Chapter 4 — Key Takeaways

The screenshot-and-reread card for focus and depth of field. Skim it before any shoot where what's sharp matters — which is every shoot.

The one rule

Decide what the photograph is about → make that the sharp thing → choose how much of the rest shares its sharpness. Open up and get close to isolate it; stop down and focus a third in to keep it all.

For any living subject, the sharp thing is the nearest eye. Always.

Definitions (the terms this chapter owns)

The three levers of depth of field (memorize as a set)

Stack for extremes: - Max background melt: long lens + widest aperture + close to subject + background far behind. - Everything sharp: wide lens + f/8–f/11 + farther back + focus at the hyperfocal distance (≈ ⅓ in).

If the background won't dissolve, the answer is almost always distance, not aperture — move closer to the subject and move the subject away from the wall.

Autofocus: choose the mode and the area

Is the subject moving toward/away from you?
   NO  → AF-S (single): focus once, lock, recompose if needed, shoot.   [portrait, landscape, still life]
   YES → AF-C (continuous): hold, let it track, fire a burst at peak.   [kids, pets, sport, traffic]
   Unsure on a one-off person who might move → AF-C.

Turn eye-detection ON for every portrait. It is the one piece of AF automation that earns its keep nearly every time.

⚙️ Settings boxes

Isolate a subject (shallow DoF):

Everything sharp (deep DoF landscape):

Phone, both cases: tap to focus, tap-and-hold to lock (AF/AE). For shallow: telephoto lens or Portrait mode + big gap to background. For deep: it's automatic — the tiny sensor keeps almost everything sharp.

When to use what (decision aid)

Top common mistakes → fixes

Two facts that explain everything

1. "In focus" is a threshold, not a fact. Only the focal plane is truly sharp; everything within the circle-of-confusion threshold reads as sharp. That tolerance band is depth of field.
2. Depth of field is ~⅓ in front of the focal plane, ~⅔ behind it. So focus into a scene, never on the farthest thing.

Sensor-size reality check (Gear Note, distilled)

• Bigger sensor → easier shallow DoF (same framing needs a longer actual focal length or closer distance — both deepen blur).
• Phone's tiny sensor → deep native DoF (very short actual focal length). That's a gift for landscapes, street, and grab shots — and the reason phones must fake background blur in software (Portrait mode; full machinery in Chapter 23).

Portfolio increment

Add one shallow-depth-of-field image — subject tack-sharp, background dissolved. Stack the three levers (widest aperture, close, longest lens, background far behind), nail focus on the most important point (the eye), and keep the frame where the subject is unmistakably sharp and the background is unmistakably gone. Label it 04-shallow-focus; write one sentence on what you wanted the viewer to look at and how focus made them. Set it beside your Chapter 3 manual frame — a controlled image that is also a creative choice.

Connections

• Built on: the camera and sensor (Chapter 2); aperture and the exposure triangle (Chapter 3) — every aperture change for DoF changes the light, so shutter/ISO must answer.
• Sets up: light and seeing (Chapter 5); composition and the eye-path (Chapter 6); perspective and compression (Chapter 9); macro's collapsed DoF and focus stacking (Chapter 19); continuous-AF tracking of action (Chapter 20); zone focusing on the street (Chapter 17); synthetic bokeh and depth maps (Chapter 23).