Zoom lens working distance extension without losing field of view

You have a modular zoom lens doing exactly what the inspection needs — a 3 × 2 mm field on a 2/3-inch sensor — but the working distance is 30–40 mm and the machine needs about 200 mm of clearance. The part that moves the working distance is the lower lens, the objective-side attachment in front of the zoom body: not the zoom body itself, and not the camera tube. The lower lens sets where the object plane sits; the camera tube and the zoom setting set the magnification; and only the magnification sets the field. Fit a 1× lower lens, keep your 2.0× camera tube, and the 3 × 2 mm field survives the move to roughly 190 mm. What follows is the multiplication, the arithmetic for your case, and the routes that fail.

The rule: the lower lens sets distance, the tube sets magnification

A modular zoom system is a chain: part, lower lens, zoom body, camera tube, sensor. A 7:1 body by itself focuses at roughly 30–40 mm at these magnifications. A lower lens is an afocal attachment that relocates the object plane: a 1× unit charts near 190 mm, and the lower-power 0.5× class near 290 mm, each contributing its own factor to the magnification.

To hold the field, hold M. A 1× lower lens multiplies M by one, so nothing downstream changes — that is why it is the right first move. A 0.5× lower lens halves M, and the camera tube must then double to compensate; on this body even that falls short, as the worked example shows.

Step by step: move the working distance out to the target clearance

1. Power the illuminator off before you open the optical path. At 30–40 mm the source sits next to your fingers, and infrared illumination stays invisible while it does its work.
2. Write down what must not change: an 8.8 × 6.6 mm sensor and a 3.0 mm wide field, so M = 8.8 ÷ 3.0 = 2.9×.
3. Read your current working point: body setting = M ÷ tube = 2.93 ÷ 2.0 = 1.47×. Mark the zoom ring position.
4. Fit the 1× lower lens to the body's objective port. Its chart distance is about 190 mm, measured from the front face of the lower lens to the part.
5. Change nothing else: the 2.0× camera tube, the camera mount and the sensor stay exactly as they are.
6. Refocus at the new distance, then return the body to the 1.47× mark. Focus against a magnified live preview rather than the full-frame view; the method is in machine vision focus preview zoom.
7. Check the scale with a dot grid such as the LS-LXR-0029, a 76 × 76 mm board with 0.5 mm dots. At 2.9× one dot should span about 17% of the frame width: 0.5 × 2.93 ÷ 8.8 = 0.17.
8. Lock the zoom and focus rings, then re-read the field after locking; ring drag moves a setting a few percent more often than people expect. The details are in lens locking screws.

Worked example: holding a 3 × 2 mm field at the new distance

Inputs, each with its source:

• Sensor: 2/3-inch format, 8.8 × 6.6 mm active area (camera data sheet).
• Field that must survive: 3.0 mm wide by about 2.2 mm high — the current system delivers roughly 3 × 2 mm at 30–40 mm (the inspection task).
• Zoom body: 7:1 span; with a 1× lower lens and a 1× tube the chart tops out at 2.3× (the body chart).
• Lower lens: 1× unit, chart working distance about 190 mm (the lower-lens chart).
• Target clearance: 200 mm (the machine layout).

Rule in words: divide the sensor width by the total magnification to get the field width; the total is the product of the three factors; choose the camera tube so the working point lands inside the body's span with margin on both sides.

The arithmetic:

• M = 8.8 ÷ 3.0 = 2.93×. At that magnification the vertical field is 6.6 ÷ 2.93 = 2.3 mm, matching today's 3 × 2 mm coverage.
• With the 2.0× tube kept, the body sits at 2.93 ÷ 2.0 = 1.47×.
• The body spans roughly 0.33× to 2.3×, a 7:1 range. The working point sits at 1.47 ÷ 2.3 = 64% of the top — comfortably mid-range.
• Reserve above the working point: 2.3 × 2.0 = 4.6× total, a smallest field of 8.8 ÷ 4.6 = 1.9 mm if a finer view is ever needed.

Result: a 1× lower lens plus the existing 2.0× tube gives about 190 mm of working distance with the field held at 3.0 × 2.3 mm. Mounting brackets and the front-face datum absorb the last 10 mm of the layout.

Decision, with the threshold: order the 1× lower lens and change nothing else. A tube is right whenever the working point lands between 25% and 75% of the body's span; 64% passes with room on both sides.

