A paper-OCR station fixed at 127 mm (5 in) from the sheet leaves no slack: the lens either reaches focus at that gap or the build stops. Whether it does is set by the lens's minimum object distance, not by its focal length, and the fix, when one is needed, is a spacer measured in tenths of a millimetre. This guide works the extension math for 6, 8 and 12 mm CS-mount lenses, the field of view each covers at that gap, the three-camera seam overlap for a 14 in sheet, and the bench checks to run before you order.
What sets the closest focus of a CS-mount lens
A CS-mount lens sits 12.526 mm from the sensor; a C-mount lens sits 17.526 mm. Focusing closer means moving the optical group farther out from that flange position, and the end of the travel is the minimum object distance (MOD). Many CS-mount board lenses list a MOD near 100 mm; others only reach 200-300 mm.
The extension past the infinity position that a working distance demands:
At the station gap this runs from 0.30 mm at the shortest focal length to 1.25 mm at the longest; the table in the next section gives each value. If the MOD, quoted to the image plane, is at or under that gap, the lens arrives on its own travel. If it stops short, a shim ring of the shortfall closes the gap; a 15 mm macro tube serves a much nearer range, as the lens extension tubes for close-up machine vision guide shows. And if a lens already refuses to focus past some near limit, run the lens focuses only at close distance checklist before changing hardware. Choose the focal length from the machine vision lenses range once the shortfall is known.
Field of view each focal length covers at the station gap
The wide-angle estimate FOV ≈ W × so / f overstates the view at close range; the exact thin-lens value runs 4 to 9 percent lower, because the lens sits fractionally farther from the sensor:
| Focal length | Magnification at the station gap | View width, 6.2 mm sensor | Extension needed |
|---|---|---|---|
| 6 mm | 0.050 | 125 mm | 0.30 mm |
| 8 mm | 0.067 | 92 mm | 0.54 mm |
| 12 mm | 0.104 | 59 mm | 1.25 mm |
The table assumes the 6.2 mm width of a typical 1/2.3-inch board sensor; scale it linearly for other widths. Run your own numbers here:
Tiling the 14 in sheet with three cameras
A 14 in sheet is 355.6 mm across. Three views of the shortest lens span 3 × 125 = 375 mm, which leaves 19 mm to share between two seams: about 10 mm, or 8 percent of each view. Fixed cameras over flat paper stitch reliably at that overlap; a seam of printed text at 31 µm per pixel carries hundreds of matchable features. Three of the middle focal length fall 80 mm short of the sheet, and four leave only 4 mm seams, so five cameras at 26 mm each is the practical layout there. Seven cameras cover the sheet at the longest focal length. Each view is also 92 mm tall, so a sheet taller than one row takes a second row of cameras on the same seam rule.
The call: the shortest focal length, three cameras, for this station. Move to five cameras at the middle focal length only if your stitching software refuses the 10 mm seams. The wider trade-off between focal length and coverage is in how to choose a machine vision lens focal length.
Worked example: three cameras, shortest focal length, on a 6.2 mm sensor
Inputs: object distance so = 127 mm (the fixed station gap); focal length f = 6 mm; sensor 6.2 mm wide, about 4000 pixels across (a typical 12 MP, 1/2.3-inch board sensor); sheet 355.6 mm (14 in); three cameras.
Rules: m = f/(so − f); view = W/m; extension e = f²/(so − f); seam = (cameras × view − sheet) / 2.
Arithmetic:
- m = 6/(127 − 6) = 6/121 = 0.0496
- View = 6.2/0.0496 = 125 mm
- e = 6²/121 = 36/121 = 0.30 mm
- Coverage = 3 × 125 = 375 mm, so seam = (375 − 355.6)/2 = 9.7 mm (7.7 percent)
- Pixel size = 125/4000 = 31 µm, so a 2 mm capital spans 2/0.031 = 64 pixels
Focus tolerance is comfortable: with DOF ≈ 2Nc(1 + m)/m² at f/2.8 and c = 1.55 µm pixel pitch, depth of field is about 3.6 mm, so ±1 mm of paper placement stays sharp.
