DoF - straight or curved?

[Bend The Light]

I was just looking through some old images, and this one taken with a MF Vivitar 90mm macro, at 1:1.
I noticed the thin "focal plane" is curved.
Is this right, and the focal plane is actually curved, or is it distortion which is not so evident in other lenses or something?


Unidentified dragonfly at Ynis-Hir 2 by http://bendthelight.me.uk, on Flickr

Thanks.

 

[Kolia]

Interesting.

Could it be a factor of lens design ? I've never noticed that with my 105mm macro...

 

[bratkinson]

When I bought my first 35mm SLR 38 years ago, the store owner talked me into a 50mm, flat-field macro lens for it. I used it for 'normal' (aka, everything) until I started buying zooms, etc 3-4 months later. While I never used it for macro work other than to 'see what it could do', I did have occassion to use the flat-field aspects of the lens while photographing historic documents at the Smithsonian Institution in Washington DC. Everything was in focus, even with a wide-open lens and the image filling the frame.

So, on that basis, I'd have to say that 'normal' lenses have a sligthly curved plane of sharpest focus (DOF). That would make sense as the individual lenses inside the lens body are all curved one way or the other. But unless one is shooting with a very, very thin DOF, the curvature is probably so minor as it is largely unnoticed.

 

[Bend The Light]

Interesting.

Could it be a factor of lens design ? I've never noticed that with my 105mm macro...

When I bought my first 35mm SLR 38 years ago, the store owner talked me into a 50mm, flat-field macro lens for it. I used it for 'normal' (aka, everything) until I started buying zooms, etc 3-4 months later. While I never used it for macro work other than to 'see what it could do', I did have occassion to use the flat-field aspects of the lens while photographing historic documents at the Smithsonian Institution in Washington DC. Everything was in focus, even with a wide-open lens and the image filling the frame.

So, on that basis, I'd have to say that 'normal' lenses have a sligthly curved plane of sharpest focus (DOF). That would make sense as the individual lenses inside the lens body are all curved one way or the other. But unless one is shooting with a very, very thin DOF, the curvature is probably so minor as it is largely unnoticed.

Yes, makes sense.

The Focal Plane is a flat plane passing through the focal distance, but this can't always be the DoF which is in focus on some lenses. I decided that the field that is in focus cannot be a plane, but a locus, so I decided to call it a "Focus Locus"

 

[Helen B]

Though most lenses have a design aim of being flat field, few achieve it - lens design is all about compromises and priorities. The field that is in focus (we are talking about the plane of focus, not DoF) is often slightly curved, and the curve can go either way. It need not be the same shape at all focus distances (it can curve forwards at close distances and backwards at a long distance, for example) and it can be wavy, going both in front and behind the theoretical flat plane. Field curvature is often the cause of lower MTF away from the lens axis (ie apparent unsharpness away from the axis). Proper macro lenses usually have a flat field as a design priority.

The DoF becomes curved because the plane (or surface) of focus is curved - DoF is about things that are out of the plane of focus (ie not focussed) but appear to be in focus when the image is viewed.

 

[jake337]

In the picture you posted the peice of wood is curved giving the illusion of a curved DOF.

 

[Helen B]

What about the focus on the wings?

 

[480sparky39 38]

In the picture you posted the peice of wood is curved giving the illusion of a curved DOF.

That's my vote.

 

[Patrice]

The plane of focus may not be as curved as it appears in this image. Notice how the camera is above and at an angle to the subject and the surface it's on. To help explain a bit of what is going on measure down at an angle to the surface of a table from a point about 3 or 4 inches above it. From that point trace where a string of constant length falls on the table, it will describe an arc much as you see in the image.

 

[jake337]

What about the focus on the wings?

Are you asking me?

Seems as he was looking somewhat down on the subject. Maybe a 15-30 degree angle from the surface?

 

[Helen B]

Wouldn't that string method show a curved surface rather than a flat one?

 

[jake337]

Like in this shot it is more pronounced. My DOF did a 90 degree bend!