Two rejections, computed:

• The 1× tube pairing: the body would need 2.93 ÷ 1.0 = 2.93×, above its 2.3× ceiling. At full zoom the field bottoms out at 8.8 ÷ 2.3 = 3.8 × 2.9 mm, and magnification is down by 2.93 ÷ 2.3 = 1.27 — 27% fewer pixels across the part. Take that pairing only if the wide, low-zoom end matters more than the fine end.
• The 290 mm route: a 0.5× lower lens halves the chain. With the 2.0× tube the ceiling is 0.5 × 2.3 × 2.0 = 2.3× — the same 3.8 mm field. Holding 2.93× would take a tube of 2.93 ÷ (0.5 × 2.3) = 2.6×, which this chart does not offer. Past that distance, change tools rather than stack lower lenses.

The four set-ups side by side

The ceiling is the chart's 2.3× body top multiplied by the tube and lower-lens factors; the field you actually use is set by the zoom mark, not the ceiling. Working distances are chart values measured from the lower-lens front face, and a blank lower-lens cell means the bare body.

The fixed-lens route: 149 mm of focal length, and no zoom

The same request in single-lens geometry: hold magnification m at working distance WD, and the focal length follows.

With m = 2.93 and WD = 200 mm: f = 200 × 2.93 ÷ 3.93 = 149 mm. That is the long end of the lenses catalog — 18 fixed lenses in the 100 mm+ group — and you give up the 7:1 span entirely. The arithmetic is the same one behind how to choose a machine vision lens focal length.

A common 35 mm C-mount lens shows why the instinct fails. At that distance it magnifies at m = f ÷ (WD − f) = 35 ÷ 165 = 0.21×, so the field is 8.8 ÷ 0.21 ≈ 42 mm wide — fourteen times too large. And at the required 2.9×, depth of field collapses: with a 7 µm circle of confusion (an 11 mm sensor diagonal ÷ 1500) at f/8, the estimate is 2 × 8 × 0.007 × 3.93 ÷ 8.58 ≈ 0.05 mm. Fine for a flat part, thin for anything with height.

That same 35 mm geometry is exactly right for the overview camera many stations add beside the inspection view: at the same distance it covers about 42 × 31 mm for part presence and coarse location. The LS-FFL-0679 is a 35 mm C-mount machine vision lens for a 2/3-inch sensor, rated at 6 MP.

Extension tubes move the opposite way

Behind the lens, a tube adds spacing that raises magnification and pulls the object in. On a 35 mm lens, 15 mm of extension sets m = 15 ÷ 35 = 0.43× with the object at about 35 × 1.43 ÷ 0.43 ≈ 116 mm: magnification up, distance in — the exact opposite of this task. The LS-LXR-0284 is a 15 mm C/CS extension tube, the right tool for close-up work on flat parts rather than for reach. The deep guide is lens extension tubes for close-up machine vision, alongside the rest of the lens accessories range.

When a telecentric is the better answer

If the 3 mm field exists to measure the part rather than to look at it, perspective magnification drift becomes the error budget, and a telecentric removes it: magnification is fixed and insensitive to object distance. The LS-TCL-1972 is a 1× object-side telecentric C-mount lens for a 2/3-inch sensor with a 440 mm working distance.

At 1× the field is the whole 8.8 × 6.6 mm — right when the part is up to about 8 mm across. A 3 mm part wants the 2.9× class instead: the telecentric lenses finder lists 409 models at 2× magnification and above, and 1,142 in the 200 mm-and-longer working distance group. You trade the zoom span for a fixed, calibration-stable magnification. The decision is covered in when you need a telecentric lens for gauging, and the accuracy check in telecentric depth of field: validate measurement accuracy.

Checks before you order

• Format coverage: the lower lens must cover a 2/3-inch sensor; a unit charted for a smaller format vignettes the corners. The formats are compared in the lens sensor format guide.
• Mechanical envelope: chart distance is measured to the lower-lens front face, and the body plus tube extend backwards from that port. Check camera clearance behind and bracket room in front before you cut metal.
• Light budget: irradiance falls with distance squared. Moving the view out from 40 mm to the target costs the square of that ratio — about 25, roughly 4.6 stops — unless the source moves with it. Plan a larger light, or a longer exposure if the part is static.
• Resolution: at 2.9×, a common 5 MP 2/3-inch sensor with 3.45 µm pixels puts 3.45 ÷ 2.93 ≈ 1.2 µm per pixel on the part. Compare that with your smallest feature; the method is in object space pixel size from magnification.
• Scale re-check: measure a known spacing after the swap, with the margin discipline in calibration target field of view and coverage margins, before releasing the station.

Next step: the lens finder filtered by focal length or the configurator walks the whole chain. If the lower-lens chart you are working from is unclear, send the target numbers through the contact page and we will pick the pairing. More like this sits on the lenses topic page.