Decision: any lens at this focal length whose MOD is at or under the station gap reaches it on its own travel; one that stops at 200 to 300 mm needs a 0.2 to 0.3 mm shim. Three cameras tile the sheet with 10 mm seams, which flat paper forgives; the comfort floor is about 15 percent, which is what makes five cameras at the middle focal length the fallback. At 64 pixels per 2 mm character, resolution sits far above the OCR floor described in minimum font size for OCR: why small text fails and how to fix it.
Fit and verify the focus on the bench
- Read the MOD line on the lens data sheet and note its reference point: image plane or lens front.
- Convert a front-quoted figure by adding the barrel length, then compare with the station gap.
- If the lens stops short, size the shim as e(station) − e(MOD). An 8 mm lens with a 300 mm MOD needs 0.54 − 0.22 = 0.32 mm.
- Fit a shim from that shortfall up to the full needed extension; beyond the full value the station falls outside the far end of the travel. Rings and shims sit in the lens accessories range.
- Mount, aim at a printed target at the station gap, rack to the close end, confirm the target snaps sharp, and lock the ring.
- Capture all three cameras on a full-size printout, stitch, and measure every seam before fixing the mounts.
Common mistakes
- Comparing a front-of-lens MOD with a board-to-paper gap; the barrel length alone can be the whole miss.
- Solving close focus with a 15 mm extension tube: on the middle focal length that drives focus to 8 + 64/15 ≈ 12 mm, an order of magnitude too near.
- Mounting a C-mount lens bare on a CS board: the missing 5 mm pulls focus far below 100 mm and gives up infinity entirely.
- Pairing a 3 to 5 MP-rated lens with a 12 MP sensor, then chasing soft character edges that the lens, not the sensor, causes.
Which lens to order
The LS-FFL-1582 is an 8 mm C-mount FA lens rated for 6-10 MP on a 2/3-inch image circle. Among short lenses that is the nearest rating match for a 12 MP board. It is C-mount, so on a CS board it takes the standard 5 mm adapter ring, which restores the 17.526 mm flange distance; the bench steps above then settle its close-focus question. Mount and clearance details are in C mount vs CS mount: adapters, focus and clearance.
The LS-CTV-0135 is a 6-15 mm C-mount varifocal rated 3 MP for 2/3-inch sensors. Zoomed toward its wide end it approaches the shortest row of the table, which makes it the part to specify while the seam layout is still fluid; its rating suits 3 to 5 MP boards. Both CCTV parts come from the CCTV lenses range.
The LS-CTV-0012 is a 12 mm fixed CS-mount lens rated 5 MP for 1/2.5-inch sensors. It threads onto a CS board with no adapter at all and suits a station that reads one text strip per frame rather than the full 356 mm width.
- Sensor format
- 2/3"
- Mount
- C-mount
- Type
- CCTV varifocal
- Sensor format
- 2/3"
- Type
- CCTV fixed
- Sensor format
- 1/2.5"
Settle the focal length first, then start the lens configurator or filter the lens finder by CS mount and sensor format. For a fixture with awkward clearances, send the drawing through the contact page and we will check the MOD and shim size with you. The lenses topic page collects the adapters, tubes and focus-travel guides in one place.
Questions
Does a C-mount lens focus closer on a CS-mount camera without an adapter?
It sits 5 mm farther from the sensor than its design position, so a 12 mm lens focuses near 41 mm instead of its normal range, and infinity is gone. The threads may also seat loosely. For a fixed station at this gap, use the 5 mm adapter and add a measured shim; keep the bare-mount trick for bench experiments.
How much overlap should I leave between cameras for stitching?
Plan at least 10 percent of one view in each seam, and 15 to 20 percent where the sheet can bow or the cameras can move. In the worked example the seams hold about 10 mm, roughly 320 pixels of text at 31 µm per pixel, which fixed cameras over flat paper match reliably.
What if the data sheet quotes MOD from the front of the lens?
Add the barrel length before comparing. A lens listed at 100 mm from its front with a 30 mm barrel reaches only 130 mm from the image plane, so it misses the station. Size the shim from the shortfall, and any value up to the full needed extension keeps the station inside the focus travel.
Should I stop the lens down for OCR at this distance?
Yes, down to about f/8. Diffraction blur is roughly 11 µm at f/8, well under the 31 µm pixel, while depth of field grows from 3.6 mm at f/2.8 to about 10 mm at f/8. Past f/11 the blur creeps toward the pixel pitch and starts to soften character edges.