How to construct a ray diagram for depth of field

[JimKasson]

The PhotonsToPhotos Thick Lens Optical Bench is intended more as a tool for the construction of educational diagrams than an instrument of education itself.

If you set Pupil Magnifiication to 1 and Internodal distance to 0 you effectively have a thin lens system.



With everything drawn to scale the full diagram is not very revealing.



But you can zoom in on the Image plane



And the Object plane



Jim, I don't report depth of focus but could add that in the future.

I'm assuming the three colors are three wavelengths. Is that right?

Consider the drawing below for one wavelength, one focal length, and one subject distance:





It seems that if I move the focal plane the same amount forward and backwards, the CoCs are the same size. What am I missing here?

--

the last word the last word - Photography meets digital computer technology. Photography wins -- most of the time.

Photography meets digital computer technology. Photography wins -- most of the time.

blog.kasson.com

 

[bclaff]

Not wavelengths, distances; far, in-focus, near.

Note here with magnification 1/40th:



No longer appears symettrical.

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

 

[OpticsEngineer]

using different colors for different field points, or points along an optical axis is kind of conventional with optical softwares.

your question made me curious so I checked just now in OSLO and it is not an option in the GUI to color code rays according to wavelength (although one could always write a short OSLO C program to make the drawings come out that way if one wanted.)

 

[JimKasson]

using different colors for different field points, or points along an optical axis is kind of conventional with optical softwares.

Gotcha. But my question relates to the situation where there is a single subject distance, as in the diagram that I drew.

your question made me curious so I checked just now in OSLO and it is not an option in the GUI to color code rays according to wavelength (although one could always write a short OSLO C program to make the drawings come out that way if one wanted.)

 

[JimKasson]

Not wavelengths, distances; far, in-focus, near.

Note here with magnification 1/40th:



No longer appears symettrical.

That's a different situation than the one that I'm talking about.

--

the last word the last word - Photography meets digital computer technology. Photography wins -- most of the time.

Photography meets digital computer technology. Photography wins -- most of the time.

blog.kasson.com

 

[bclaff]

Not wavelengths, distances; far, in-focus, near.

Note here with magnification 1/40th:



No longer appears symettrical.

That's a different situation than the one that I'm talking about.

I thought that you were making a generalization. I suspect the lines are of equal length at 1x but not at other magnifications.

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

 

[JimKasson]

Not wavelengths, distances; far, in-focus, near.

Note here with magnification 1/40th:



No longer appears symettrical.

That's a different situation than the one that I'm talking about.

I thought that you were making a generalization. I suspect the lines are of equal length at 1x but not at other magnifications.

I was confused by your original diagram because the definition that I had been using for depth of focus is different than what you are using. The situation that you are describing is one where both the sensor and the subject can be in several places. I now understand what you mean, and I have no problem with it. I am talking about a different situation. I talk about depth of field in a situation where the lens and sensor remain in the same place and the subject can be at various distances. I talk about depth of focus in a situation where the lens and subject remain in the same place and the sensor can be at various distances from both. I asked above what I should be calling what I am now calling depth of focus. I am still unclear on that point.

--

the last word the last word - Photography meets digital computer technology. Photography wins -- most of the time.

Photography meets digital computer technology. Photography wins -- most of the time.

blog.kasson.com

 

[OpticsEngineer]

"...the lens and sensor remain in the same place and the subject can be at various distances."

this leads to the conventional definition of depth of field.

sometimes people accidentally say depth of focus when they mean depth of field.

I haven't been following this thread from the start, so I don't know what kind of discussions there have been.

Here is the picture out of Greenleaf "Photographic Optics" that has the conventional depth of field diagram. I think every place I have worked there has been a copy of this book in some kind of central bookshelf all the engineers had access to.

 

[Tom Axford]

...

Consider the drawing below for one wavelength, one focal length, and one subject distance:



It seems that if I move the focal plane the same amount forward and backwards, the CoCs are the same size. What am I missing here?

Hi Jim,

In this diagram, you are considering the image of a fixed point and seeing what happens if the sensor plane is moved. The blur size is the same on both sides of the focus point for equal distances away.

However, if you change the angle between those two marginal rays, then the distance that the sensor has to move (to get a certain blur size) will change.

The depth of focus situation is different because it is considering three different points of focus. The angle of convergence of the rays will be slightly different for each of those three points.

 

[Tom Axford]

"...the lens and sensor remain in the same place and the subject can be at various distances."

this leads to the conventional definition of depth of field.

sometimes people accidentally say depth of focus when they mean depth of field.

I haven't been following this thread from the start, so I don't know what kind of discussions there have been.

Here is the picture out of Greenleaf "Photographic Optics" that has the conventional depth of field diagram. I think every place I have worked there has been a copy of this book in some kind of central bookshelf all the engineers had access to.

The diagram in this book is probably typical of many DoF diagrams.

However, it relies on the use of the lens equation to work out where to put the object points. The diagram itself contains no clues about where to put them.

If you go back and read my OP, you will see that I was suggesting the use of a more complete diagram that does not rely on the lens equation. The location of the object points is constructed purely by the geometry.

 

[chrisfisheye]

"...the lens and sensor remain in the same place and the subject can be at various distances."

this leads to the conventional definition of depth of field.

sometimes people accidentally say depth of focus when they mean depth of field.

I haven't been following this thread from the start, so I don't know what kind of discussions there have been.

Here is the picture out of Greenleaf "Photographic Optics" that has the conventional depth of field diagram. I think every place I have worked there has been a copy of this book in some kind of central bookshelf all the engineers had access to.

The diagram in this book is probably typical of many DoF diagrams.

