science

Lens hood

Lens hood is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Lens hood rather than just read about it. In short: In photography, a lens hood or lens shade is a device used on the front end of a lens to block the Sun or other light source(s) to prevent glare and lens flare. Lens hoods may also be used to protect the lens from scratches and the elements without having to put on a lens cover.

Key takeaways

Lens hood belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Lens hood to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lens hood from memory before moving on to harder problems.

Reference excerpt

In photography, a lens hood or lens shade is a device used on the front end of a lens to block the Sun or other light source(s) to prevent glare and lens flare. Lens hoods may also be used to protect the lens from scratches and the elements without having to put on a lens cover. The geometry of a lens hood is dependent on three parameters: the focal length of the lens, the size of the front lens element and the dimensions of the image sensor or film in the camera.

Cause of lens flare

Flare occurs when stray light strikes the front element of a lens and then bounces around within the lens. This stray light often comes from very bright light sources, such as the Sun, bright studio lights, or a bright white background. If a light source is in the lens' angle of view, a lens hood will hardly have any effect, but the light does not have to cause lens flare. It is sufficient that stray light from a bright light source enters the lens. Multi-layer coatings in newer lenses also help to reduce lens flare.

Types The shape of a lens hood can vary from a plain cylindrical or conical section (much like a lamp shade) to a more complex shape, sometimes called a petal, tulip, or flower hood. These more complex shapes take into account the final image's shape and aspect ratio. This allows the lens hood to block stray light with the longer portions of the lens hood, while allowing more light into the corners of the image through the shorter portions of the hood, thereby reducing the amount of mechanical vignetting (reduction of light around the periphery) in the final image. The geometry of a lens hood is dependent on three things:

focal length of the lens, size of the front lens element, dimensions of the image sensor or film in the camera. Ideally, lens hoods should increase in length, and therefore in efficiency, as the focal length of the lens increases and the angle of view reduces. Lens hoods are more prominent in long focus lenses because they have a smaller viewing angle than that of wide-angle lenses. For wide angle lenses, the length of the hood (away from the end of the lens) cannot be as long as those for telephoto lenses, as a longer hood would enter the wider field of view of the lens. Maximum aperture also affects the shape of a lens hood. As the aperture gets larger the amount of light and consequently the amount of the frame the sensor "sees" increases. This can be seen when comparing two lens hoods of the same focal length but with differing apertures – compare the lens hood of a telephoto f/4 lens with that of the same lens but with a maximum aperture of f/2.8. Correctly made rectangular or square lens hoods are generally more efficient than cylindrical or conical ones because those shapes closely resemble the shape of the photograph. However, rectangular or square lens hoods should not be used with zoom lenses whose front elements rotate as the focal length is changed, as the hood will rotate as well, blocking parts of the angle of view. The same also applies to petal lens hoods. For these types of lenses, only cylindrical or conical lens hoods will work effectively. In addition, lens hoods can offer some degree of physical protection for the lens due to the hood extending farther than the lens itself.

Lens hoods with an extending bellows design (much like the bellows of a medium or large format camera) can be adjusted for depth. This means that the depth can be increased when used on longer focal length lenses, and reduced as necessary for shorter focal length lenses.

Storage Lens hoods that are supplied by the manufacturer of the lens are often designed to fit onto the matching lens facing either forward, for normal use, or backwards, so that the hood may be stored with the lens without occupying much additional space. Rubber lens hoods are flexible and generally collapse for storage. However other lens hoods must be removed if these features are not available and length extension of the lens is not preferred during storage.

See also Barn doors – similar device used for stage lights Lens cover Matte box – similar, adjustable device used in still photography and video

References

External links

lenshoods.net (»Free printable cardboard lens hoods«)

Illustrations

Worked examples

Example 1 — a first encounter with Lens hood

Start with the simplest possible case. Write down what Lens hood claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lens hood before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lens hood ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lens hood

In research: Lens hood appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lens hood in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lens hood is common in secondary-school and first-year university syllabi. It links to neighbouring topics Photography equipment, so understanding it makes those chapters shorter.
In everyday life: Look for Lens hood outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Lens hood in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lens hood means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Lens hood out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lens hood in simple terms?

Why does Lens hood matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lens hood?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lens hood.