[walternewton]

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.

[ronh]

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

[Kevin Purcell]

Originally Posted by walternewton

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?

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.

Originally Posted by walternewton

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?

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.

Originally Posted by walternewton

Does the "Depth of Field" people talk about have anything to do with this issue of center vs edge focus?

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".

[H K]

Depth of field is more of a camera term , meaning more depth to the focused area . The opposite would be shallow DOF in a photo where only the object focused on is sharp .This will vary with power and size of objectives .

Flat field is more optical quality, Yes its total focus at one range . Some lenses have a pin cushion focus ,the center is sharper than the edges .Its More bragging rights , as we move the glasses toward sharp center naturally .


I'm using more telescope terms for this . The key aspects are large objectives ( within reason ) crisp focus that pops in and out with a definite sharp point . The dim / soft focus point that covers a half turn of the knob indicates a lower performing unit .

While brand names and warranties mean more to some people . A good report from a consumer has more quality than a large ad .

[John S.]

Hi. Depth of field is the range in front of and behind an object that you have focused on that remains reasonably well in focus. You can shift that range back and forth by focusing on another object. Depth of field is a term more commonly used by photographers but it has applicability with binocular users too.

[Kevin Purcell]

Originally Posted by John S.

Hi. Depth of field is the range in front of and behind an object that you have focused on that remains reasonably well in focus. You can shift that range back and forth by focusing on another object. Depth of field is a term more commonly used by photographers but it has applicability with binocular users too.

Photographic DoF has a precise definition (so you don't need the "reasonably sharp" fuzziness). It is the depth of focus for which the circle of confusion is smaller than the resolution of the detector i.e. where a focused point remains smaller than the resolution of the eye for binoculars. The only thing that varies photographic DOF in bins in the magnification (not the focal ratio ... an example showing binoculars are not cameras).

This is not what people use when they're talking about "DOF" in bins (as I mentioned above) as the true DoF is tiny. If you can see the bird being slightly fuzzy eithe rside of focus that's the true DoF.

The perceived snap into focus will be affected by focuser rate too and corelates to what some people call DoF.

The quality of the out of focus aberrations also affect how people perceive the DoF. If the astigmatism and coma stay low out of focus then the eye's accommodation can overcome the effect.

The amount of field curvature that can be "focused out" by the observer varies with the age of the viewer so the perceived DOF varies for the same bin.

Ultimately the perceived DOF is so observer dependent that it's very difficult to say anything useful about it unless you deal with it in components (i.e. the focuser rate and speed).

A check on threads in BF about DOF show these effects and variations!

[John S.]

Originally Posted by Kevin Purcell

Photographic DoF has a precise definition (so you don't need the "reasonably sharp" fuzziness). It is the depth of focus for which the circle of confusion is smaller than the resolution of the detector i.e. where a focused point remains smaller than the resolution of the eye for binoculars. The only thing that varies photographic DOF in bins in the magnification (not the focal ratio ... an example showing binoculars are not cameras).

This is not what people use when they're talking about "DOF" in bins (as I mentioned above) as the true DoF is tiny. If you can see the bird being slightly fuzzy eithe rside of focus that's the true DoF.

perceived snap into focus will be affected by focuser rate too and corelates to what some people call DoF.

The quality of the out of focus aberrations also affect how people perceive the DoF. If tThe he astigmatism and coma stay low out of focus then the eye's accommodation can overcome the effect.

The amount of field curvature that can be "focused out" by the observer varies with the age of the viewer so the perceived DOF varies for the same bin.

Ultimately the perceived DOF is so observer dependent that it's very difficult to say anything useful about it unless you deal with it in components (i.e. the focuser rate and speed).

A check on threads in BF about DOF show these effects and variations!

For some reason you choosing to make a very simple concept seem very complex. It isn't. Anyone who has viewed a bird in the wild knows that some of the foliage in front and behind the bird will be in focus but the further you get from the bird without refocusing the less clear it will be.

[Kevin Purcell]

John S

I'm not making it complicated. I'm clarifying a situation that most people think the depth of field varies between different bins of the same magnification.

It doesn't. The DoF only varies with the magnification. Nothing else. e.g. it's the same for all 8x bins.

But people still perceive different bins of the same magnification have different "DoFs" (perceived depth of field).

That's the difference I'm explaining.

It's subtle difference and not as simple as you want to make out.

Is that clear enough?

May I suggest you review some of the many threads we've already had on this topic on BF?

[John S.]

Originally Posted by Kevin Purcell

John S

I'm not making it complicated. I'm clarifying a situation that most people think the depth of field varies between different bins of the same magnification.

It doesn't. The DoF only varies with the magnification. Nothing else. e.g. it's the same for all 8x bins.

But people still perceive different bins of the same magnification have different "DoFs" (perceived depth of field).

That's the difference I'm explaining.

It's subtle difference and not as simple as you want to make out.

Is that clear enough?

May I suggest you review some of the many threads we've already had on this topic on BF?

Sigh. Before you get too carried away in your justification please go back and re-read what you originally posted.

[lilcrazy2]

Walter
Depth of field is generally referred to by most viewers as the distance in front of and behind the object being viewed that is in sharp focus. Thus, if you look at an object at 10yds, 100 yds and 1000yds, this apparent DOF will get larger as the distance to the object increases.
Kevin is correct in what he says about the perceived DOF among different viewers of different bins, in that field curvature can give a perception of a larger DOF. However, " perceived" DOF is also dependent on the visual acuity of the user as well as the distance to the object being viewed. Actual DOF is only governed by magnification, so as magnification increases, actual DOF decreases.

To address your question on center sharpness, all binos have a "sweetspot" where the center is sharp for a certain percentage of the field of view, and then starts to soften and becomes unsharp as you move to the edges. As a general rule, more expensive binos have a larger sweetspot, but there are many roof prisms binos in the $200 range that have very large sweetspots and give extraordinaly good optical performance, such as the Zen Ray ZRS HD, Bushnell Legend Ultra HD, Alpen Apex, just to name a few.

Here is a link to some charts that will show you percentages of the sweetspot, compliments of another member.
http://www.birdforum.net/showthread.php?t=173767

[walternewton]

Thanks guys for the replies.