How is the distace from subject to focal plane called?

J

Jose Rafael Perez Balen

New member
When I zoom, I'm changing the focal length. When I do a focus pull, what am i changing? How is that called?; the distance from the focal plane to the subject (as opposed to the distance from the optical center of the lens, to the focal plane)?
 
You are changing the focus distance. Sometimes referred to as the focal plane or subject to camera distance.

You keep saying focal plane to subject. It seems you are referring to (what I call) the sensor plane as the focal plane. To me, the sliver of space that the lens is focusing on is the focal plane.


What do you mean by "optical center of the lens, to the focal plane"?
 
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Jose Rafael Perez Balen said:
When I do a focus pull, what am i changing?
Click to expand...

You're changing the location of the focal plane. You could say you're changing its distance from the film plane or sensor.

The distance of the focal plane from the sensor/film determines what will be in focus and by how much. Take a look at this graphic.

depth-of-field.png

The depth of field is distributed such that 2/3 of the depth of field - what is in focus (or "acceptable" focus) - extends beyond the focal plane, and 1/3 of it extends back toward the camera.

Also, the further the focal plane from the camera, the greater the depth of field, since depth of field is determined by a fraction of focal plane's distance from camera.


I don't know if any of that helps. I thought a little background information might help you on your way to an answer.
 
WHOA!!! Hold on a second!

WHOA!!! Hold on a second!

Going back to my days as a Navy photographer (back in the early 70's), we were taught that the Focal Plane referred to the location of the film. You might have heard of "focal plane shutters". Cameras having that type of shutter had a shutter located immediately in front of the film. This differentiated them from cameras having "between-the-lens shutters". The old Speed Graphic press camera had a focal plane shutter, allowing it to have faster shutter speeds than it's less expensive cousin, the Crown Graphic, which used lenses with between-the-lens shutters. When 35mm SLRs came out, almost all had focal plane shutters. Only a very few used between-the-lens shutters (the Kodak Medalist might have been one). As for a term for where the lens is focused, I haven't heard of a consistently used term. You could say "the subject", but that assumed that the subject is in focus. :huh: Maybe "point of focus" or "plane of focus".

Getting back to the OP's question. When you're pulling focus, you're .... well, you're changing focus. With a simple lens, you're changing the distance between lens and film or sensor (I'll avoid saying "focal plane", to avoid confusion). As you change that distance you're "sort of" changing the effective focal length of the lens (kind of like zooming). This effect is called "breathing", and some modern cinema lenses are engineered to minimize this effect, since it can be distracting when doing a slip focus.

-- Ken
 
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Jordan Scott Price said:
You're changing the location of the focal plane. You could say you're changing its distance from the film plane or sensor.

The distance of the focal plane from the sensor/film determines what will be in focus and by how much. Take a look at this graphic.

View attachment 57365

The depth of field is distributed such that 2/3 of the depth of field - what is in focus (or "acceptable" focus) - extends beyond the focal plane, and 1/3 of it extends back toward the camera.

Also, the further the focal plane from the camera, the greater the depth of field, since depth of field is determined by a fraction of focal plane's distance from camera.


I don't know if any of that helps. I thought a little background information might help you on your way to an answer.
Click to expand...

just wanted to jump in and point out, for anyone reading, the diagram is a little misleading. The film/sensor plane is actually what you measure to for accurate focusing distance, rather than the "camera plane" that the diagram shows-- the sensor plane being a point IN the camera, behind the shutter (where the actual sensor or film is). Not a big difference, but the extra five or six inches between the front of the lens (as the diagram shows) and the actual sensor plane could put something out of focus.
 
As I understand it, the Focal Plane is where the film or sensor is,

convexlens1.jpg


which sometimes is called Image Plane, and you can find it's exact location in a camera by looking for a greek letter Phi.

film-plane.jpg


The thing is, I've heard a lot of people refer to the plane where the subject stands, as the Focal Plane. But all the articles and film manuals I found say the Focal Plane is the distance from lens to film plane/sensor; which leads me to believe there is a wide misconception of the term.

Why does this matter? Well, if the focal length -or distance- is the distance from lens to focal plane, changing the focal distance does different things depending on what you understand Focal Plane is (it either narrows -or widens- your field of view; or it makes sharper objects that are further away -or closer-).

I'm teaching a class on film directing, and we touch the subject of lens decisions for narrative impact, and there we touch on the subject of optics from a non-photographer point of view. But not having a clear definition -a common term for this concepts- is really doing a number on my students.

So, I guess my question is:

When you move the focus ring, you change the measurement is shows (let's say from 200 ft to 60 ft).

Which is the proper term for that distance?

focus-MVC-696S.jpg
 
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Jordan Scott Price said:
Bam! That is a lot less ambiguous. :)

And, yea, the graphic I posted was misleading - it was the result of a Google search at 2am. The film plane/sensor is not in front of the lens :)
Click to expand...

yeah i figured you were aware of it, just wanted to point it out for anyone who happened to be reading the thread
 
Jose,
I think the problem is that there is no one-word term for the distance from film plame to subject. In aerial reconnaissance, they use the term "altitude", but that's too specific to that field. For general photography, you could say "I am focused at 60 feet", or maybe "the focus distance is 60 feet". But it seems there is no universally agreed upon word for what you want.

-- Ken
 
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