固定焦点・可変焦点・ズームのマシンビジョンレンズ：焦点特性、トレードオフ、および選定
3種類のレンズは、ある動作上の点において異なります。それは、「焦点距離が変わった場合、焦点はどうなるか」という点です。このガイドでは、各レンズの定義、トレードオフ、試作から量産までのワークフロー、およびカタログで確認済みの選択肢について解説します。
A fixed focal length lens has one focal length and typically gives the best sharpness, distortion control, and calibration stability per dollar. Choose it once working distance and field of view are confirmed. A varifocal lens changes focal length but loses focus with every adjustment, so it must be refocused after any focal-length change. A parfocal lens, the true zoom, holds focus through the change within a specified tolerance.
Adjustable lenses earn their cost during prototyping and in systems that reframe during operation. Everywhere else, fixed focal is the production answer.
What Is the Difference Between Fixed, Varifocal, and Zoom Lenses?
Fixed focal has one focal length. Varifocal changes it but loses focus. A parfocal zoom changes it while holding focus to a stated tracking tolerance. The deciding question is not image quality but when, and how often, the focal length has to change. The table below is the reference for every decision on this page.
| プロパティ | 固定焦点 | 可変焦点 | 同焦点ズーム |
|---|---|---|---|
| 焦点距離 | 製造時に設定されたEFLが1つ | 調整可能範囲 | 調整可能範囲 |
| 焦点距離の変更によるピント合わせ | 該当なし。フォーカスは一度設定される | 失われた；変更のたびに焦点を再調整する | Held within a stated tracking tolerance |
| サイクル半ばでの視点の転換 | いいえ | Possible if motorized, with a refocus and recalibration step in the cycle | Yes, manual or motorized, with no refocus step |
| 光学的な複雑さ | 最低 | 穏やかで、移動中のグループが1つある | 最高レベル、連携した移動グループによる |
| 相対的なコスト | 最低 | 低～中程度 | 最高 |
| 特定の焦点距離におけるMTF | ある動作点で最適化されている | 全ラインナップでバランスが取れている | 全ラインナップでバランスが取れている |
| 歪み | 1つのプロファイル、1回のキャリブレーション | 焦点設定による変化 | ズーム位置による変化 |
| 校正の負担 | 通常、1回の校正で有効です | 調整を行うたびに再校正を行ってください | 使用するズーム位置に応じてキャリブレーションを行う |
| パッケージのサイズと重量 | Smallest and lightest | 大きい | 最大 |
| 最適なサイズ | 安定した生産形状 | Commissioning, then locked; motorized reframing between runs | 動作中に変化するフレーミング |
Fixed focal designs make up the bulk of machine vision catalogs and true parfocal zooms the smallest group, because the compensating mechanics cost the most to build well. That availability pushes production toward fixed focal, which is why the Commonlands catalog runs mostly fixed focal M12 and C-mount lenses with two manual varifocal M12 lenses for bring-up.
What Is a Fixed Focal Length Lens?
A fixed focal length lens has one effective focal length (EFL) and no zoom ring. For a given sensor, the lens produces one angle of view. It is the machine vision default because the optical design is corrected at a single operating point instead of compromised across a range, and because one calibration typically holds until the lens is disturbed or the environment shifts.
The focal length you need is a calculation from working distance, active sensor width, and required scene width, not a preference. The focal length selection guide walks the geometry. Once the number is verified on a real target, buying flexibility you will never use has no return.
Fixed Focal Does Not Mean Fixed Focus
Fixed focal refers to the focal length, not the focus setting. Most fixed focal industrial lenses keep a focus ring set at the working distance during installation and then locked. Some compact M12 lenses ship pre-focused from the factory, a separate property called fixed focus. Aperture is independent too: many fixed focal C-mount lenses carry an adjustable iris, while M12 lenses typically have a fixed aperture set at manufacture.
Photography calls a fixed focal lens a prime lens, and the optics match. Industrial lenses document sensor format coverage, distortion, resolution, and minimum working distance. Verify image circle against the sensor diagonal, corner MTF at the operating aperture, and distortion against the measurement tolerance before ordering. Commonlands documents all three on every fixed focal M12 and C-mount product page.
What Is a Varifocal Lens?
バリフォーカルレンズは、内部のレンズ群を移動させることで焦点距離を調整し、それに伴って焦点面も移動します。ズームリングとフォーカスリングは連動していません。そのため、焦点距離を変更した後、フォーカスリングを再調整するまでは、像がぼやけたままになります。バリフォーカル設計では、焦点距離の決定が購入段階から設置段階へと先送りされます。
That deferral is the value. Mount the camera, turn the zoom ring until the framing is right, restore sharpness with the focus ring, and lock both. From that point the lens behaves like a fixed focal lens. Engineers reach for varifocal lenses during proof-of-concept work at untested mounting positions and at multi-product stations where framing changes between production runs.
