The Focal Plane Error: Most of us understand that a lens can focus on only one distance at a time. The multiple “focus points” shown on many cameras represent the distance measurements...

IDguy Loc: Idaho

Most of us understand that a lens can focus on only one distance at a time. The multiple “focus points” shown on many cameras represent the distance measurements that the camera is using to determine what distance to focus at…which may not be any of the points. That misleads some photographers and contributes to some complaints about focus with some cameras or lenses. The issues are usually caused by the user.

But there is another point I can’t find any references to. There actually is no such thing as a focal PLANE. There is a focal distance. That distance represents a sphere around the camera. It isn’t flat. Knowing this can help set up some shots, like groups, especially if using narrow DOF (low fstop). You should curve your line to keep the distance to the camera the same.

Comments invited.

Siemienczuk

Relevant concept when using reframe technique (after focusing on center point) on a subject outside the center of the frame with narrow DoF.

crphoto8 Loc: Anaheim, California

A plane can be curved which is what you referred as focal distance.

KindaSpikey Loc: English living in San Diego

IDguy wrote:

Most of us understand that a lens can focus on onl... (show quote)

Thank you Idguy, nicely explained. I'm sure there's many of us who didn't realize this. Very helpful post.

BebuLamar

IDguy wrote:

Most of us understand that a lens can focus on onl... (show quote)

It's a plane and not part of a sphere. Otherwise you would need a lot of DOF to copy flat artwork. Because if it's a sphere enlarger would have to stop down to f/128??

brontodon

The focal plane is a flat plane onto which the lens focuses its image. The film or digital sensor is located on this plane. This is where the term "focal plane shutter" gets its name -- the shutter is located just in front of the focal plane, as opposed to an in-lens shutter, which is located (not surprisingly) somewhere within the lens.

JD750 Loc: SoCal

IDguy wrote:

Most of us understand that a lens can focus on onl... (show quote)

Yep that’s right, it’s a sphere not a plane. However given a small angle on the sphere it is approximately flat so I guess that’s where “focus plane” comes from. Maybe we should change the term to focus sphere? I’m certainly willing to do that.

And flash lighting works the same way. A given constant exposure value forms a sphere at some distance from the flash. But it’s not omnidirectional for a conventional speed light as they tend to be focused into a narrower beam.

rlscholl Loc: California

crphoto8 wrote:

A plane can be curved which is what you referred as focal distance.

A plane, by definition, is not curved. Rather it is a flat surface defined by any three points not in a line. The focal distance reference is to a portion of a spherical surface equidistant from a point.

IDguy Loc: Idaho

brontodon wrote:

The focal plane is a flat plane onto which the lens focuses its image. The film or digital sensor is located on this plane. This is where the term "focal plane shutter" gets its name -- the shutter is located just in front of the focal plane, as opposed to an in-lens shutter, which is located (not surprisingly) somewhere within the lens.

You are confusing terms.

IDguy Loc: Idaho

JD750 wrote:

Yep that’s right, it’s a sphere not a plane. However given a small angle on the sphere it is approximately flat so I guess that’s where “focus plane” comes from. Maybe we should change the term to focus sphere? I’m certainly willing to do that.

And flash lighting works the same way. A given constant exposure value forms a sphere at some distance from the flash. But it’s not omnidirectional for a conventional speed light as they tend to be focused into a narrower beam.

Right about the flash. Focusing the beam is another matter.

Light enters the lens from all directions. It is correct to use focal distance for the camera.

KindaSpikey Loc: English living in San Diego

IDguy wrote:

You are confusing terms.

... And confusing my mind!

IDguy Loc: Idaho

JD750 wrote:

Yep that’s right, it’s a sphere not a plane. However given a small angle on the sphere it is approximately flat so I guess that’s where “focus plane” comes from. Maybe we should change the term to focus sphere? I’m certainly willing to do that.

And flash lighting works the same way. A given constant exposure value forms a sphere at some distance from the flash. But it’s not omnidirectional for a conventional speed light as they tend to be focused into a narrower beam.

The curve can be significant if using low fstop causing shallow DOF. Some like to do that for groups to blur background. Then they complain the folks at the edges are out of focus.

IDguy Loc: Idaho

BebuLamar wrote:

It's a plane and not part of a sphere. Otherwise you would need a lot of DOF to copy flat artwork. Because if it's a sphere enlarger would have to stop down to f/128??

The curvature depends on distance. If taking artwork at close range you should account for it. Otherwise the edges will be OOF: fuzzy.

Digital enlargement doesn’t use fstop.

If you back up with a longer lens the effect is less.

pkipnis

There is a little circle with a line through it symbol on the top of your camera that indicates where the focal plane ie: is where the light rays converge that point is the focal point. From this distance you would measure to subject if you wanted to do critical focusing such as scientific imaging. Cinematographers Focus not by image, but but with thay distance to the subject. Often times they'll set the shot up using that symbol on the camera to measure to the subject to get accurate focusing I do sit there. The feels

Orphoto Loc: Oregon

IDguy wrote:

You are confusing terms.

Not so fast. The lens does have front and back focal planes. The back plane is located at the sensor/film.