Chapter 4 — Quiz

Twenty-six questions to check that focus and depth of field have become controls you command rather than accidents you survive. Mix of multiple choice, true/false (always justify), short answer, and several "what's wrong here?" seeing questions. Answer before checking the key; the explanation matters more than the letter.

Multiple choice

Q1. A lens can render in true, perfect sharpness: - (a) the entire scene at once - (b) everything within the depth of field - (c) a single distance — one flat plane parallel to the sensor - (d) only the nearest object

Q2. To make depth of field shallower (less in focus), you should: - (a) stop down to a larger f-number - (b) open up to a smaller f-number, get closer, and use a longer focal length - (c) move farther from the subject and use a wider lens - (d) raise the ISO

Q3. Which f-number gives the deepest depth of field? - (a) f/1.8 - (b) f/2.8 - (c) f/5.6 - (d) f/16

Q4. You are photographing a child running toward you. The correct autofocus mode is: - (a) AF-S / single / One-Shot - (b) AF-C / continuous / AI Servo - (c) manual focus locked at one distance - (d) it does not matter as long as the shutter is fast

Q5. The circle of confusion is best described as: - (a) the blurry ring some cheap lenses produce - (b) the largest blur disc a point can render and still look acceptably sharp to a viewer - (c) the aperture's iris when it is wide open - (d) the zone of the scene that is out of focus

Q6. Bokeh refers to: - (a) the amount of the scene that is in focus - (b) the aesthetic quality of the out-of-focus areas - (c) a specific f-number - (d) the camera's autofocus accuracy

Q7. For a front-to-back-sharp landscape, the recommended aperture sweet spot on most lenses is about: - (a) f/1.4–f/2 - (b) f/4–f/5.6 - (c) f/8–f/11 - (d) f/22–f/32

Q8. The depth of field around the focal plane is distributed roughly: - (a) evenly — half in front, half behind - (b) about one-third in front of the plane, two-thirds behind it - (c) about two-thirds in front, one-third behind - (d) entirely behind the plane

Q9. The single most reliable rule for focusing on a living subject is: - (a) focus on the center of the body - (b) focus on the nearest eye - (c) focus on the brightest part of the frame - (d) let auto-area AF decide

Q10. Stopping all the way down to f/22 to maximize sharpness can backfire because of: - (a) motion blur - (b) noise - (c) diffraction softening the whole image - (d) chromatic aberration only

Q11. Why does a phone keep almost everything sharp without trying (deep native depth of field)? - (a) its software sharpens everything - (b) its very small sensor uses a very short actual focal length, which deepens depth of field - (c) its lenses are always set to f/16 - (d) it focuses on infinity by default

Q12. A bright point of light in the out-of-focus background of a wide-aperture shot renders as: - (a) a sharp point - (b) a soft glowing disc or orb (bokeh) - (c) a black dot - (d) a lens flare streak

Q13. The most precise way to focus a static subject on a tripod is usually: - (a) auto-area autofocus - (b) focus-and-recompose at the widest aperture - (c) manual focus using magnified live view (and/or focus peaking) - (d) trusting the green focus-confirm dot in the viewfinder

Q14. Eye-detection autofocus is most worth turning on for: - (a) landscapes - (b) portraits of people (and pets, where animal-eye AF exists) - (c) architecture - (d) star photography

True / False — and justify each in one sentence

Q15. A wide aperture (small f-number) alone guarantees a dissolved background.

Q16. Focus-and-recompose is always safe, at any aperture and distance.

Q17. "In focus" is an exact physical property of the light, the same for every viewer.

Q18. Moving closer to your subject makes the depth of field shallower.

Q19. Manual focus is obsolete now that autofocus exists.

Q20. Focusing on the most distant object in a landscape gives you the most front-to-back sharpness.

Short answer

Q21. Name the three levers of depth of field and, for each, state which direction makes depth of field shallower.

Q22. Explain in two sentences the difference between AF-S and AF-C, and give one subject suited to each.

Q23. What is back-button focus, and name one concrete advantage it gives the photographer.

Q24. Define the hyperfocal distance, and state where the near and far edges of sharpness fall when you focus there.

What's wrong here? (seeing questions)

Q25. A photographer wants a head-and-shoulders portrait with melted background. They set f/1.8 and a 85mm-equivalent lens, but stand with the subject pressed up against a brick wall. The result: the subject is sharp but the wall behind their head is still distractingly in focus, and opening the aperture further does nothing. What is the actual problem, and what is the fix? (One sentence each.)