Not sure

However, it relies on the use of the lens equation to work out where to put the object points. The diagram itself contains no clues about where to put them.

If you go back and read my OP, you will see that I was suggesting the use of a more complete diagram that does not rely on the lens equation. The location of the object points is constructed purely by the geometry.

There are plenty of diagrams equivalent to your one which show dof by geometry construction.

https://graphics.stanford.edu/courses/cs178/applets/dof-depth-of-field.jpg

https://www.mdpi.com/2076-3417/14/7/2748

or even at dpreview:

https://www.dpreview.com/forums/thread/4468001

 

[Tom Axford]

"...the lens and sensor remain in the same place and the subject can be at various distances."

this leads to the conventional definition of depth of field.

sometimes people accidentally say depth of focus when they mean depth of field.

I haven't been following this thread from the start, so I don't know what kind of discussions there have been.

Here is the picture out of Greenleaf "Photographic Optics" that has the conventional depth of field diagram. I think every place I have worked there has been a copy of this book in some kind of central bookshelf all the engineers had access to.

The diagram in this book is probably typical of many DoF diagrams.

Not sure

However, it relies on the use of the lens equation to work out where to put the object points. The diagram itself contains no clues about where to put them.

If you go back and read my OP, you will see that I was suggesting the use of a more complete diagram that does not rely on the lens equation. The location of the object points is constructed purely by the geometry.

There are plenty of diagrams equivalent to your one which show dof by geometry construction.

https://graphics.stanford.edu/courses/cs178/applets/dof-depth-of-field.jpg

https://www.mdpi.com/2076-3417/14/7/2748

or even at dpreview:

https://www.dpreview.com/forums/thread/4468001

I wasn't claiming it to be original. The idea probably dates back to the nineteenth century.

 

[JimKasson]

...

Consider the drawing below for one wavelength, one focal length, and one subject distance:



It seems that if I move the focal plane the same amount forward and backwards, the CoCs are the same size. What am I missing here?

Hi Jim,

In this diagram, you are considering the image of a fixed point and seeing what happens if the sensor plane is moved. The blur size is the same on both sides of the focus point for equal distances away.

However, if you change the angle between those two marginal rays, then the distance that the sensor has to move (to get a certain blur size) will change.

The depth of focus situation is different because it is considering three different points of focus. The angle of convergence of the rays will be slightly different for each of those three points.

I understand that. What I have been asking is what to call the quantity that I am defining if it’s not depth of focus.

--

the last word the last word - Photography meets digital computer technology. Photography wins -- most of the time.

Photography meets digital computer technology. Photography wins -- most of the time.

blog.kasson.com

 

[Jack Hogan]

I understand that. What I have been asking is what to call the quantity that I am defining if it’s not depth of focus.

It seems closer to the concept of focus tolerance Jim, I've seen it used that way by e.g. Hopkins.

Jack

 

[JimKasson]

I understand that. What I have been asking is what to call the quantity that I am defining if it’s not depth of focus.

It seems closer to the concept of focus tolerance Jim, I've seen it used that way by e.g. Hopkins.

That sounds like a reasonable description. Or maybe focus plane tolerance? I will stop using the term depth of focus for that, but I don't want to discard the metric, since it is so easy to calculate, and so useful for stacking calculations.

 

[chrisfisheye]

I understand that. What I have been asking is what to call the quantity that I am defining if it’s not depth of focus.

It seems closer to the concept of focus tolerance Jim, I've seen it used that way by e.g. Hopkins.

That sounds like a reasonable description. Or maybe focus plane tolerance? I will stop using the term depth of focus for that, but I don't want to discard the metric, since it is so easy to calculate,

and so useful for stacking calculations.

Exactly.

This means that for focus bracketing you need to move the sensor plane with this distance (if you want the blur size to be smaller than CoC). Just do it as many times as needed for the scene.

With a focus ring, you can estimate the move required for each step.

Similarly, you can focus on a plane in the scene. This time, you move your focus point in the EVF with an equal distance for every step. Will not develop this method, it has been explained in other threads.

 

[Tom Axford]

Maybe you should stick to your original definition, Jim?

I have just noticed that Edmund Optics define depth of focus in the same way as you do.

To quote:

Due to similarity in name and nature, depth of field (DOF) and depth of focus are commonly confused concepts. To simplify the definitions, DOF concerns the image quality of a stationary lens as an object is repositioned, whereas depth of focus concerns a stationary object and a sensor’s ability to maintain focus for different sensor positions, including tilt.

 

[JimKasson]

Maybe you should stick to your original definition, Jim?

I have just noticed that Edmund Optics define depth of focus in the same way as you do.

To quote:

Due to similarity in name and nature, depth of field (DOF) and depth of focus are commonly confused concepts. To simplify the definitions, DOF concerns the image quality of a stationary lens as an object is repositioned, whereas depth of focus concerns a stationary object and a sensor’s ability to maintain focus for different sensor positions, including tilt.

Hmm…

Thanks.

 

[JimKasson]

Maybe you should stick to your original definition, Jim?

I have just noticed that Edmund Optics define depth of focus in the same way as you do.

To quote:

Due to similarity in name and nature, depth of field (DOF) and depth of focus are commonly confused concepts. To simplify the definitions, DOF concerns the image quality of a stationary lens as an object is repositioned, whereas depth of focus concerns a stationary object and a sensor’s ability to maintain focus for different sensor positions, including tilt.

It may be looking for your keys under the lamppost (ask me if you don't know the joke), but my definition has the advantage that it's really easy to calculate. You just need to know the f-stop and the CoC diameter, and you can do it in your head.