What is the correct term for the distance a lens is focused at?
Asked 1/31/2016
8 views
2 answers
0
I thought “focal length” meant the distance to the subject that will appear sharp, but it seems focal length actually describes a property of the lens and its field of view rather than the subject distance.
What is the correct term for the distance you set with the focus ring? For example, if I want a subject 3 meters away to appear sharp, what parameter is set to 3 meters? Also, what do related terms like depth of field and working distance mean?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
17
Focal length is the distance between the lens and the sensor when the subject is in focus, not the distance to the subject.
The term for the distance to the subject in focus is the focus distance and is measured from the image plane (sensor/film plane). The distance from the lens to the subject is called the working distance which can be significantly less within the context of macro photography. The zone which appears in focus either side (front and back) of the subject is the depth of field. This varies with the aperture - depth of field increases as the aperture gets smaller (f-number gets larger). All else being equal, depth of field is greater at f/4 than at f/2.
So if you focus on an object 3 meters away with a focal length of 18mm and aperture of f/11, everything from 1m to infinity will be in focus. However, if you focus on the same subject with the same aperture with a focal length of 135mm, the near focus limit is 2.9m and the far focus limit is 3.1m - the depth of field is only 20cm deep, in other words.
Originally by user3205. Source · Licensed CC BY-SA 4.0
user3205
10y ago
0
Generated from our catalog & community — verify before relying on it.
The term you’re looking for is focus distance (also called subject distance in some contexts): the distance from the camera’s image plane/sensor to the subject that is in focus.
Focal length is different. It’s an optical property of the lens, related to its angle of view/magnification, not the distance to the subject you’re focusing on.
A few related terms:
- Focus distance: the distance at which the lens is focused.
- Depth of field: the range in front of and behind that focus distance that appears acceptably sharp.
- Working distance: the physical distance from the front of the lens to the subject; this can matter a lot in macro photography.
At infinity focus, very distant subjects are sharp, and for a simple lens this corresponds to the sensor being about one focal length behind the lens. To focus on closer subjects, the lens elements move so the image stays sharp on the sensor.
So if you want something 3 meters away to be sharp, you set the lens’s focus distance to about 3 m.
Recommended products
UniqueBot
AI10y ago
Your Answer
Related Questions
What determines depth of field in a photograph?
What does a lens’s focal length actually mean, and why isn’t it always the lens-to-sensor distance?
Does focal length change perspective if camera-to-subject distance stays the same?
Is a lens’s stated focal length a physical property, or a 35mm-equivalent value?
How do I calculate the fixed copy-stand height for photographing an A5 page with a smartphone?