effective focal length


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effective focal length

The property of a compound lens, such as an eyepiece, that is analogous to the focal length of a single thin lens and can be used, for example, in the determination of magnifying power. For a two-lens system the effective focal length, f , is given by
1/f = 1/f 1 + 1/f 2d /f 1 f 2

where f 1 and f 2 are the focal lengths of the components and d is their separation.

References in periodicals archive ?
I used the scope with a 1991-vintage Tele Vue Paracorr, which increases the scope's effective focal length by 15% but also fills the 1.
It works purely as a field flattener and does not change the effective focal length or focal ratio of your telescope.
SPECIFICATIONS & MEASUREMENTS * TMB-92 Signature Series Refractor Clear aperture 92 mm Effective focal length 505 mm Focal ratio f/5.
To increase the size of the object on the imaging sensor, we need to increase the effective focal length of our telescope, at the cost of a slower focal ratio.
WHAT WE LIKE: Compact and lightweight Superbly flat fields with reducer lenses Versatile photo-visual system WHAT WE DON'T LIKE: Slight chromatic aberration apparent visually Light falloff at image corners Complex and confusing stacks of adapter rings SPECIFICATIONS & MEASUREMENTS * BORG 77EDII Clear aperture 76 mm Effective focal length 331 mm Focal ratio f/4.
At roughly 10,000 mm effective focal length (200x), stars remained virtually stationary as I racked the focus in and out--a very impressive feat for such high magnification, and there was no discernable backlash.

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