Questionnaire #6

Eleven Questions




1. What is the maximum lens aperture?
The limit to how wide a lens can be opened is called the maximum aperture. It is indicated on the front, outer edge of a lens (1:1.4) What does this 1:1.4 indicate?
Ratio = Maximum diameter : lens focal length.
(e.g. if focal length is 50mm with a maximum aperture of 1.4, the aperture diameter is 35.7mm) and for every 1mm of aperture diameter there is 1.4mm of focal length.

2. If a 50mm lens has an aperture diameter of 25mm what f stop is it set to?
umm f/2 . . . ?


3. What is a lens aberration? and why do they exist?
Lens aberration is wavelengths of light not focusing at the same point.


4. What is spherical aberration? and how can it be reduced?
Spherical aberration fails to bring light to a single focus point. Stopping down your aperture can reduce this affect.


5. What are chromatic aberrations?
Chromatic aberrations happen when light passes through a solid (glass lens) and its individual colours are dispersed at slightly different focus points.


6. What do these lenses do?
a) achromatic.
b) apochromatic.
c) superachromatic.


7. What is coma?
Coma is an off-axis spherical aberration that creates a cone shaped point. It is a phenomenon where the diagonal light rays do not focus on one point of the image surface. Stopping down can reduce this slightly.


8. What is astigmatism?
This happens when off-axis aberration causes vertical and horizontal lines to be focused at two different planes.


9. What is distortion? and geometric distortion?
There are two main types of distortion found in lenses. Pin-cusion and Barrel distortion.


Geometric distortion occurs when things at the edge of a field of view are 'stretched' to fit (display) on a flat film plane. 


10. What is curvature of field?
Curvature of field happens when trying to focus on a flat image on a flat film plane. The result gives a blurred outer edge when focus is set to the centre and visa-versa. I t is commonly seen when enlarging a photograph.


11. What is a sweet spot on your lens?
Most lenses have one. It refers to the aperture range that produces the sharpest image. Normal rule of thumb is two stops from maximum aperture (e.g. maximum of f/2.8 - sweet spot of f/5.6).












D.