Q26. A portrait shot at f/1.4 looks almost sharp, but on close inspection the eyelashes and tip of the nose are crisp while the eyes themselves are very slightly soft — and the ear is perfectly sharp. The camera was in AF-S, and the photographer used focus-and-recompose, swinging the camera up and over after locking on the center of the face. Diagnose what happened and prescribe the fix.

Answer Key

Q1 — (c). A lens focuses exactly one distance perfectly; that flat sheet is the focal plane. Everything else is, strictly, blurred, and only accepted as sharp within the depth of field.

Q2 — (b). All three levers pushed toward shallow: open up, get closer, go longer. (a) and (c) deepen DoF; ISO (d) has nothing to do with depth of field.

Q3 — (d). Bigger f-number = smaller hole = deeper depth of field. f/16 is the deepest of the four.

Q4 — (b). A subject moving toward you needs continuous AF (AF-C / AI Servo) to keep re-driving the focal plane; AF-S would lock on the first position and the child would walk out of the sharp zone.

Q5 — (b). The circle of confusion is the largest blur disc that still reads as a sharp point at normal viewing size — the threshold that defines "in focus."

Q6 — (b). Bokeh is the quality (smooth vs. busy) of the blur, especially of out-of-focus highlights; the amount of blur is depth of field.

Q7 — (c). f/8–f/11 gives deep DoF without significant diffraction softening; f/22+ loses sharpness to diffraction.

Q8 — (b). Roughly one-third of the depth of field falls in front of the focal plane and two-thirds behind — which is why you focus into a scene, not on the horizon.

Q9 — (b). Focus on the nearest eye; it is where viewers look first, and a sharp eye reads the whole portrait as sharp.

Q10 — (c). Past about f/11–f/16, diffraction bends light at the aperture edge and softens fine detail across the entire frame.

Q11 — (b). The tiny sensor uses a very short actual focal length to fill its frame, which yields a deep depth of field — almost everything is sharp, so phones fake shallow DoF in software.

Q12 — (b). Out-of-focus highlights render as soft discs/orbs — the visible signature of bokeh.

Q13 — (c). Magnified live view (with focus peaking) is the most precise focusing method for static subjects and beats autofocus for landscape and product work.

Q14 — (b). Eye-detection AF finds and tracks the subject's near eye — transformative for portraits.

Q15 — False. Aperture is only one of three levers; if the subject is close to the background or the photographer is far from the subject, even f/1.8 leaves the background recognizable. Distance matters at least as much.

Q16 — False. It fails at very shallow depth of field and close distances, where swinging the camera to recompose shifts the focal plane and softens the subject; use a moved focus point or back-button focus.

Q17 — False. "In focus" is a threshold judgment — any blur disc under the circle of confusion reads as sharp — and it depends on viewing size and the viewer, not just the light.

Q18 — True. Closer focus distance gives shallower depth of field; this is why macro has almost none.

Q19 — False. Manual focus wins where autofocus can't lock — low light, through glass/fences, macro, stars, and deliberate pre-focus/zone focusing.

Q20 — False. Focusing on infinity wastes the two-thirds of DoF that extends behind the plane and loses the foreground; focusing nearer (the hyperfocal distance, or one-third in) gives more total front-to-back sharpness.

Q21. Aperture (open up / smaller f-number → shallower); distance to subject (closer → shallower); focal length (longer → shallower). (Bonus: a nearer background also reads sharper, so pushing the background away makes it dissolve more.)

Q22. AF-S focuses once and locks (for still subjects — portraits, landscapes); AF-C focuses continuously to track motion (for moving subjects — sports, kids, wildlife).

Q23. Back-button focus moves focusing to a rear button so the shutter only fires; advantages include recomposing freely without refocusing, switching between locked and tracking focus on the fly, and never firing a frame while the lens is hunting.

Q24. The hyperfocal distance is the focus distance at which depth of field extends from infinity to its nearest possible point; focus there and sharpness runs from half that distance (H/2) out to infinity.

Q25. Problem: the background is too close to the subject — it sits inside the sharp zone, so no aperture will dissolve it. Fix: move the subject away from the wall (and/or step closer to the subject), so the background falls far behind the depth of field.

Q26. Diagnosis: at f/1.4 the depth of field is paper-thin; locking on the center of the face and then arcing the camera up shifted the focal plane off the eyes (focus-and-recompose failing at shallow DoF up close), landing it on the nose/lashes and ear instead. Fix: place the AF point directly on the near eye (use eye-detection AF, or move the single point) and do not recompose — or use back-button focus to hold it.

Topics to review by question