With my binoculars, if I've focused on the center of the field, there is some softness to the view at the edges, as would be expected...however, if I refocus, I can bring the edges into a good deal sharper view (then putting the center out of focus, of course).
Is this what's generally meant when people talk about center vs. edge sharpness or resolution or performance - that the edges are "soft" because they are not actually in focus at the same time as the center - or are they comparing in-focus center vs. in-focus edge?
Is the "edge-to-edge sharpness" you often hear about with regards to Swarovski obtained because their center and edges are more in-focus with each other at the same time? Is this what's also referred to as a "flatter field"?
Are any other brands or designs generally considered better or worse with respect to this aspect of performance?
Does the "Depth of Field" people talk about have anything to do with this issue of center vs edge focus?
Thanks.
Hi Walter,
I will try to answer your questions.
Edge sharpness is judged by focusing on something in the middle of the field, and observing how sharp that same object, or others at the same distance, appears when positioned at the edge, without adjusting the focus.
What you describe is, its pure form, called field curvature. It is the only blurring aberration affecting the edge that can be eliminated by adjusting the focus.
The other common edge aberrations that cause edge blurring are astigmatism and coma. These will cause the image of a point source like a star to be deformed into characteristic shapes, which allow a skilled user to judge which of the two is dominant. These two cannot be eliminated by refocusing.
Most binoculars have a mixture of all three. There are some exceptions, in my limited experience. Fujinon FMT-SX 7x50, for example, has rather strong field curvature but almost no coma nor astigmatism at the edge. This can be somewhat useful, as objects at different distances will appear in focus at different positions within the field of view. The Swarovision EL is virtually free from all edge defects. Other people can give more examples of extreme cases. The original EL, and the Nikon SE are also very good, although I can't tell you how the various effects are mixed.
The absence of a curved field is, of course, a "flat field". But, this term is often misused to describe a binocular that is relatively free of all edge blurring effects.
There is no relationship, in the mathematical sense, between depth of field and any of these. But the subjective perception of depth of field can indeed depend on them, as well as the lighting, your eyesight, the phase of the moon and what you ate last.
Ron
[QUOTE=walternewton;2097911]With my binoculars, if I've focused on the center of the field, there is some softness to the view at the edges, as would be expected...however, if I refocus, I can bring the edges into a good deal sharper view (then putting the center out of focus, of course).
Is this what's generally meant when people talk about center vs. edge sharpness or resolution or performance - that the edges are "soft" because they are not actually in focus at the same time as the center - or are they comparing in-focus center vs. in-focus edge?[/quote]
Yes, that's field curvature. The focal plane of the objective and eyepiece don't line up so only one part is focused at a time.
The other feature that contributes to "bluriness" at the edge of field is astimatism: different focus points for tangential and saggital (radial) focus.
[QUOTE=walternewton;2097911]Is the "edge-to-edge sharpness" you often hear about with regards to Swarovski obtained because their center and edges are more in-focus with each other at the same time? Is this what's also referred to as a "flatter field"?
Are any other brands or designs generally considered better or worse with respect to this aspect of performance?[/quote]
Flat field and "edge to edge" sharpness are not the same thing. You can optimize a binocular design so it has flat field but astigmatism increase at the ege of field so it's not edge to edge sharp. Zeiss, I think, are like this.
Swaro and Fujinon and Nikon and Meopta (I must have missed someone) all have made "flat" (i.e. flatter field) and "edge to edge sharp bins to varying degrees. Henry Link as talked about some of the distinctions in their different approaches.
[QUOTE=walternewton;2097911]Does the "Depth of Field" people talk about have anything to do with this issue of center vs edge focus?[/QUOTE]
True DoF depends only on the magnification but this is never what people talk about when they say "depth of field".
Field curvature can have an effect and that effect varies with how much accommodation you have but I suspect that focuser speed (and user preference) has a larger (often unconscious) impact on how people perceive "depth of field".