Photography Lens Optics and Selection 5 — Questions and Answers
Question 1: What happens to depth of field as you move closer to your subject, assuming aperture and focal length remain constant?
- Depth of field increases
- Depth of field decreases (Correct answer)
- Depth of field remains the same
- Depth of field is only affected by aperture, not distance
Correct answer: Depth of field decreases
As subject distance decreases, depth of field decreases—at very close focusing distances like macro photography, depth of field can be just millimeters deep.
Question 2: What does OIS (Optical Image Stabilization) do inside a lens?
- Electronically amplifies the signal from the sensor to reduce noise
- Uses gyroscopic sensors and moving lens elements to counteract camera shake (Correct answer)
- Locks the focus point so it cannot drift during handheld shooting
- Vibrates the lens at a frequency that cancels shutter vibration
Correct answer: Uses gyroscopic sensors and moving lens elements to counteract camera shake
OIS uses gyroscopic sensors to detect camera movement and shifts a floating lens element in the opposite direction to counteract handshake blur.
Question 3: A 'fast' lens refers to one that:
- Has a high-speed autofocus motor
- Has a large maximum aperture (low f-number) (Correct answer)
- Is physically lightweight and easy to carry
- Produces images with a high dynamic range
Correct answer: Has a large maximum aperture (low f-number)
A 'fast' lens has a large maximum aperture (such as f/1.4 or f/1.8), allowing more light to reach the sensor in a given time, enabling faster shutter speeds in low light.
Question 4: What is 'focus shift' in a lens?
- When autofocus moves the focal plane to a different subject accidentally
- A change in the point of focus when the aperture is stopped down from the focusing aperture (Correct answer)
- The time lag between pressing the shutter and achieving focus
- When the lens loses focus accuracy after being dropped
Correct answer: A change in the point of focus when the aperture is stopped down from the focusing aperture
Focus shift occurs in some lenses when spherical aberration changes as the aperture is stopped down, causing the focused image to shift slightly from where it was composed.
Question 5: What is the 'circle of confusion' in relation to depth of field?
- The circular bokeh shape produced by a lens at wide apertures
- The maximum acceptable blur spot diameter on the sensor for a point to be considered 'in focus' (Correct answer)
- A flare pattern that looks like overlapping circles
- The area of a scene that a fisheye lens cannot resolve
Correct answer: The maximum acceptable blur spot diameter on the sensor for a point to be considered 'in focus'
The circle of confusion defines how large a blur spot can be while still appearing acceptably sharp; depth of field calculations are based on this threshold.
Question 6: Which optical design reduces ghosting and flare in backlit situations?
- Aspherical elements
- Nano-crystal or multi-layer anti-reflective coatings (Correct answer)
- Fluorite elements
- Extra-low dispersion glass
Correct answer: Nano-crystal or multi-layer anti-reflective coatings
Multi-layer anti-reflective coatings (like Nikon's Nano Crystal Coat or Canon's ASC) reduce internal reflections between glass elements, minimizing ghosting and flare.
Question 7: When would a photographer choose a 'decentering' test for their lens?
- To check if the lens produces correct color rendition
- To determine whether the optical elements are perfectly aligned on the optical axis (Correct answer)
- To verify that the autofocus motor responds at the correct speed
- To measure the maximum magnification ratio of the lens
Correct answer: To determine whether the optical elements are perfectly aligned on the optical axis
A decentering test checks if lens elements are perfectly aligned; a decentered lens shows one corner or side sharper than the opposite side, which is a manufacturing defect.
Photography Lens Optics and Selection 5
What happens to depth of field as you move closer to your subject, assuming aperture and focal length remain constant?