These lenses focus parallel beams to a light spot. This spot is a real image that can be projected on a screen or manipulated with a space filter like a diaphragm. The asymmetry of a plano-convex lens minimizes spherical aberrations when either the object distance or the image distance is more than 5 times the other. To minimize aberrations a plano-convex lens should be placed with the plane side pointing to the closest conjugate point – an object or an image whatever is closer.

These symmetrical lenses also possess positive focal lengths and produce real images of distinct objects. However theses lenses minimize the spherical aberrations for nearly symmetrical positions of the conjugate points with respect to the lens. For exactly symmetrical imaging with a 1:1 magnification not only the spherical aberration is minimized, but also coma and distortion are completely eliminated. As a rule of thumb the bi-convex lenses are best used for magnifications between 1/5 and 5.

Concave lenses convert incident parallel rays into diverging bundles of rays that seem to originate from a single point in object space – the so-called imaginary focal point. In doing so, aberrations are minimized for absolute conjugate ratios greater than 5 or less than 1/5.

These lenses are similar to plano-concave lenses and are best used with absolute conjugate ratios between 1/5 and 5.

These lenses focus parallel beams to a light spot. This spot is a real image that can be projected on a screen or manipulated with a space filter like a diaphragm. The asymmetry of a plano-convex lens minimizes spherical aberrations when either the object distance or the image distance is more than 5 times the other. To minimize aberrations a plano-convex lens should be placed with the plane side pointing to the closest conjugate point – an object or an image whatever is closer.

These symmetrical lenses also possess positive focal lengths and produce real images of distinct objects. However theses lenses minimize the spherical aberrations for nearly symmetrical positions of the conjugate points with respect to the lens. For exactly symmetrical imaging with a 1:1 magnification not only the spherical aberration is minimized, but also coma and distortion are completely eliminated. As a rule of thumb the bi-convex lenses are best used for magnifications between 1/5 and 5.

Concave lenses convert incident parallel rays into diverging bundles of rays that seem to originate from a single point in object space – the so-called imaginary focal point. In doing so, aberrations are minimized for absolute conjugate ratios greater than 5 or less than 1/5.

These lenses are similar to plano-concave lenses and are best used with absolute conjugate ratios between 1/5 and 5.