Objective
Applies a compact focusing thin-lens model; ƒ also sets the distance to the back focal plane (BFP) behind the lens — the plane a telescope or scan relay should image onto for pupil-matched scanning.
Open in the canvas →In the real world
A real microscope or camera objective is a highly corrected assembly of many lens elements, not a single piece of glass — the element count exists almost entirely to cancel spherical and chromatic aberration, flatten the field, and reach a high numerical aperture without the image falling apart. Numerical aperture NA is the single number that matters most: it sets the objective's light-gathering cone and, through diffraction, the finest detail it can ever resolve, regardless of magnification:
Modern objectives are almost always infinity-corrected: a point at the sample (the front focal plane) emits a cone that leaves the back of the objective as a collimated beam, which a separate tube lens then focuses onto a camera or eyepiece — nothing focuses light directly behind an infinity objective on its own. The reference plane a focal length f behind the objective, on that tube-lens side, is the back focal plane (BFP) — where the objective's entrance pupil (radius above) is imaged. It matters most in laser-scanning microscopy: a scan mirror, or its relayed image via a scan lens and tube lens, is deliberately positioned at a plane conjugate to the BFP, so that as the mirror tilts, the beam pivots around a fixed point inside the pupil instead of walking across it — keeping the full aperture illuminated at every scan angle.
In OpticalSetup
Optically this is the same single thin-lens surface as the plain lens element, using the same paraxial ray-transfer relation u' = u − h/f at the lens plane marked by the housing's flat front glass — just with a short default focal length and a drawn clear aperture typical of a real objective. What's modeled precisely, though, is where the back focal plane actually sits: toggle "Show focal points" (the ƒ button) or select the objective, and a marker labeled BFP appears exactly f behind the lens plane, on the side the beam arrives from — the real coordinate to position (or image, via a telescope) a scan mirror onto for correct pupil-matched scanning, not just an illustrative icon.
The BFP's position is geometrically exact for this thin-lens model, but its size is not modeled — there's no numerical aperture parameter, so the pupil radius formula above isn't computed or enforced anywhere, and no aberration correction, immersion media, or field-flatness limits exist either. It's a single idealized thin lens wearing an objective's housing, with one genuinely precise feature: the BFP marker's location.