Parfocal lens

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A parfocal lens is a lens that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but small enough not to be considered significant.

1 Microscopy
2 Photography
3 Telescopes
4 References
5 External links

[edit] Microscopy

Parfocal microscope objectives stay in focus when magnification is changed; i.e., if the microscope is switched from a higher power objective (e.g., 40×) to a lower power objective (e.g., 10×), not necessarily vice versa, the object stays in focus.

[edit] Photography

Zoom lenses (sometimes referred to as "true" zoom) are parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and then zooming back to a shorter focal length to compose the image. However, not all so-called "zoom" lenses are actually parfocal.^[1]

Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation essentially the same as a parfocal zoom.

[edit] Telescopes

Parfocal telescope eyepieces stay in focus when magnification is changed; i.e., if the telescope is switched from a lower power eyepiece (e.g., 10×) to a higher power eyepiece (e.g., 20×), or vice versa, the object stays in focus.