John A Roberts
Well-known member
B) Field Curvature
i.e. does the sharpness of focus on an object change, depending on the object’s location in the FOV?
Spherical optics typically project a curved image for viewing, rather than a flat one. This is known as Petzval field curvature, see at: Petzval field curvature - Wikipedia
n.b. while a 2 dimensional representation of the projected image is shown as a curve, in 3 dimensional space the image forms part of a spherical surface.
The first image shows the application to photography, with a flat sensor/ film surface. Moving away from the centre of the optical axis, the focus curve is increasingly in front of the flat surface, and so the image is increasingly out of focus. And although this is less of a problem where a receptor is also curved in a similar manner - such as the interior surface of the human eye - the problem still remains.
The second image shows that with more complex optics, as is common with many modern photographic lenses, the projected image may take on flatter but more complex characteristics *
In practice, as with geometric distortion, there are also varying degrees of field curvature/ field flatness. In addition, an increasing deterioration of sharpness towards the edge of the FOV, is often a combination of both focus variation and astigmatism. The difference being:
• the first can be corrected by refocusing (though with a consequent loss of central focus), but;
• the second cannot.
Roger Vine routinely addresses these considerations in his binocular reviews (and usually also image distortion), under the heading 'Flat field?'. They can be found at: Binocular Reviews
Field flatness may be an advantage for certain uses e.g. especially astronomy, or where a binocular is mainly used from a tripod. However, for many other uses it does not offer a significant advantage, and with hand-held use there is a natural tendency to position an object of interest in the centre of the FOV. So for many users the choice of a flat field of view is as much personal preference as practical advantage.
* This presumedly can be related to the complex optics of the Swarovski EL SV eyepiece - where at least early on - there was the presence of an area of reduced sharpness aka the 'Absam Ring' e.g. see the first post at: Distortion Characteristics of Three Swarovision Models
- - - -
Traditionally, binoculars have had more simple eyepieces compared to other optics, including many telescopes and camera lenses. Consequently, even well after WWII, binocular optics often exhibited significant degrees of field curvature.
As indicated, Swarovski introduction the flat field EL Swarovision in 2010, which led to the proliferation of similar featured binoculars from other manufacturers. However, there are also much earlier examples.
In the mid 1980’s Nikon launched the 8x40 DCF roof prism (with the SE Porro prism following in 1995), see a cross-section image of the DCF in post #37 by Henry at: Flat Field technology: the hows, the why's, the consequences
And a set of images of eyepieces of other early examples can be found in posts #35 and 36 also by Henry at: Field flattener binoculars
As can be seen from the images, flat field optics typically require more complex eyepieces.
Finally, circling back to my opening comments . . .
The distinction between geometric distortion and field flatness - as described by Swarovski in its own literature (both immediately following the introduction of the EL SV, and continuing to now) - would not be clear to many casual readers.
John
Images from:
1st & 2nd: What is Field Curvature?
3rd and 4th from 2011 Swarovski Optik catalogue
5th from 2012 EL Swarovision brochure
6th from 2021 EL Legend brochure
i.e. does the sharpness of focus on an object change, depending on the object’s location in the FOV?
Spherical optics typically project a curved image for viewing, rather than a flat one. This is known as Petzval field curvature, see at: Petzval field curvature - Wikipedia
n.b. while a 2 dimensional representation of the projected image is shown as a curve, in 3 dimensional space the image forms part of a spherical surface.
The first image shows the application to photography, with a flat sensor/ film surface. Moving away from the centre of the optical axis, the focus curve is increasingly in front of the flat surface, and so the image is increasingly out of focus. And although this is less of a problem where a receptor is also curved in a similar manner - such as the interior surface of the human eye - the problem still remains.
The second image shows that with more complex optics, as is common with many modern photographic lenses, the projected image may take on flatter but more complex characteristics *
In practice, as with geometric distortion, there are also varying degrees of field curvature/ field flatness. In addition, an increasing deterioration of sharpness towards the edge of the FOV, is often a combination of both focus variation and astigmatism. The difference being:
• the first can be corrected by refocusing (though with a consequent loss of central focus), but;
• the second cannot.
Roger Vine routinely addresses these considerations in his binocular reviews (and usually also image distortion), under the heading 'Flat field?'. They can be found at: Binocular Reviews
Field flatness may be an advantage for certain uses e.g. especially astronomy, or where a binocular is mainly used from a tripod. However, for many other uses it does not offer a significant advantage, and with hand-held use there is a natural tendency to position an object of interest in the centre of the FOV. So for many users the choice of a flat field of view is as much personal preference as practical advantage.
* This presumedly can be related to the complex optics of the Swarovski EL SV eyepiece - where at least early on - there was the presence of an area of reduced sharpness aka the 'Absam Ring' e.g. see the first post at: Distortion Characteristics of Three Swarovision Models
- - - -
Traditionally, binoculars have had more simple eyepieces compared to other optics, including many telescopes and camera lenses. Consequently, even well after WWII, binocular optics often exhibited significant degrees of field curvature.
As indicated, Swarovski introduction the flat field EL Swarovision in 2010, which led to the proliferation of similar featured binoculars from other manufacturers. However, there are also much earlier examples.
In the mid 1980’s Nikon launched the 8x40 DCF roof prism (with the SE Porro prism following in 1995), see a cross-section image of the DCF in post #37 by Henry at: Flat Field technology: the hows, the why's, the consequences
And a set of images of eyepieces of other early examples can be found in posts #35 and 36 also by Henry at: Field flattener binoculars
As can be seen from the images, flat field optics typically require more complex eyepieces.
Finally, circling back to my opening comments . . .
The distinction between geometric distortion and field flatness - as described by Swarovski in its own literature (both immediately following the introduction of the EL SV, and continuing to now) - would not be clear to many casual readers.
John
Images from:
1st & 2nd: What is Field Curvature?
3rd and 4th from 2011 Swarovski Optik catalogue
5th from 2012 EL Swarovision brochure
6th from 2021 EL Legend brochure
Attachments
Last edited:
