Reference sheet, formats, accuracy, and answers.
The practical facts: what to print, what to photograph, what comes out, and how we know the numbers are right.
Print it, put the part on it, photograph it.
The sheet carries four printed markers at known spacing. LensCAD finds them in the photo, corrects the perspective, sets the scale, crops to the paper and ignores the markers and the dashed border in every trace method. One tap replaces both the perspective and the scale steps.
Download the reference sheet (PDF) Download the small card (PDF)
Tips for a good photo
- Print at 100 percent, no "fit to page" scaling. The marker spacing is the scale.
- Lay the sheet flat, in even light, and keep all four markers in frame.
- Shoot from roughly above. Perspective is corrected, but a steep angle costs resolution on the far side.
- A small part traces sharper on the small card (4 x 6 in, printed on Letter paper): the camera can come close to it, so the part fills more of the photo.
- A part larger than the sheet is clipped at the paper's edge. Use the Tabloid or large sheet, or photograph it in two views.
- For a 3D model, photograph each side with the part resting on the sheet, following the illustrated guide in the Add View dialog.
What goes in, and what comes out.
| Photos in | JPEG, PNG, TIFF, HEIC and HEIF from an iPhone, and DNG. A 12 MP frame is uploaded at full resolution. Metadata (location, timestamps, serial numbers) is stripped before the photo is stored. Faces are not blurred, so keep people out of the frame. |
|---|---|
| Reference sheets | A small card for small parts, Letter, Tabloid and a large-format sheet, each with its own marker set, plus inverted (white-on-black) variants. A custom sheet size can be typed in. |
| 2D export | DXF at 1:1 scale with the corrected photo embedded, named component layers with colours, and a dimensions layer. Curves as true splines or as points, your choice at download. A multi-view part exports as a third-angle sheet or as 3D planes, each view registered to the part, photos optionally cropped to it. A one-view drawing downloads free; multi-view downloads with Pro or Studio. |
| 3D export | STEP (true solid, mm), 3DM (the finished solid plus editable pieces on hidden layers, written as a version 7 .3dm file so older software opens it), STL (the solid's tessellation, in your units), OBJ with a photo atlas (zip), and 3D DXF with curves on hidden layers and optional photos. An add-on body travels as a separate solid or object in every one. Details → |
| Project files | A LensCAD project file (.lcad) holds every view, all geometry, dimensions, layers and colours, and every 3D operation: extrudes, edge fillets, the add-on fit and simplification. Free to save and reopen on any device. |
| Every export | Carries a plain-text debug report: the resolution the photo arrived at, the rectified scale, and how each Smart Trace run went. |
| Units | Inches, millimetres and centimetres. Standardize rounds to 1/16 inch or 0.5 mm steps, coarser on each further tap. |
| Runs on | Any current browser on a phone, tablet or desktop, in portrait or landscape, with the complete toolset on each. Nothing to install. Tested most on iPhone and iPad Safari and on desktop Chrome. How the interface adapts → |
Against hand-measured ground truth, through the real app.
The number on the homepage, roughly 0.1 mm median error, is not a model's opinion of itself. This is how it is produced.
The benchmark
A set of real parts photographed on the reference sheet with an ordinary phone: a blade, a hook, a bolt cutter, a torch and others. For each, a ground-truth outline was made by hand from the rectified photo and kept as a fixture that is never regenerated.
The test
Every measurement is taken through the app itself, exactly as a user's photo goes through it, never on a hand-prepared image. The traced outline is then compared with the hand-made one along its whole length, so the result does not depend on how densely either outline was drawn.
What the numbers say
On the blade, the app's own pipeline returns a median error of 0.11 mm, a 90th-percentile error of 0.23 mm, and every point within 0.5 mm of the hand-made outline. Across the set, outlines land within a few tenths of a millimetre where the sheet and the photo are good.
What limits it
- The photo's resolution at the part. LensCAD works at the photo's own resolution, so a 12 MP frame uploaded whole beats the same frame shrunk on the way up.
- The sheet. A face standing well above the paper photographs slightly large; the 3D reconciliation corrects for that, a single 2D view does not.
- The edge. Translucent, glossy and shadowed edges are where hand correction still earns its keep, which is why the editor is there.
These tests run before every release. When a change makes a number worse, it does not ship.
Common questions.
Do I need the reference sheet?
No. Four corners, two perpendicular lines or a circle correct the perspective, and a known dimension or a ruler sets the scale. The sheet is the fastest and most accurate route, and the one the 3D workflow is designed around.
Which CAD programs open the files?
The DXF opens in any program that reads DXF. STEP opens in mechanical CAD programs. The .3dm opens in 3D modelling programs that read that format. STL and OBJ open in slicers and 3D tools.
Does it work on my phone?
Yes. The complete toolset runs in the browser on a phone or tablet, in portrait or landscape, with touch gestures and a loupe for precise points. You can also add it to your home screen.
What does Smart Trace cost, and what if it fails?
Re-running the same view uses half a run, and a run that fails or that you cancel is given back automatically. New accounts start with 3 free runs; Pro includes 30 a year and Studio 100, added monthly, and a top-up adds 20 for $15. Pricing →
What happens to my photos?
Before a photo is stored, its location, timestamp and serial-number metadata are stripped. Faces are not blurred, so keep people out of the frame. Photos and drawings are never used to train a model, ours or a provider's. Trust center → and Privacy Policy →.
Can the 3D model carve a pocket or an undercut?
Not on its own. The model is a visual hull, the shape your views have in common, so it sees the outer silhouette from each side. Inner shapes you traced can be cut through, recessed or raised by an amount you set. A feature no silhouette shows is one you add in your CAD from the STEP.
Something looks wrong. What do I do?
Every error in the app offers a Report button with the details already attached, and the help menu has one for anything else. Every export also carries a debug report that says what the app saw. Reports land in front of a person.