Field correction question (1 Viewer)

There's been some discussion of field correction choices lately. I don't mean to run it into the ground, but while folks are thinking about it seems like a good time to raise my question.

I use a Fujinon FMT-SX 7x50, mostly for astronomy. The F stands for "flat field", according to Fujinon. It shows almost no distortion of long straight lines, like telephone poles, near the edge of the field. It also exhibits the "rolling globe" effect when panning in the daytime. As Henry Link explained, this is like the perspective seen looking down on a flat checkerboard. So, it makes sense to call the field flat.

But, the binocular exhibits another effect, related to the sharpness at the edge of the field. My eyes have very little focus accomodation, and I find that if I focus an object in the center, and move it to the edge, it appears blurred. But, it can be made sharp again, by refocusing to a more distant setting, different by 2 diopters, read off the eyepiece scales. This makes sense in light of the above checkerboard, since the edge of the flat checkerboard is farther away than the center.

Now, I don't really have any problem accepting the reality of this situation, but I don't know what to call it. Over on the CN forum, this kind of edge blur that can be focused away, unlike astigmatism or coma, is called "field curvature".

So it appears that the binocular could be said to simultaneously possess a field that is flat, and curved. Of course that is nonsense, but it points out a semantic question. What are these things properly called, so we can communicate clearly when talking about this stuff?

Thanks, Ron

The F apparently stands for "sorta flat for older people"

But, the binocular exhibits another effect, related to the sharpness at the edge of the field. My eyes have very little focus accomodation, and I find that if I focus an object in the center, and move it to the edge, it appears blurred. But, it can be made sharp again, by refocusing to a more distant setting, different by 2 diopters, read off the eyepiece scales. This makes sense in light of the above checkerboard, since the edge of the flat checkerboard is farther away than the center.

Click to expand...

What you describe is field curvature.

http://www.mellesgriot.com/products/optics/fo_3_2_4.htm
http://www.telescope-optics.net/curvature.htm

A "flat field" is describing the flatness of the focal plane of the objective (and the eyepiece ... both need to be flat or to compensate for each other to get the edge of the field in focus at the same time as the edge).

Flat is relative especially when used in marketing and perhaps targeted at the young who have enough accommodation to make this "go away" (i.e. you change the focus in your eyes to compensate).


Astigmatism (in this sense usually coma) is a aberration that maps one point to many points in the image.

You can separate field curvature and astigmatism at the edge of the field by looking at how the image sharpens as you change focus. You can compensate for field curvature with the focus. What's left is astigmatism.

Distortion, like pincushion distortion, is the moving of points about in the image plane. It has nothing to do with focus or bluriness (i.e. the mapping is one point to one point not one to many).

http://www.telescope-optics.net/distortion.htm

People do seem to confuse the field curvature/flat field and no distortion. Perhaps this is why the marketers choose to confuse the two, as well?

More reading. The wikipedia article is OK (not perfect)

http://scienceworld.wolfram.com/physics/OpticalAberrations.html
http://en.wikipedia.org/wiki/Optical_aberration
http://toothwalker.org/optics/astigmatism.html
http://www.mellesgriot.com/products/optics/fo_3_2.htm

ronh said:

There's been some discussion of field correction choices lately. I don't mean to run it into the ground, but while folks are thinking about it seems like a good time to raise my question.

I use a Fujinon FMT-SX 7x50, mostly for astronomy. The F stands for "flat field", according to Fujinon. It shows almost no distortion of long straight lines, like telephone poles, near the edge of the field. It also exhibits the "rolling globe" effect when panning in the daytime. As Henry Link explained, this is like the perspective seen looking down on a flat checkerboard. So, it makes sense to call the field flat.

But, the binocular exhibits another effect, related to the sharpness at the edge of the field. My eyes have very little focus accomodation, and I find that if I focus an object in the center, and move it to the edge, it appears blurred. But, it can be made sharp again, by refocusing to a more distant setting, different by 2 diopters, read off the eyepiece scales. This makes sense in light of the above checkerboard, since the edge of the flat checkerboard is farther away than the center.

Now, I don't really have any problem accepting the reality of this situation, but I don't know what to call it. Over on the CN forum, this kind of edge blur that can be focused away, unlike astigmatism or coma, is called "field curvature".

So it appears that the binocular could be said to simultaneously possess a field that is flat, and curved. Of course that is nonsense, but it points out a semantic question. What are these things properly called, so we can communicate clearly when talking about this stuff?

