
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Which aberration causes off-axis sources to appear wedge-like?
Which aberration causes off-axis sources to appear wedge-like?
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What is a potential downside of adding a stop behind a lens to reduce aberrations?
What is a potential downside of adding a stop behind a lens to reduce aberrations?
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Which combination of lenses makes up the Taylor-Cooke triplet?
Which combination of lenses makes up the Taylor-Cooke triplet?
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Which of these explains astigmatism in lenses?
Which of these explains astigmatism in lenses?
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What is a more recent alternative to correcting spherical aberrations using multiple lenses?
What is a more recent alternative to correcting spherical aberrations using multiple lenses?
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What is the result of applying a single converging lens at a low f/ number?
What is the result of applying a single converging lens at a low f/ number?
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Which parameter of an aspherical lens changes with distance from the optical axis?
Which parameter of an aspherical lens changes with distance from the optical axis?
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What is one benefit of using aspherical lenses over lens combinations?
What is one benefit of using aspherical lenses over lens combinations?
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Study Notes
Camera Lenses and Aberrations
- Using a single converging lens results in poor-quality images, especially at low f/numbers.
- Conventional ray-tracing rules only work for truly thin lenses and paraxial rays.
- Real lenses suffer from multiple "aberrations" that can be corrected using more complex combinations of spherical lenses.
Aberrations
- Coma: off-axis sources acquire a wedge-like distortion.
- Astigmatism: focal length depends on orientation in the image plane.
- Both coma and astigmatism are usually the result of imperfection in the lens.
Spherical Aberration
- Application of Snell's Law to a ray moving parallel to the optical axis of a hemispherical lens reveals that the location where it crosses the optical axis depends on how close it is to the optical axis.
- The best focus achievable is a "disc of minimum confusion".
Spherical Aberration Correction
- Adding a "stop" behind the lens to block non-paraxial and marginal rays significantly reduces aberration.
- The disc of minimum confusion is reduced in size, but less light enters the lens.
- An alternative to a stop is a combination of lenses that collectively compensate for variation in focal point, such as the Taylor-Cooke triplet.
- The Taylor-Cooke triplet consists of two converging crown glass lenses and one diverging flint glass lens in between, addressing many aberrations and distortions.
Chromatic Aberration
- The focal length of a lens depends on the index of refraction of the medium of which it's made and the surrounding medium.
- Dispersion means the focal length depends on wavelength, resulting in blurriness with a coloration at the edges.
Chromatic Aberration Correction
- A compound lens can be used to compensate for the dispersion, such as an achromatic doublet consisting of a converging crown glass element cemented to a diverging flint glass element.
- The result is a (almost) common focal length for long and short wavelengths.
Capturing Images
- The camera was of limited use until a mechanism became available to capture and preserve images.
- The first permanent(-ish) image was captured by Niépce in 1826.
- The process greatly improved by Daguerre, as shown in the 1839 "daguerreotype".
- The daguerreotype was an image on a silver-plated copper sheet, requiring an exposure time of 10-15 minutes.
Capturing Images - Black & White Film
- A plastic/cellulose backing is coated with an emulsion consisting of tiny crystals of silver-bromide suspended in gelatin.
- Each crystal is about one micron in size (varies from film to film).
- Bromine atoms can absorb visible light (mostly at the blue end of the spectrum), releasing an electron into the crystal.
- The result is a small buildup of metallic silver in the crystals that have been exposed to light.
- The buildup of metallic silver constitutes a latent image.
- Processing of the film to produce a usable image involves two separate steps: development and treatment with a developer solution.
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Description
Learn about the limitations of single converging lenses, ray-tracing rules, and common aberrations in lenses such as coma and astigmatism.
