What does turning a lens focus ring physically change inside the lens?
Asked 9/7/2014
5 views
2 answers
0
When you turn a lens focus ring in manual focus, what is actually happening inside the lens? Does it just change the distance at which the image is focused, so that subjects at that distance are sharp? And is the range that still looks sharp what we call depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
3
Yes, the manual focus mode on lenses lets you manually change the point of focus. Actually what you're setting is the plane of focus, an entire plane, perpendicular to the optical axis, where the image is in focus.
The distance to the plane of focus is not constant, though - the larger the angle from the optical axis, the longer light has to travel to get to the camera from the plane of focus.
The "tolerance" you're referring to is called the depth of field which is the distance between the closest and furthest objects in an image that appear acceptably sharp.
What physically is going on in the lens is that by turning the ficus ring, is that lens elements within the lens move towards the front or back of the lens, changing the plane of focus. That the plane of focus does change when moving the lens elements is a result of optics.
Originally by user21986. Source · Licensed CC BY-SA 4.0
user21986
11y ago
0
Generated from our catalog & community — verify before relying on it.
Turning the focus ring moves one or more optical elements inside the lens forward or backward. That changes where the lens projects a sharp image onto the camera’s sensor.
In practical terms, you are setting the plane of focus: an imaginary plane in the scene where subjects are sharply focused. It is often described as being perpendicular to the optical axis of the lens.
Anything not exactly on that plane is not perfectly focused, but some objects in front of or behind it may still look acceptably sharp. That acceptable range is the depth of field.
So yes: adjusting the focus ring changes the focusing distance, but more precisely it changes the optical relationship between the lens and sensor by repositioning lens elements, which shifts the plane of focus closer or farther away.
Recommended products
UniqueBot
AI11y ago
Your Answer
Related Questions
What changes when you focus a fixed-focal-length lens, and how does that relate to depth of field?
What does a lens’s minimum focus distance actually mean if depth of field extends closer?
How can a prime lens focus at different distances if its focal length is fixed?
Can turning the focus ring in AF mode damage a lens?
Can a Logitech C615 webcam be adjusted for better infinity focus?