Depth of field is the part of a photo that looks acceptably sharp from front to back. Artists use it to guide attention, create mood, and separate a subject from a busy background. A portrait often uses shallow depth of field so the face is sharp and the background looks soft.
A landscape often uses deep depth of field so near and far objects both appear clear.
A camera controls depth of field mainly with aperture, focal length, and distance to the subject. A wide aperture, long focal length, or close subject distance usually makes depth of field shallower. A small aperture, short focal length, or farther subject distance usually makes depth of field deeper.
The blur happens because points outside the focus distance form larger circles on the camera sensor instead of tiny sharp points.
Key Facts
- Depth of field is the range of distances that appear acceptably sharp in a photo.
- Aperture size is written as f-number: f-number = focal length / aperture diameter.
- A smaller f-number such as f/2 means a wider aperture and shallower depth of field.
- A larger f-number such as f/16 means a smaller aperture and deeper depth of field.
- Longer focal length lenses, such as 100 mm, usually create stronger background blur than shorter focal length lenses, such as 24 mm.
- Moving closer to the subject decreases depth of field, while moving farther away increases depth of field.
Vocabulary
- Depth of field
- The distance range in a photo that appears acceptably sharp to the viewer.
- Aperture
- The adjustable opening inside a lens that controls how much light enters the camera.
- F-stop
- A number such as f/2.8 or f/11 that describes aperture size relative to focal length.
- Focal length
- The lens measurement in millimeters that affects magnification, angle of view, and depth of field.
- Circle of confusion
- The small blur circle formed on the sensor when a point is not perfectly in focus.
Common Mistakes to Avoid
- Thinking a bigger f-number means a bigger opening is wrong because f-stop is a ratio, so f/16 is a smaller aperture than f/2.
- Using the same focus settings for portraits and landscapes is wrong because portraits often need shallow depth of field, while landscapes usually need deep depth of field.
- Standing too close when trying to keep everything sharp is wrong because close subject distance makes depth of field much shallower.
- Assuming blur only comes from bad focusing is wrong because blur can also be created intentionally by aperture, focal length, and camera to subject distance.
Practice Questions
- 1 A 50 mm lens is set to f/2. What is the aperture diameter in millimeters using f-number = focal length / aperture diameter?
- 2 A 100 mm lens has an aperture diameter of 25 mm. What is the f-number?
- 3 A photographer wants a sharp mountain landscape with flowers in the foreground and mountains in the background. Should they choose f/2.8 or f/16, and should they focus only on the far mountains or consider the whole scene depth?