Field flattener lenses effect? (1 Viewer)

[Steve Napier]

Until recently, I didn"t realize that binoculars that use Field flattener lenses have a rolling ball effect at the periphery of vision.I found this out when reading a review of the Fujinon 10x50 in "Astronomy Now" magazine.
I understand that the Nikon SE and HGL {LXL in U.S.A} range also use field flattener lenses.
According to the Fujinon review,this "Rolling Ball" effect is only seen when the binoculars are being panned,my question is, how does this effect the Nikon binoculars when used in daylight for handheld panning? When a bird is being followed in flight is it bothersome?
This Rolling ball effect is something that seems to have been given scant regard.
Hope you can help.
Thankyou.
Steve.

 

[Pileatus]

Until recently, I didn"t realize that binoculars that use Field flattener lenses have a rolling ball effect at the periphery of vision.I found this out when reading a review of the Fujinon 10x50 in "Astronomy Now" magazine.
I understand that the Nikon SE and HGL {LXL in U.S.A} range also use field flattener lenses.
According to the Fujinon review,this "Rolling Ball" effect is only seen when the binoculars are being panned,my question is, how does this effect the Nikon binoculars when used in daylight for handheld panning? When a bird is being followed in flight is it bothersome?
This Rolling ball effect is something that seems to have been given scant regard.
Hope you can help.
Thankyou.
Steve.

Steve,

I find the SE to be as transparent a tool as one could imagine and optical distortion has never been a problem. My Leica has more obvious curvature than my SE.

Actually, the SE is perfect for panning because its wide, sharp field keeps the target in near-perfect focus as its position in the field varies. The SE challenge is eye placement, something one can master with continued use.

John

 

[henry link]

Steve,

The "Astronomy Now" review may be correct about a "rolling ball" effect in the Fiujinon 10x50 which I haven't seen, but there is no consistent amount or type of distortion in binoculars with "field flatteners".

The classic rolling ball effect comes from barrel distortion which I hardly ever see in any significant amount in modern binoculars. I have some WWII military binoculars with Kellner eyepieces that have plenty of it. Almost all modern binoculars have at least a little pincushion distortion which is sometimes intentionally added by designers who consider a little pincushion more comfortable for panning than no distortion at all.

I have four binoculars with so called field flatteners. The distortion in them ranges from very slight barrel distortion in a Fujinon 8x30 FMT-SX to virtually no distortion in a Nikon 8x32 SE to slight pincushion in a Nikon 7x50 Prostar and Pentax 10x50 PIF. Some old Fujinon literature I have on the 7x50 FMT-SX has a diagram of distortion comparing the MT-SX without field flattener to the FMT-SX with field flattener. It shows about half as much distortion in the FMT compared to the MT.

BTW, I'll take this as another opportunity to mention that the "field flatteners" in most of my binoculars that use them don't actually correct field curvature as the name implies. Instead they correct off-axis astigmatism. Only the Prostar has both low astigmatism and low field curvature.

Henry

 

[Pileatus]

Steve,

The "Astronomy Now" review may be correct about a "rolling ball" effect in the Fiujinon 10x50 which I haven't seen, but there is no consistent amount or type of distortion in binoculars with "field flatteners".

The classic rolling ball effect comes from barrel distortion which I hardly ever see in any significant amount in modern binoculars. I have some WWII military binoculars with Kellner eyepieces that have plenty of it. Almost all modern binoculars have at least a little pincushion distortion which is sometimes intentionally added by designers who consider a little pincushion more comfortable for panning than no distortion at all.

I have four binoculars with so called field flatteners. The distortion in them ranges from very slight barrel distortion in a Fujinon 8x30 FMT-SX to virtually no distortion in a Nikon 8x32 SE to slight pincushion in a Nikon 7x50 Prostar and Pentax 10x50 PIF. Some old Fujinon literature I have on the 7x50 FMT-SX has a diagram of distortion comparing the MT-SX without field flattener to the FMT-SX with field flattener. It shows about half as much distortion in the FMT compared to the MT.

BTW, I'll take this as another opportunity to mention that the "field flatteners" in most of my binoculars that use them don't actually correct field curvature as the name implies. Instead they correct off-axis astigmatism. Only the Prostar has both low astigmatism and low field curvature.

Henry

Henry,

Why don't all high-end optics provide the type of sharpness across the field that we see in the SE?

John

 

[Pinewood]

I would guess that the high end binoculars do not have sharpness across the whole field because they have wider fields than the SE. Every binocular design is a compromise. As Henry pointed out, rectilinearity leads to some rolling ball effect. Purposefully introduced pincushion distortion compensates for the rolling ball effect. Fields wider than the 8x32 SE's 7.5º field have poorer resolution at the edges, but may provide a more relaxed view, as well as a broader view. Even the SE has poorer resolution at the edges than at its center. For bird watching this may not be disturbingly fuzzier as the eye's diaphragm is narrower when used in daylight. I am now writing about the eye and binocular interface, more Elkcub's expertise, where all sorts of matter of perception creep in beyond physics.

