Eric_D
Well-known member

Curvature of field is when you have to refocus if you move the binoculars such that the target is moved from the center to the edge of the field of view.First, I do not mean angular magnification distortion (AMD): that is perceived as straight lines bending as you move them away from diametrically across the center of the field, towards an edge. Not pincushion or barrel distortion. That is not 'curvature of field', IMO.
If you don't try refocussing, it can be perceived as unsharpness towards the edges, or a small 'sweet spot'.
In astronomy, everything is at infinity, near enough, so yes, they want a flat field. (After all, the night sky is a flat plane ... no ?)
Even photographers, when they set the lens at "50 meters", do not expect 50m object distance to be sharp from edge-to-edge: that would be a spherical focus surface, yet lenses are designed and tested on plane (flat) objects like brick walls.
Some binoculars, notably Zeiss Jenoptem, have very noticeable curvature of field, such that nearer objects away from the center are in focus.
Is this a bad thing? Have they deliberately left it in, perhaps to avoid compromising other things (FOV, off-axis sharpness)?
We rarely look at flat surfaces from a straight-on perpendicular angle.
Usually we are standing on a horizontal plane, where the foreground, below the horizon, is progressively closer to us as we look downwards.
I would even argue that we spend more time in streets and valleys, where things to either side are closer than central, distant objects.
We spend less time on ridges.
So "Is curvature of field a good thing?"
I have informally tested many (predominantly mid/low-end Japanese late-20th century Porro I ) binoculars, and found very few with the opposite, undesirable field curvature.
Is focussing closer at the edges seen as beneficial, a bit like the preference for pincushion rather than barrel distortion (AMD)?
Is it perhaps welcomed as (pseudo) 'Depth of field'?
I do not believe binoculars with the same '8x30' specification can really have greater or less depth-of-field: it is determined by the '8x30' numbers, lighting conditions and the user's eyes.
The entire discussion of edge sharpness often fails to recognize the most important optical element in the system: the human eye. Great eyes probably don't see much edge softness, whereas eyes that have lost the ability to readily accomodate see it immediately. I think the same can be said for our differing FOV perceptions.
John
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