Lesson 8 : 3.5.2010 - Maximum Lens Aperture
Maximum Lens Aperture
- How large the hole opens
- Figure is shown on the lens
- Ratio 1:1.4 = Max Aperture Diameter : Lens Focal Length
eg. If lens focal length = 50mm the max aperture would be 1:4 and the aperture diameter would be 35.7
- For every 1mm of aperture diameter there is 1:4mm of focal length.
Relative Lens Apertrue - Each f number is related to the lens focal length
f2 on a 25mm lens has aperture diameter of 12.5mm
f2 on a 50mm lens has aperture diameter of 25mm
f2 on a 100mm lens has aperture diameter of 50mm
Lens Aberations is an optical error
- no lens is perfect
- many aberrations can be corrected by inducing
- alternative errors in the lens
- some errors are welcomed - soft focus lens
Why do aberrations exist?
- light is bent or refracted as it passes through glass
- amount of refraction varies with wavelength of light
- different wavelengths do not focus at same point
Chromatic Aberration (Colour)
- light passing through glass is dispersed into its colours
- axial (logitudinal) and transverse (lateral)
- common in long and wide lenses
- addition of flourite in glass reduces this
Coma
- an off-axis spherical aberration creating cone shaped points
- stopping down can slightly reduce effect
- can be reduced through deliberate use of other aberrations
Astigmatism
- off-axis aberration causing vertical and horizontal lines to to be focused in two different planes
- reduced slightly by stopping down
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