Thanks, Ron

Click to expand...

The first reference you make, that to long lines appearing straight at the edges, is a reference to distortion. Either pincushion or barrel distortion will bow lines at the edges. This is not a reference to flat field, or field curvature. Distortion is present because the magnification at the edges is different than the magnification in the center. Distortion does not affect the image of the star, it simply moves it in the field of view. Generally, distortion cannot even be seen in astronomy, but can usually be seen when the binocular is used for looking at terrestrial objects where we are accustomed to seeing nice straight line edges of things like buildings and poles.


Field curvature appears when the outside of the field of view has a different focal point than the center. The eyepiece is moved in or out and the focal plane of the eyepiece, "plane" being the key word, can only focus on one plane at a time. Well, the objective has one focal length and it delivers a curved arc of points. The eyepiece plane cannot coincide with the objective focal "plane" arc for all points at the same time. In other words, you can't get both the center and the edge in focus at the same time but you can get one or the other in focus. That is a "lack" of flat field. That is field curvature.

Curvature of field (change in focus off-axis) can be reduced by using a field flattener lens. Without the flattener, the outer portions of the field will be slightly out of focus when the center is in focus. Instead of pinpoints in the outer field, this will have the tendency to enlarge and blur the circles of light that form the stars in the outer field. They would be slightly out of focus. (Slightly enlarged circles, that is assuming no other aberrations are present). In a terrestrial view, the objects are not points, so its much more difficult to measure, but the out of focus is there and can easily be seen. A field flattener can be used to insure that the entire field is in focus at the same time. Stars in the outer field will be focused and appear just as stars in the center (again, assuming no other aberrations are present).

The field flattener does not eliminate spherical aberration, astigmatism, coma or chromatic aberration. The field flattener has no affect on distortion (lines).

edz

(Ron)...So it appears that the binocular could be said to simultaneously possess a field that is flat, and curved. Of course that is nonsense, but it points out a semantic question. What are these things properly called, so we can communicate clearly when talking about this stuff?

Click to expand...

Correct. A binocular simultaneously possess some degree of (1) distortion and (2) field curvature. They can be thought of as independent aberrations, and what we see is a combination of the two. Usually a "flat field" is associated with eliminating field curvature, which means that object points located on a plane surface would be in focus on a plane surface in image space (i.e., the Gaussian surface). This can be accomplished by a combination of lenses, or a corrective lens know as a field flattener. Nikon uses them a lot.

What makes straight lines change shape, or be un-flat, so to speak, is distortion. This is due to a gradient in image magnification from the center of the lens to the edge. The result (for positive or pincushion distortion) is that individual points on an extended line will show up proportionately further from the center of the (Gaussian) image plane than in object space. This is what produces the "fun house" perceptual effect, but has nothing to do with the points being in or out of focus.

One aspect of distortion, which Holger Merlitz has also discussed at some length, is that it interferes with exact determination of the apparent field of view. If there is uneven magnification across the field it must effect how many spatial points can be imaged through the field stop of the instrument. Hence, to know FOV one must first know distortion (Nikon's "new" AFOV metric not withstanding.)

To address your semantic question, then, we should probably talk about field curvature and distortion, rather than a "flat field," at least when referring to the properties of the instrument.

Unfortunately, however, there is also the human perceptual side of things, which is quite a bit more confusing. Here, I would posit that the perception of flatness depends on what is being judged, lines and/or surfaces, or even free-floating impressions. How these perceptions relate to the instrument aberrations (and other properties) is the business of psychophysics. Much of that research is either not done or buried in arcane journals. In general, though, reduced field curvature yields a perception of static flatness. Distortion mostly effects dynamic percepts, as when moving the eyes or the head (panning).

Whew. Hope this helps,
Ed

PS. I could have saved a lot of time if I had seen edz's post sooner.

Kevin, EdZ and Ed,
I am still thinking about Henry's instructive example of the checkerboard, so I'll put it in those terms to see if I have it right.

If the appearance of the checkerboard viewed from above is similar to what the naked eye would see, with lines looking straight everywhere, ie, the checkerboard is flat and looks flat, and as a result the binocular incurs a defocus at the edge, corresponding to the greater distance to the edge of the board, that is called "field curvature". So, the common usage of the term is accepted. This didn't seem obvious, but here we have a case where experts actually agree!
Thanks guys,
Ron