Product Photog Lenses Focal Length Depth Field F Stop
Focal Length
Focal length is a core concept in optics and photography. It defines the distance (in millimeters) between the lens’s optical center and the camera sensor (or film) when the lens is focused at infinity.
🧠 Simple Definition:
Focal length = the distance from the lens to the image sensor when focused on a subject far away.
🔍 What It Actually Means:
- It determines how much of a scene the lens captures (field of view).
- It also affects how large objects appear (magnification).
📏 Units:
Focal length is measured in millimeters (mm) and is always specified for a lens:
- Wide angle = short focal length (e.g. 14mm, 24mm)
- Normal = around 50mm (approximates human eye)
- Telephoto = long focal length (e.g. 85mm, 200mm, 600mm)
🔭 Examples:
| Focal Length | Type | Field of View | Effect on Image |
|---|---|---|---|
| 14mm | Ultra-wide | Very wide | Expansive view, distorted edges |
| 35mm | Wide angle | Wide | Good for street, documentary |
| 50mm | Normal | Natural perspective | What the eye sees |
| 85mm | Short tele | Narrow | Great for portraits, soft background |
| 200mm+ | Telephoto | Very narrow | Brings far things close |
📐 Relationship to Perspective:
Focal length does not change perspective — camera position does.
But:
- Wide lenses (short focal lengths) exaggerate space — things appear farther apart.
- Telephoto lenses compress space — things appear closer together.
📸 In Summary:
- Focal length tells you the angle of view and magnification of a lens.
- It’s the key factor in whether you’re capturing a wide scene or zooming in tight.
- It directly influences composition, background blur, and depth perception.
The f-stop (or f-number) is a fundamental concept in photography that relates to the aperture size of a lens and how it affects exposure and depth of field.
📷 What is F-Stop?
The f-stop is a ratio that describes the size of the aperture opening in a camera lens relative to the focal length of the lens.
✅ Formula:
$$
f\text{-number} = \frac{f}{D}
$$
- f = focal length of the lens (in mm)
- D = diameter of the aperture opening (in mm)
So:
- A lower f-number (e.g. f/2.0) = larger aperture
- A higher f-number (e.g. f/16) = smaller aperture
🔍 Meaning in Practical Terms:
| f-Stop | Aperture Size | Light Allowed In | Depth of Field |
|---|---|---|---|
| f/1.4 | Very large | A lot | Very shallow |
| f/2.8 | Large | More | Shallow |
| f/8 | Medium | Moderate | Moderate |
| f/16 | Small | Less | Deep |
| f/22 | Very small | Very little | Very deep |
🎯 Relation to Distance to Focus Plane (Depth of Field)
The f-stop indirectly affects how much of the scene, from front to back, appears in focus — this is called depth of field (DOF).
- Large aperture (small f-stop, e.g. f/1.8) → shallow DOF → only a narrow range (near focus plane) is sharp.
- Small aperture (large f-stop, e.g. f/16) → deep DOF → wide range (near and far from focus plane) is sharp.
Distance to focus plane = how far the subject you’re focusing on is from the lens.
- The farther the focus distance, the deeper the DOF — even at the same f-stop.
- At a given f-stop, focusing at 5 feet gives shallower DOF than focusing at 50 feet.
🧠 Intuition:
Think of the f-stop as controlling a cone of sharpness radiating out from the camera:
- A small f-stop (large hole) gives a narrow cone — only a thin slice of distance is in focus.
- A large f-stop (small hole) gives a wide cone — more distances from the camera are in focus.
🗝️ In Summary:
- F-stop = focal length / aperture diameter
- It sets how wide the aperture opens
- Controls both:
- Exposure (light entering the lens)
- Depth of Field (how much front-to-back appears sharp)
- The focus plane is where objects are crisply sharp; DOF defines how much distance around it also appears sharp — and this is strongly shaped by your f-stop setting.