Photography Lens Optics and Selection 4 — Questions and Answers
Question 1: What is 'spherical aberration' and how does it affect image quality?
- Light rays passing through lens edges focus at a different point than those through the center, causing softness (Correct answer)
- The image curves at the edges due to field curvature
- Highlights bloom into large circular shapes
- The lens produces circular flare at all apertures
Correct answer: Light rays passing through lens edges focus at a different point than those through the center, causing softness
Spherical aberration occurs because curved lens surfaces cannot focus all incoming rays to a single point, resulting in reduced sharpness and contrast, especially at wide apertures.
Question 2: An 85mm lens on a crop sensor camera with a 1.5x crop factor has an equivalent focal length of:
- 57mm
- 85mm
- 127.5mm (Correct answer)
- 170mm
Correct answer: 127.5mm
Multiplying 85mm by the 1.5x crop factor gives an equivalent full-frame field of view of 127.5mm.
Question 3: What is the primary purpose of a lens hood?
- To protect the front element from rain and scratches only
- To reduce flare by blocking stray light from entering the lens at oblique angles (Correct answer)
- To act as a counterweight for telephoto balance
- To mark the correct way to mount the lens
Correct answer: To reduce flare by blocking stray light from entering the lens at oblique angles
A lens hood blocks stray light from outside the frame from hitting the front element at angles that cause flare and reduce contrast.
Question 4: What does 'field curvature' mean as a lens aberration?
- The inability of a lens to focus on objects at the edges of the frame simultaneously with the center (Correct answer)
- The bending of straight lines at the edge of the frame
- A color shift that appears only in the corners
- Uneven exposure brightness across the image circle
Correct answer: The inability of a lens to focus on objects at the edges of the frame simultaneously with the center
Field curvature means the plane of sharp focus is curved rather than flat, so when the center of an image is sharp, the corners may be out of focus and vice versa.
Question 5: Which lens mount characteristic allows a lens to be adapted to multiple camera systems?
- The focal length of the lens
- The rear flange distance (flange focal distance) (Correct answer)
- The maximum aperture of the lens
- The number of electrical contacts on the mount
Correct answer: The rear flange distance (flange focal distance)
A shorter flange focal distance on a camera body (common in mirrorless systems) allows adapters to be used for lenses with longer flange distances from other systems.
Question 6: What is 'coma' as an optical aberration?
- A lens defect causing point light sources near edges to appear comet-shaped or smeared (Correct answer)
- Loss of contrast in the center of the frame due to reflections
- Color fringing that only affects blue wavelengths
- A focus shift when switching from live view to viewfinder
Correct answer: A lens defect causing point light sources near edges to appear comet-shaped or smeared
Coma causes off-axis point light sources—like stars—to appear as comet-like smears or fan shapes rather than sharp points, and is most visible at wide apertures.
Question 7: For environmental portraits that include background context, which focal length is typically preferred?
- 14mm
- 35mm (Correct answer)
- 200mm
- 400mm
Correct answer: 35mm
A 35mm lens provides a natural perspective with moderate field of view, allowing the subject and their surrounding environment to be included in the frame without excessive distortion.
Photography Lens Optics and Selection 4
What is 'spherical aberration' and how does it affect image quality?