In photography, some of the edge problems are reduced when the lens is stopped down, an option not really available for binocular users.

In any case, there are design compromises and we have to choose, among those compromises, binoculars to optimize our own needs.

Happy bird watching,
Arthur Pinewood

 

[henry link]

Beats me, John. The FMT and Prostar designs designs are over 20 years old, the SE about 12. Certainly Zeiss and Leica coulld do this if they wanted to. The Swarovski 8.5x42 EL's do pretty well even without apparently using a field flattener.

There are at least some theoretical arguments against the SE, FMT. Their approach doesn't correct field curvature and might even make it a bit worse. The Zeiss 8x42 FL and others might be said to actually have lower field curvature than the SE, FMT in the sense that their "best" edge sharpness (the mid point between the sagittal and tangential foci at the edge) is close to the focal plane of the center focus. Trouble is that the "best" edge sharpness in the FL isn't very good because no one can achieve really good focus at the midpoint between two foci that are at least a few diopters apart. In the Nikon SE the sagittal and tangental foci at the edge have been brought close together, but that focus point is 2-3 diopters different from the center focus. The edge in the SE can only be brought to pretty good focus at the same time as the center if the eye has sufficient focus accomodation. Personally I find the SE design works better for me even though I only have about 1 diopter of accomodation. It should work even better for young people with 3 or more diopters. I've posted about this same thing recently, but once again I think a better approach would a more complex correction like a photographic anastigmat in which a little more astigmatism is allowed in exchange for better field flatness. This may be what is done in the Prostar and perhaps some of the Canon IS binoculars with doublet field flatteners.

Arthur,

Field curvature and astigmatism do get worse as AFOV gets wider, but at 30 or even 15 degrees off-axis the binoculars with "field flatteners" have lower astigmatisn than binoculars without them. I think the main benefit from these things is a larger sweet spot between perhaps 10 and 20 degrees off-axis were the field curvature is still pretty low.

Henry

 

[Steve Napier]

Thanks for all your replies.
I wonder if this "Rolling Ball" effect is a bit like CA,in as much that some people see a lot of it and other people using the same binocular,don"t see it at all.
Thankyou,
Steve.

 

[Chhayanat]

Thanks for all your replies.
I wonder if this "Rolling Ball" effect is a bit like CA,in as much that some people see a lot of it and other people using the same binocular,don"t see it at all.
Thankyou,
Steve.

It would appear that the "Rolling Ball" effect is noticeable by more users than is chromatic aberration. It certainly applies to me. I cannot see chromatic aberration through binoculars but I can certainly see it it photographs taken with telephoto lenses, when enlarged on Adobe Photoshop.
After inquiries on Birdforum, I understand that the "rolling ball" effect is very apparent in the NIkon LX/HG series during panning. It is certainly very apparent in my 10x42lx/hg. If interested, please see my posts under "Nikon 10x42Hg: Not satisfied". I am less disturbed by it than I was when I first started using the glasses. This bears out John Traynor's advice. He is also correct when he says that older users with less accommodation in their vision are more sensitive to various visual effects in field glasses than are younger people whose vision compensates far better.
Chhayanat

 

[Steve Napier]

I certainly will have a look at that post,thankyou.
Steve.

 

[Swedpat]

When we are talking about distorsions, I am still not completely sure about the differences. Is "barrel distorsion" the same as field curvature?
Are there any images available who clearly demonstrate the differences?

Patric

 

[henry link]

Patric,

Distortions result from uneven magnification across the field. If the magnification increases toward the field edge that causes "pincushion" distortion so that lines near the edge appear to bow in toward the center. If the magnification decreases toward the field edge that causes "barrel" distortion so that lines appear to bow out toward the edge. Field curvature causes an object which is in focus at the center of the field to be increasingly out of focus as it gets nearer to the field edge.

Henry

 

[Pileatus]

When we are talking about distorsions, I am still not completely sure about the differences. Is "barrel distorsion" the same as field curvature?
Are there any images available who clearly demonstrate the differences?

Patric

http://www.monkoptics.co.uk/General/about-binoculars.html

 

[henry link]

Thanks for the link, John. I've seen this one before, but couldn't remember the address.

However, please note that the line illustrations of pincushion and barrel distortion are wrong, they should be reversed. The photo illustrations are correct.

 

[elkcub]

When we are talking about distorsions, I am still not completely sure about the differences. Is "barrel distorsion" the same as field curvature?
Are there any images available who clearly demonstrate the differences?

