Focus and depth of field
Focusing means deciding which part of the image will be sharp and, therefore, where the viewer will look. Depth of field is how far that sharp zone extends in front of and behind the focus point. Mastering both is what separates a photo where everything competes from one with a clear subject.
Where and how the camera focuses
Autofocus (AF) needs you to tell it where to focus. You can let the camera choose, but as soon as the scene has several elements it often gets it wrong. The most reliable option to start with is a single focus point that you place on the subject; on a phone, just tap the screen.
There are two basic modes. AF-S (One Shot on Canon) focuses once and locks: it's ideal for still subjects and for focusing and recomposing. AF-C (AI Servo on Canon) keeps recalculating focus while you hold the shutter button halfway, and it's essential for moving people, sports or animals. Today's cameras add face and eye detection, which is very useful for portraits.
In a portrait, always focus on the eye closest to the camera.
Focus areas
As well as the mode, the camera lets you choose the focus area, that is, which part of the frame it uses to focus. The names vary by brand, but the options are always the same:
| Area | How it works | Use it for |
|---|---|---|
| Single point | You choose one point and the camera focuses exactly there. | Landscapes, posed portraits, products and architecture: maximum precision. |
| Zone or dynamic | You choose a point or a group of points; if the subject moves, the neighboring points help track it. | Sports, animals and street, combined with AF-C. |
| Automatic | The camera decides what to focus on across the whole frame, usually the closest or most contrasty thing. | Quick, simple situations. With several elements, it often gets it wrong. |
| Tracking with subject detection | Recognizes faces, eyes, animals or vehicles and follows them around the frame. | Portraits, wildlife and sports on recent cameras: the big help of the last few years. |
Separating focus from the shutter button
By default, half-pressing the shutter button makes the camera focus and meter the light. Many professionals prefer back-button focus: they assign focusing to a button on the back of the camera, usually AF-ON, and leave the shutter button just for taking the picture.
The advantage is twofold: you can focus once and shoot as many times as you like without the camera refocusing, or hold the button down to track a subject in AF-C. It takes a few days to get used to, but afterwards it's hard to go back.
Same aperture, f/2. Only the focus point changes.
The four factors of depth of field
One important point before we start: in reality, only one plane is perfectly in focus. Depth of field is the zone where the blur is so small that the eye can't tell, which in optics is called the circle of confusion. That's why a photo that looks sharp on a phone may not be sharp printed large.
The sharp zone depends on four factors worth remembering together. Aperture: the wider it is, the shallower the depth of field. Distance: the closer you focus, the shallower. Focal length: with the subject the same size in the frame, a telephoto blurs and isolates the background better. And sensor size: with the same framing and aperture, a larger sensor blurs more.
This explains why a phone, with a tiny sensor and very wide-angle optics, gets almost everything sharp and needs portrait mode to simulate blur, while a full-frame camera with an 85 mm at f/1.8 easily isolates a person.
To calculate it precisely there are depth of field calculators, like the one in the PhotoPills app, which tell you where the sharp zone begins and ends for your camera, lens, aperture and focus distance.
Wide aperture, short distance and long focal length: maximum background blur.
Which aperture to use in each situation
These are common starting points among professionals, not fixed rules:
| Situation | Aperture | Why |
|---|---|---|
| Single-person portrait | f/1.8 to f/2.8 | Separates the person from the background |
| Group of people | f/5.6 to f/8 | Every face falls inside the sharp zone |
| Landscape | f/8 to f/11 | Sharp from front to back without diffraction |
| Close objects and products | f/8 to f/16 | Up close, the sharp zone is tiny |
| Sports | f/2.8 to f/4 | Allows fast shutter speeds and isolates the athlete |
Manual focus and common mistakes
Manual focus (MF) is still useful in low light, through glass or in macro. To do it precisely, magnify the image on the screen and turn on focus peaking, which highlights in-focus edges in color.
The most common mistake isn't technical but one of method: focusing on the center, recomposing at a very wide aperture and not checking the result. At f/1.8 and close range, that small turn of the camera is enough to throw the eye out of focus. Move the focus point instead of recomposing.
Why a photo isn't sharp
When a photo comes out soft, zoom in to 100% and look for clues. Each cause leaves a different fingerprint:
| Symptom | Likely cause | Fix |
|---|---|---|
| Something is sharp, but not what you wanted | Missed focus: the camera focused in front of or behind the subject. | Use a single point on the subject or eye detection. |
| Everything is blurred in the same direction | Camera shake: the camera moved during the exposure. | Use a faster shutter speed, turn on stabilization or use a tripod. |
| The background is sharp and the subject is blurred | The subject moved faster than the shutter speed could freeze. | Speed up the shutter: 1/500 or faster for action. |
| The subject is sharp, but nearby parts aren't | Not enough depth of field. | Stop down or step back a little. |
| The whole photo looks soft at f/16 or f/22 | Diffraction. | Go back to f/8 or f/11. |
| Fine detail smeared and grainy | Noise from high ISO. | Lower the ISO and compensate with aperture or shutter speed. |
| Milky image, with halos and little contrast | A dirty lens or filter, or light hitting the lens head-on. | Clean the lens and use the lens hood. |
Real and perceived sharpness
The sense of sharpness depends on two different things. Resolution is the amount of fine detail the lens and sensor record. Acutance is edge contrast: the more abrupt the transition between a light and a dark area, the sharper the image looks.
That's why the sharpening slider in editing programs works by increasing edge contrast: it improves acutance but doesn't create detail that wasn't recorded. AI tools can hide slight blur, but they can't rescue a badly focused photo.
Sharpness is achieved when you shoot; editing only finishes it off.
Your turn: two planes, two readings
A table and two objects are all you need.
- Place one object about 20 inches (50 cm) from the camera and another about 5 feet (1.5 m) behind it.
- Focus on the first and shoot; focus on the second and shoot.
- Repeat both photos at f/2.8 and at f/11 (on a phone, with and without portrait mode).
- Compare the four images: which one tells what you wanted to show best, and why?
- Check the photos zoomed to 100% and, if any isn't sharp, identify the cause with the table in this lesson.