- Ratio = max aperture diameter : lense focus length
- for every 1mm of aperture diameter there is 1.4mm focal length
Relative lens Aperture
- Each f-number is related to the focal length
Lens Aberrations
-an aberration is an optical error
-no lense is perfect
many aberrations can be corrected by inducing alternative errorsin the lens
-some errors are welcomed (soft focus lens)
-different wavelengths do not focus at the same point
Chromatic Aberations
-light passing through glass is dispersed into colours
-Axial (logitudinal) and transverse (lateral)
-common in long and wide lenses
Axial chromatic aberations
-can be reduced by careful use of other aberations
-chromatic lense types
*Achromatic = blue and green
*Apochromatic = blue green red
*Superachromatic = IR B G R
Coma
-an off axis aberation creating cone shaped points
-stopping down can slightly reduce effect
can be reduced through deliberate use of other aberations
Asigmatism
-off axis aberation causing vertical and horizontal lines to be focused in two different planes
-reduced slightly by stopping down
Distortion
-sharpness is not affected
Geometric distortion
-when the objects at the edge of a field of view are stretched to fit a film plane
Curvature of field
-lens cannot focus on image of a flat object on flatplane
-focused image is formed in a bowl shape
-stopping down slightly reduces this