The optical cost is real. A designer correcting one focal length can target aberrations at that exact configuration. A varifocal designer balances correction across the whole range instead. A varifocal set to 8mm will typically show softer corners and more distortion than a fixed 8mm lens from the same price tier.
A varifocal does not solve an unstable working distance. If the target moves toward or away from the camera, the fix is sufficient depth of field, not a lens a technician has to readjust. It also does not preserve calibration: changing the focal setting shifts image scale, the distortion profile, and the principal point, so coefficients captured at one setting are invalid at another. Fix the focal length before calibrating. Commonlands stocks two manual varifocal M12 lenses, the 4mm-9mm CIL436 and the 6mm-42mm CIL461, for this commissioning role.
What Is a Zoom Machine Vision Lens?
A zoom machine vision lens changes focal length while a compensating optical group holds the image plane in place, so the picture stays sharp through the range. Holds is a tolerance, not an absolute: cam tracking error, assembly tolerances, and thermal drift leave a residual focus shift that the datasheet should bound. That stability is what separates a true zoom from a varifocal, and it is why zoom lenses carry more elements, tighter mechanical tolerances, and higher prices.
Inside the barrel, one group changes focal length while a second cancels the focus shift, coupled by a cam or driven by motors (Kingslake, Lens Design Fundamentals, 2nd ed., §18.3). Zoom ratio is the longest focal length over the shortest, so 6mm to 42mm is 7X. On many zoom designs the entrance pupil cannot grow in step with focal length once it reaches the front element clear aperture, so the F# climbs toward the telephoto end:
So any loop that commands a zoom move must also command exposure, gain, or illumination. Datasheet practice varies: some list both endpoints or a range, some publish a constant aperture, and some give one number without saying where it applies, so ask which end it describes. A motorized zoom makes runtime reframing possible without a technician, but adds failure modes: motor step accuracy, position tracking, and settling time. A system commanding moves mid-cycle must confirm the move has settled before it trusts the next frame.
What Is a Parfocal Lens?
A parfocal lens maintains focus while its focal length changes, within a specified focus tolerance rather than perfectly. The word names that behavior, nothing else. A true zoom is parfocal by design. A varifocal is not, because it has no compensating group. The Commonlands CIL461, sold as a 7X zoom M12 lens, is a manual varifocal, so it loses focus through a focal change: set the focal length, restore focus, lock both rings, and treat it as fixed from then on.
Varifocal vs Zoom Lenses in Machine Vision
Both change focal length. Only the parfocal zoom does it without a refocus step. A varifocal reframe has to adjust, refocus, and recalibrate, which a motorized varifocal can do under software between parts if the cycle time absorbs it. What a varifocal cannot do is reframe with focus and calibration intact. If the focal length is set once and locked, the cheaper varifocal wins; the parfocal zoom is what removes the refocus step. Pay for that behavior when the application uses it, not as insurance.
How Do You Choose Between Fixed, Varifocal, and Zoom Lenses?
Choose by when the focal length changes, then confirm against the table. Moving from an adjustable prototype lens to a fixed focal production lens should be an explicit design-review item, not an accident.
| あなたの状況 | レンズの種類 |
|---|---|
| 作動距離と視野が確認されました。形状は固定されました。 | 単焦点レンズ |
| 設置までは固定位置が未定。一度調整した後、固定される | 可変焦点 |
| 動作中に視野が変化する。再焦点合わせの手順は行えない。 | 同焦点ズーム。ソフトウェアによる指令があれば電動駆動 |
| 動作中は、ターゲットまでの距離自体が変動します | ズームではなく、十分な被写界深度を持つ固定焦点レンズ |
| 歪み許容範囲が狭い高精度測定 | 固定焦点、設置時に一度だけ校正済み |
Fixed and Varifocal Lens Picks Compared
Commonlands stocks two manual varifocal M12 lenses for bring-up and a set of fixed focal lenses for production, across M12 and C-mount. The table ranks them the way an integrator reaches for each: the CIL461 6mm–42mm varifocal to find a focal length, then a fixed focal lens to lock it in. The fixed picks run $39 to $149 and hold a single calibration. The varifocal trades that stability for a 7X focal-length range.
