A reticle is an internal optical reference
A reticle or graticule is installed near a focal plane so its pattern can be seen together with a target image, specimen or field. A resolution target is placed in the object scene and imaged by the system; a reticle is part of the instrument itself.
The measured quantity determines the pattern
A crosshair answers where an axis or centre is. A graduated scale answers how far a feature extends. A grid or point field supports sampling, calibration or coordinate registration. A circle, quadrant or protractor adds radial or angular information.
| Function | Reference geometry | Result to read |
|---|---|---|
| Aiming and alignment | Crosshair, quadrant or circle | Centre, axis or angular offset |
| Measurement | Scale, crossed scale or protractor | Length, angle or orthogonality after calibration |
| Counting and sampling | Grid, points or test lines | Count, area fraction or sampling result |
Focal-plane position controls apparent alignment
Pattern-side orientation, substrate thickness, holder shoulder and optical power determine where the pattern appears. A sharp reticle viewed through an eyepiece does not prove that the target image and reticle occupy the same conjugate plane.
- Define the film side, clear aperture, thickness, seating surface and the reference plane in the drawing.
- For transmitted systems, state which side is illuminated and which surface is observed.
Parallax is relative motion at the eye
Fix the instrument and move the eye across the exit pupil. If the reticle moves against the target, the reticle and target image are not in the intended conjugate relationship. Eyepiece dioptre adjustment sharpens the reticle for the observer; it does not by itself remove instrument parallax.
Check aiming and measurement in the installed instrument
A reticle drawing can establish line position, pitch and substrate geometry. Final alignment also depends on holder runout, optical axis, focus, magnification and viewing direction. Accept the component and the installed instrument as separate checks.
Illumination and polarity change what the eye sees
Etched, chrome-coated, reflective and illuminated patterns can produce different contrast at the same nominal geometry. State the working wavelength or colour, observed side and whether the useful feature is bright or dark.
Convert pattern displacement into optical angle
For a simple focal-plane reference at small angles, θ ≈ s/f, where s is pattern displacement and f is the effective focal length mapping the angle onto that plane. Use the system’s actual mapping for relayed, distorted or variable-magnification optics.
First and second focal planes in variable optics
In a conventional variable-magnification telescope, the first-focal-plane reticle is before the variable erector and the second-focal-plane reticle is after it; both are ahead of the eyepiece. FFP markings scale with the target image. SFP markings remain approximately constant to the observer while the target changes size.
For an SFP pattern calibrated at magnification M₀, its angular interval at M is approximately the calibrated interval × M₀/M. At half the reference magnification, the angular interval doubles. The relationship applies to angular subtension; it cannot be converted directly into a doubled target distance.
Diagnose sharpness, parallax and orientation separately
A blurred pattern can result from incorrect eyepiece focus or mounting-plane position. Relative movement against a sharp target points to a conjugate-plane mismatch. A mirrored scale or reversed numbering points instead to film-side orientation or image inversion.
- Assess the reticle with a relaxed eye before adjusting the target focus.
- Fix target distance and magnification while checking movement across the exit pupil.
- Use an asymmetric mark to verify orientation from the specified observation side.