​

Take a picture of a marble hanging in the air at your widest aperture at 1:1. Your DOF will seem to wrap completely around it.

 

[Bend The Light]

Though most lenses have a design aim of being flat field, few achieve it - lens design is all about compromises and priorities. The field that is in focus (we are talking about the plane of focus, not DoF) is often slightly curved, and the curve can go either way. It need not be the same shape at all focus distances (it can curve forwards at close distances and backwards at a long distance, for example) and it can be wavy, going both in front and behind the theoretical flat plane. Field curvature is often the cause of lower MTF away from the lens axis (ie apparent unsharpness away from the axis). Proper macro lenses usually have a flat field as a design priority.

The DoF becomes curved because the plane (or surface) of focus is curved - DoF is about things that are out of the plane of focus (ie not focussed) but appear to be in focus when the image is viewed.

In the picture you posted the peice of wood is curved giving the illusion of a curved DOF.

What about the focus on the wings?

In the picture you posted the peice of wood is curved giving the illusion of a curved DOF.

That's my vote.

The plane of focus may not be as curved as it appears in this image. Notice how the camera is above and at an angle to the subject and the surface it's on. To help explain a bit of what is going on measure down at an angle to the surface of a table from a point about 3 or 4 inches above it. From that point trace where a string of constant length falls on the table, it will describe an arc much as you see in the image.

What about the focus on the wings?

Are you asking me?

Seems as he was looking somewhat down on the subject. Maybe a 15-30 degree angle from the surface?

Wouldn't that string method show a curved surface rather than a flat one?

The wood is not curved.

I didn't call it a focal plane, because to me a plane is flat. This is not. Angling the camera down will angle a flat plane down at the same angle and will intersect the wood in a straight line, just as the camera being perfectly horizontal with a proper flat Focal plane will mean the focal plane draws a straight line perpendicular to the camera's axis.

Since this "area" where there is focussed points is not in a plane, I will call it a Focus Locus.

and one day I will maybe afford a lens with optics giving a focal plane rather than a focus locus.

 

[BrianV]

I'm amazed that a Macro lens shows such high field curvature. A Sonnar- this is typical.

Macro lenses are designed for flatness of field. This one is showing a lot of field curvature. Lenses designed for use with film had to make assumptions about film flatness, and allow for some bend. I can't imagine a lens designer depending on curvature of film to produce flatness of field in the image- but it is possible.

 

[jake337]

Though most lenses have a design aim of being flat field, few achieve it - lens design is all about compromises and priorities. The field that is in focus (we are talking about the plane of focus, not DoF) is often slightly curved, and the curve can go either way. It need not be the same shape at all focus distances (it can curve forwards at close distances and backwards at a long distance, for example) and it can be wavy, going both in front and behind the theoretical flat plane. Field curvature is often the cause of lower MTF away from the lens axis (ie apparent unsharpness away from the axis). Proper macro lenses usually have a flat field as a design priority.

The DoF becomes curved because the plane (or surface) of focus is curved - DoF is about things that are out of the plane of focus (ie not focussed) but appear to be in focus when the image is viewed.

In the picture you posted the peice of wood is curved giving the illusion of a curved DOF.

Are you asking me?

Seems as he was looking somewhat down on the subject. Maybe a 15-30 degree angle from the surface?

Wouldn't that string method show a curved surface rather than a flat one?

The wood is not curved.

I didn't call it a focal plane, because to me a plane is flat. This is not. Angling the camera down will angle a flat plane down at the same angle and will intersect the wood in a straight line, just as the camera being perfectly horizontal with a proper flat Focal plane will mean the focal plane draws a straight line perpendicular to the camera's axis.

Since this "area" where there is focussed points is not in a plane, I will call it a Focus Locus.

and one day I will maybe afford a lens with optics giving a focal plane rather than a focus locus.

Are you sure that wood is not curved. I'll beleive you if you say yes but how is it the wood has field curvature, but the wings are not showing the same bend?