| ランク | 種類 | レンズ | マウントとEFL | 校正と再現性 | 最適なサイズ |
|---|---|---|---|---|---|
| 1 | Manual varifocal | CIL461 | M12, 6mm–42mm, F/2.3 | 被写界深度を変更した後は、必ずピントを合わせ直し、調整し直してください。構図が決まったら、両方のリングを固定してください。 | ブリングアップ：実際の取り付け位置で範囲をスキャンし、焦点距離を特定する。 |
| 2 | 修正済み | CIL059 | M12、5.9mm、F/1.7 | -4%の歪みプロファイルが1つ。一度キャリブレーションを行えば、レンズに触れない限りその設定が維持されます。 | Wide, locked production geometry on 1/1.7-inch sensors. The F/1.7 build resolves 4-6MP at 3µm pixels, and the slower F/4.0 and F/5.6 builds carry the 8-12MP ratings. |
| 3 | 修正済み | CIL125 | M12、12.5mm、F/2.4 | The focal length is fixed, but focus rides on the M12 x 0.5 thread: tighten the lock nut against the holder after focusing, or the barrel walks under vibration. A light M12 body keeps mounting stress low. | 重量制限のある組み込みシステム、ドローン、ロボット工学の構築における望遠撮影の構図。 |
| 4 | 修正済み | CIL522 | Cマウント、12mm、F/1.4 | The iris tunes depth of field, not field of view, so a stored distortion correction usually stays close enough to reuse. Aperture, focus, and temperature can still move distortion and the principal point, so measure the shift before trusting it in a metric task. | 4.2µmのピクセルピッチで、低照度環境に対応した大口径Cマウント。 |
| 5 | 修正済み | CIL532 | Cマウント、12mm、F/2.0～F/16の絞り | Stopping down buys depth of field without changing field of view, so a stored distortion correction usually carries over; measure the shift before a metric task relies on it. On its 2.2µm pixels the Airy disk (2.44λN at 550nm) spans two pixel pitches near F/3.3, so past that marker the depth of field keeps growing with F# while contrast at fine detail falls off gradually. | 2/3インチ、12MPの量産工程で、ライン上でのアイリス制御が必要な作業。 |
| 6 | Manual varifocal | CIL436 | M12, 4mm–9mm (2.25X), F/1.9–F/3.0 | Refocus and recalibrate per setting. The wide end is specified at about -65% barrel distortion, a display figure whose metric (TV, optical, or F-theta) the datasheet does not state. | 立ち上げ時の広角リフレーミング；メトリック関連の作業を行う前に、歪みを補正してください。 |
For motorized C-mount zoom, vendors like Computar and Fujinon carry deeper catalogs. The fixed focal picks above are where Commonlands competes on published MTF, US stock, and price. Confirm image circle against your sensor before ordering.
よくある質問
The Commonlands catalog runs mostly fixed focal M12 and C-mount lenses, so these distinctions decide what actually ships in a build.
バリフォーカルレンズとズームレンズの違いは何ですか？
Both adjust focal length. The difference is focus behavior. A true zoom (parfocal) lens holds focus through the focal-length change within a specified tolerance, so it can reframe mid-operation. A varifocal lens loses focus with every adjustment and needs a separate refocus step, which a motorized varifocal can run between parts when the cycle time allows. Catalogs blur the two terms, so verify focus behavior in the datasheet.
ズームレンズでは、ズーム操作を行うと有効口径は変化しますか？
Often, yes, though the amount is design-specific. On many variable-aperture zoom designs the entrance pupil cannot grow in step with focal length once it reaches the front element clear aperture, so the F# climbs toward the telephoto end, commonly by one to two stops. Constant-aperture designs hold the geometric F# across the range, which is not the same as constant light: transmission and relative illumination still vary, so check the T-stop or a measured flat field. Where the aperture drifts, exposure or illumination must compensate.
バリフォーカルレンズの焦点設定を変更する場合、再調整は必要ですか？
はい、ほぼすべてのケースでそうです。焦点距離が変わると、画像のスケール、視野、歪みの特性が変化し、焦点位置もずれます。以前の設定で測定された歪み係数、1mmあたりのピクセル数、およびホモグラフィは、もはや適用されません。システムが再度測定を行う前に、焦点を合わせ直し、再校正を行い、再検証を行ってください。
単焦点レンズとプライムレンズは同じものですか？
Optically, yes: both terms mean one focal length with no zoom. Prime lens is photography vocabulary. Fixed focal length lens is the machine vision term. Industrial fixed focal lenses are specified for sensor format coverage, distortion, and resolution across the image circle, which photography primes typically do not document.
量産用のマシンビジョンシステムで、バリフォーカルレンズを使用することは可能ですか？
Yes, when field reframing is required (different part sizes at one station, for example). Lock the rings after setup and budget a refocus-plus-recalibration procedure for every adjustment. If the focal setting will never change in production, a fixed focal lens gives better repeatability with no drift-prone adjustment point.
Lock in the Right Focal Length
Send your working distance, scene size, and sensor part number to the Commonlands engineering team in San Diego. They will confirm whether a fixed focal lens covers it or a zoom earns its place, including motorized zoom requirements. Orders placed before 12 PM PT ship the same day.