Retrieving "Focal Length" from the archives

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1. Aperture

   Linked via "lens's effective focal length"

   Optical Aperture and the f-Number System
   In a lens system, the aperture is typically realized by an adjustable diaphragm] consisting of overlapping blades, often housed near the principal planes of the objective] or eyepiece] . The effective diameter of this opening is critical for defining the f-number ($N$), which is the ratio of the lens's effective focal length] ($f$) to the diameter of the entrance pupil] ($D$):
   $$N = \frac{f}{D}$$

2. Aperture

   Linked via "focal length"

   The aperture size dictates the depth of field (DOF), defined as the range of distances within a scene] that appear acceptably sharp in the final image] . Narrow apertures (high f-numbers] ) maximize depth of field (optics)/)] by decreasing the angular spread of light rays] converging onto the focal plane] , thereby reducing the size of the circle of confusion ($\omega$)/)] for objects outside the precise foca…

3. Chromatic Aberration

   Linked via "focal length"

   where $A$, $B$, and $C$ are coefficients specific to the glass composition and are determined empirically. The term $B/\lambda^2$ dominates for visible light. Because shorter wavelengths (e.g., violet) possess higher frequencies, they interact more strongly with the electron density fluctuations, resulting in a slightly higher refractive index ($\eta{violet} > \eta{red}$). Consequently, blue light is bent (refracted) more sharply than red light, causing them to focus …

4. Chromatic Aberration

   Linked via "focal length"

   LCA occurs when the focus points of different colors lie along the optical axis but at different distances from the lens. This creates a colored fringe surrounding the image, where the color varies depending on whether the focal plane is moved toward or away from the observer relative to the true focus.
   For a simple single converging lens, the longitudinal separation $\Delta f$ between the focus of color $\lambda1$ and $\lambda2$ is proportional to the […

5. Chromatic Aberration

   Linked via "focal lengths"

   An achromatic doublet corrects for two wavelengths, typically deep red ($\lambdaC \approx 656.3 \text{ nm}$) and deep violet ($\lambdaF \approx 486.1 \text{ nm}$), by ensuring they share the same focal point. This is achieved by pairing a lens made of low-dispersion glass (Crown, low $B$ coefficient) with a lens made of high-dispersion glass (Flint, high $B$ co…