How to specify a custom lens, step by step

01Anchor the job: sensor, distance, field of view

A lens spec starts from what it must image, not from lens jargon. Name the sensor (model or format + pixel pitch), the working distance, and the field of view — either the angle or “I need to see an object this wide at this distance.” Those three facts anchor everything else.

With sensor width w and full field of view θ, the focal length is fixed: EFL ≈ w / (2·tan(θ/2)). A 7.6 mm-wide sensor that must cover 40° needs EFL ≈ 10.4 mm — no catalog browsing required.

02Choose the f-number with the diffraction limit in view

The f-number sets how much light the lens collects and how deep the focus zone is. Smaller f-number = more light, shallower depth of field. But there is a floor nobody escapes: diffraction.

The diffraction-limited spot is about 2.44·λ·N across. At f/16 in green light that is ≈ 21 µm — on 2.2 µm pixels you have specified a blur filter, not a lens. Keep the spot comparable to the pixel: on small mobile-class pixels that means staying near f/2–f/5.6.

03Specify sharpness as MTF at Nyquist

“Sharp” is not quotable. The measurable request is contrast (MTF) at the highest spatial frequency your sensor can use — its Nyquist frequency, which is 1/(2·pixel pitch).

A 2.2 µm-pixel sensor has Nyquist ≈ 227 lp/mm. A solid, buildable ask is MTF ≥ 0.3 at half-Nyquist, ≥ 0.15 at Nyquist across the field. Asking for high MTF at frequencies past Nyquist buys nothing the sensor can record.

04Name the waveband — and stick to it

Glass dispersion, coating design, and even which glasses are allowed all follow from the spectrum: visible (roughly 420–680 nm), visible+NIR (to 950 nm for IR illumination), SWIR, or a single laser line.

A lens color-corrected for visible light will typically shift focus in NIR — “works with our 850 nm illuminator too” is a real requirement that changes the design. Say it up front; retrofitting it is a redesign, not a tweak.

05Bound the envelope, mount, and back focal distance

The mechanical facts vendors get burned by: maximum barrel length and diameter, the mount (M12, C/CS, custom thread), and the back focal distance — the clearance between the last glass surface and the sensor, including any filter glass in the path.

State filters explicitly: a 0.5 mm IR-cut filter in the optical path shifts focus by roughly a third of its thickness. The vendor needs to design it in, not discover it.

06Set the environment and quantity

Temperature range, humidity, vibration/shock, and washdown/sealing silently drive glass choice, housing material, and adhesives. Quantity drives everything about price: a one-off prototype and a 10k/year production part are different designs.

Give quantity as: prototype quantity now, expected annual volume if it goes to production. Vendors quote completely differently against that one line.

07Defer the rest — by name, to the vendor

Element count, glass selection, aspheres vs. spheres, coating recipe, exact tolerancing: that is the lens designer's craft. A good spec lists these explicitly as “vendor's choice, meeting the requirements above” — an empty section reads as an oversight; a deferred list reads as competence.

The litmus test for a finished spec: a vendor who has never met you can quote it without a clarifying phone call, and later prove the delivered lens meets it — every requirement is a number with a tolerance and a named test.

Worked example: a machine-vision inspection lens

The whole chain, numbers shown — the same derivations the SpecBuildr interview performs and prints in your spec's math appendix.

Sensor1/2.5″, 5.7 × 4.3 mm, 2.2 µm pixels

Field of view needed60 mm-wide scene at 200 mm working distance → θ ≈ 17°

Focal lengthEFL ≈ 5.7 / (2·tan(8.5°)) ≈ 19 mm

Nyquist1 / (2 × 0.0022 mm) ≈ 227 lp/mm

MTF ask≥ 0.3 @ 114 lp/mm, ≥ 0.15 @ 227 lp/mm, full field

f-numberf/4 — diffraction spot ≈ 5.4 µm, acceptable; f/11 (≈15 µm) would not be

Wavebandvisible + 850 nm NIR (shared illuminator)

Envelope / mount≤ 30 mm long, M12; BFD ≥ 6.5 mm incl. 0.7 mm filter stack

The mistakes that make quotes expensive

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Specifying magnification when you mean field of view

Magnification only means something with a named sensor at a named distance. FOV + sensor + distance is unambiguous.

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“As sharp as possible”

Unquotable — it invites either a padded price or a guess. MTF at Nyquist is the same wish, made testable.

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Stopping down to fix everything

Depth of field grows, but past the diffraction floor resolution falls. The check is one multiplication before you send the spec.

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Forgetting the filter and cover glass

Every millimeter of glass between lens and sensor moves focus. List the whole optical path.

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A bare “TBD” anywhere

A TBD without an owner stalls the quote. Every open item gets a name: yours, or explicitly the vendor's.

Frequently asked questions

What does a lens manufacturer actually need to quote a custom lens?

Seven things: sensor (or detector) details, working distance and field of view, f-number, a measurable sharpness target (MTF at a stated frequency), waveband, mechanical envelope and mount, and environment plus quantity. With those, a vendor can quote without a round of clarifying questions.

How do I work out the focal length I need?

From the sensor width and field of view: EFL ≈ sensor width / (2·tan(FOV/2)). You almost never pick focal length first — it falls out of what you must image.

What f-number should I ask for?

The smallest (brightest) your depth-of-field allows, bounded below by diffraction: the spot is about 2.44·λ·N, and once that exceeds your pixel pitch you are past the useful floor. On small pixels that usually means f/2–f/5.6.

What is MTF at Nyquist and why should my spec say it?

MTF is measured contrast at a spatial frequency; Nyquist (1/(2·pixel pitch)) is the finest detail your sensor can record. Asking for a stated MTF at Nyquist turns “sharp” into a number a test bench can verify.

Can I write a lens spec without an optics background?

Yes — if something asks the right questions in your language. That is exactly what the SpecBuildr interview does: you answer in plain engineering terms, and it derives the optical quantities and runs the physics checks.

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