science

Parfocal lens

Parfocal lens 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 Parfocal lens rather than just read about it. In short: A parfocal lens is a lens that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but it is too small to be considered significant.

Key takeaways

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

Reference excerpt

A parfocal lens is a lens that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but it is too small to be considered significant.

Microscopy Parfocal microscope objectives stay in focus when magnification is changed; i.e., if the microscope is switched from a lower power objective (e.g., 10×) to a higher power objective (e.g., 40×), the object stays in focus. Most modern bright-field microscopes are parfocal.

Photography Zoom lenses (sometimes referred to as "true" zoom) are ideally parfocal, in that focus is maintained as the lens is zoomed (i.e., focal length and magnification changed), which is convenient and has the advantage of allowing more accurate focusing at maximal focal length then zooming back to a shorter focal length to compose the image. Many zoom lenses, particularly in the case of fixed-lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (e.g., focal length range, maximal aperture, size, weight, cost) than parfocal zoom, which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming"), making operation practically indistinguishable from a parfocal zoom.

Cinematography Zoom lenses used for moviemaking applications must be parfocal to be of practical use. It is almost impossible to stay in correct focus (as done manually by the focus puller) while zooming. This could be possible theoretically, but if the subject and/or camera also move, there are too many variables to correct consistently.

Telescopy Parfocal telescope eyepieces stay in focus when magnification is changed; i.e., if the telescope is switched from a lower-power eyepiece (e.g., 10×) to a higher-power eyepiece (e.g., 20×), or vice versa, the object stays in focus.

References

"Lens Glossary". Tokina Industrial, Inc. Archived from the original on February 12, 2004. Retrieved 2007-11-18.

Worked examples

Example 1 — a first encounter with Parfocal lens

Start with the simplest possible case. Write down what Parfocal lens 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 Parfocal lens 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 Parfocal lens 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 Parfocal lens

In research: Parfocal lens 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 Parfocal lens 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: Parfocal lens is common in secondary-school and first-year university syllabi. It links to neighbouring topics Lenses, so understanding it makes those chapters shorter.
In everyday life: Look for Parfocal lens 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 Parfocal lens in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Parfocal lens 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 Parfocal lens out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Parfocal lens in simple terms?

A parfocal lens is a lens that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but it is too small to be considered significant.

Why does Parfocal lens 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 Parfocal lens?

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 Parfocal lens.