Lesson 07: Focus and Depth of Field

1. What Is Focus?

Focus (Focusing) is the process of adjusting the lens so that a specific part of the scene appears at its sharpest on the sensor. It is one of the most fundamental skills every photographer must master.

An image may be correctly exposed, but if the focus is off, it is still considered a failed shot. In many situations, nailing focus matters even more than getting the exposure right.

2. Autofocus Modes (AF)

Most modern cameras offer three main autofocus modes:

2.1. AF-S (One-Shot AF on Canon)

Single Autofocus — the camera locks focus once when you half-press the shutter button. Once focus is locked, if the subject or you move, the focus will not readjust automatically.

• Use for: Static subjects — portraits, landscapes, still life, architecture.
• Pros: Accurate, with time to confirm focus.
• Cons: Not suitable for moving subjects.

2.2. AF-C (AI Servo AF on Canon)

Continuous Autofocus — the camera continuously adjusts focus as the subject moves. Half-press the shutter button and the camera will track the subject continuously until you fully press the shutter.

• Use for: Moving subjects — sports, children, animals, birds in flight.
• Pros: Accurately tracks motion.
• Cons: Uses more battery; focus may hunt if the subject stops moving.

2.3. AF-A (AI Focus AF on Canon)

Automatic Autofocus — the camera automatically switches between AF-S and AF-C depending on subject movement. It starts in AF-S and automatically switches to AF-C when it detects motion.

• Use for: Mixed shooting — subjects that alternate between still and moving.
• Pros: Convenient, fully automatic.
• Cons: Sometimes the camera guesses wrong — less accurate than manual selection.

Focus Area Modes

Modern cameras offer several focus area modes:

• Single Point: A single focus point. Most precise with the best control.
• Zone: An area (group of points). A balance between precision and flexibility.
• Wide / Auto: The camera selects the point automatically. Fast but sometimes not what you intended.
• Eye AF / Face Detection: The camera automatically detects eyes/faces. Extremely useful for portraits.

3. Manual Focus (MF)

Manual Focus — you rotate the focusing ring on the lens yourself to adjust focus. It does not rely on the camera's AF system.

When to Use MF?

• Macro: AF often struggles to be accurate at very close distances. MF lets you control focus down to the millimeter.
• Low light: AF may fail to find enough contrast to lock on. MF lets you choose the focus yourself.
• Complex subjects: When there are multiple objects at different distances. AF may hunt between them.
• Landscape: Sometimes MF + hyperfocal distance is faster and more accurate than AF.
• Video: MF allows smooth, intentional focus pulls.

Manual Focus Aids

• Focus Peaking: A bright colored outline highlights the in-focus area on the EVF/monitor.
• Focus Magnification: Magnifies the focus area by 5×–10×.
• Split Screen: Splits the screen to compare focus.

4. Depth of Field (DOF)

Depth of Field (DOF) is the distance from the nearest to the farthest point that is considered acceptably sharp in the image.

In other words, when you focus on a single point, not only that point is sharp — there is a zone in front of and behind that point that is also sharp. That zone is the DOF.

Three Related Concepts

• Plane of Focus: The plane where everything is critically sharp.
• Near limit: The closest point still considered acceptably sharp.
• Far limit: The farthest point still considered acceptably sharp.

DOF = Far limit − Near limit

An important characteristic: DOF is not symmetrical. The distance from the plane of focus to the far limit is typically greater than the distance to the near limit. The typical distribution is 1/3 in front, 2/3 behind.

5. Three Factors That Affect DOF

5.1. Aperture

This is the most important and most easily controlled factor.

• Wide open aperture (f/1.4, f/2.8) → shallow DOF.
• Small stopped-down aperture (f/11, f/16) → deep DOF.

Example: At the same distance and focal length, f/1.8 produces a much shallower DOF than f/8.

5.2. Focal Length

• Short focal length (24mm, 35mm) → deep DOF.
• Long focal length (85mm, 135mm) → shallow DOF.

That is why telephoto lenses (85mm+) are often used for portraits with blurred backgrounds — at the same aperture, they produce a shallower DOF than wide-angle lenses.

5.3. Subject Distance

• Subject close to the camera → shallow DOF.
• Subject far from the camera → deep DOF.

This is why macro shooting (at very close distances) always yields an extremely shallow DOF — just a few millimeters. While landscape shooting (at far distances) yields a very deep DOF — nearly infinite.

Summary Table

6. Hyperfocal Distance

Hyperfocal distance is the focus distance at which everything from half that distance to infinity is considered acceptably sharp.

This is a key technique in landscape photography — it maximizes DOF without having to stop down to an extremely small aperture (avoiding diffraction).

Formula

Example

How to Apply It

1. Switch to Manual Focus (MF).
2. Look up the hyperfocal table for your focal length + aperture (or use an app).
3. Focus at the hyperfocal distance.
4. Shoot. Everything from half of H to infinity will be sharp.

7. Demo: DOF Simulator

Adjust the three sliders to see how the in-focus zone (DOF) changes. Trees inside the DOF zone will appear sharp; trees outside will gradually blur. Near/Far/DOF values update according to the optical formula.

8. Summary

After this lesson, you have learned:

• Autofocus: AF-S (static subjects), AF-C (moving subjects), AF-A (automatic).
• Manual focus: Used for macro, low light, and complex subjects.
• DOF: The in-focus zone before and after the plane of focus, asymmetrical (1/3 in front, 2/3 behind).
• Three factors that affect DOF: Aperture (primary), focal length, and subject distance.
• Hyperfocal distance: The optimal focus distance that keeps everything from H/2 to infinity sharp.

Hands-On Exercises

1. Shoot the same subject (portrait or still life) at three apertures: f/1.8 (or f/2.8), f/5.6, f/16. Compare the DOF.
2. Shoot the same subject at three focal lengths: 35mm, 50mm, 85mm — keep the same aperture and framing (move the camera so the subject occupies the same proportion of the frame). Compare the DOF.
3. Shoot a landscape using hyperfocal distance: set MF to H (look it up in a table), shoot and check whether the zone from H/2 to infinity is sharp.
4. Enable AF-C and photograph a person walking toward you. Then switch to AF-S and repeat. Compare the keeper rate.