Patric

I don't mean to go overboard here (although I have), but there are five basic monochromatic aberrations that are corrected by the manufacturer: spherical aberration, coma, astigmatism, curvature of field, and distortion. They each have specific definitions, but distortion refers to non-constant magnification of the image from the center to the periphery. This results in the misshapen grids as shown (if one is looking at a rectangular grid). It also has other effects when looking at different targets.

The rolling ball effect is an optical illustion, which is induced only under the right circumstances. Many people intuit that their eyes are locked forward when they pan, let's say from right to left. Actually, that's not the case. If the panning motion is slow enough the eye first moves in the direction of head motion (left) and picks up an object to fixate upon; it then follows the object with counter-rotating eye movement until it leaves the field at the right. Subsequently the eye jumps again to the left, picks up another object, and so on. When barrel distortion (decreasing magnification towards the edge) is present, an object first seen on the left increases in size and then decreases again before it leaves the field. The next object does the same thing, all of which the brain perceives as a rolling ball. The reverse would be true for pincushion distortion, but apparently no comparable illusion has been named. The reason some people experience the illustion and others don't is that it's fragile. It depends upon the visual field flow rate, and the tendency of the observer to follow objects in the manner described. It should be evident from the nature of the phenomenon that barrel distortion is the culprit, but I have my doubts that pincushion is needed to supress it — as the urban legend would have us believe.

The two aberrations, astigmatism and field curvature, are antagonistic. Field curvature tends to increase as astigmatism decreases. This being the case, when the design calls for agressive astigmatism control, it is necessary to bring in additional lens elements to re-flatten the field — the infamous field flattener. In this case urban legend has unfortunately led us to believe that field flatteners were intended to produce super-flat fields, which Henry's superlative observations have shown to be a misconception. Indeed, there is nothing particularly new about field flatteners either, even though it is hyped as a technology innovation.

My own opinion is that agressive astigmatism control was motivated primarily for astronomical observations. This would support off-axis viewing with tripod mounted instruments. Indeed, the dark adapted eye sees best off-axis (i.e., non-foveally). Although it might (and will no doubt) be disputed, I am also of the opinion that hand-held birding instruments do not need this level of astigmatism control and benefit more from a flatter field (as described above). This is because head motion is an efficient way to center a target on the fovea of a light adapted eye, and a flat field minimizes the need to refocus as the target approaches the fovea. Putting it all together, then, I have a strong sense that optimal design has evolved to be highly task related, and a terrific binocular created for one purpose may not be as sensational for another.

All of the above is intended to stimulate thought and discussion, perhaps with corrections or additional insights that I have missed.

Ed
PS. In re-reading, most if not all of my comments are consistent with what Henry mentioned in post #3, although from a somewhat different perspective.

 

[Swedpat]

Thank you all for the replies!

When looking at the images of the site http://www.monkoptics.co.uk/General/about-binoculars.html
I see that the rolling ball effect seems to be related to barrel distorsion, but to a certain extent also to Curvature of field?

But when I look at the pictures, don't the mixed the images? I want it this way:



Patric

 

[elkcub]

Thank you all for the replies!

When looking at the images of the site http://www.monkoptics.co.uk/General/about-binoculars.html
I see that the rolling ball effect seems to be related to barrel distorsion, but to a certain extent also to Curvature of field?

But when I look at the pictures, don't the mixed the images? I want it this way:



Patric

Patric,

Try not to confuse the ball shape of the barrel distorted image with the rolling ball illusion.

As explained in the article, field curvature refers to the (three dimensional) surface containing the scene elements that are simultaneously in focus. Barrel distortion refers to the bulging of the sides of the image due to magnification differences from center to edge. The rolling ball effect (i.e., illusion) requires a panning action by the observer with barrel distorted binoculars. In other words, it's a dynamic effect and not a static one, so it can't be seen in the illustrations. I don't think field curvature reinforces the illusion, but it's always possible I guess.

Hope this helps.

Ed

 

[Swedpat]

Ed,

I very well know that the rolling ball effect is visible when panning.
If you look at the image I have attached, isn't it obvious that they have mixed the images? I am not really sure what is what...

Regards, Patric

 

[elkcub]

Ed,

I very well know that the rolling ball effect is visible when panning.
If you look at the image I have attached, isn't it obvious that they have mixed the images? I am not really sure what is what...

Regards, Patric

Patric

Henry Link commented on post #13 that:

... please note that the line illustrations of pincushion and barrel distortion are wrong, they should be reversed. The photo illustrations are correct.

and your drawing shows with arows the proper relationship that he mentioned. Is that what you mean by "isn't it obvious that they have mixed the images"? Yes, indeed, they mixed the images. I give you my full support.

Ed

 

[kabsetz]

A couple of comments, nothing new here.

Firstly, concerning the "rolling ball" effect and pincushion/barrel distortion (and I may note here that I do not own any binoculars that have barrel distortion), it seems that at least for most idividuals, their brains adjust to the type of distortion there is in the binocular they use regularly, and they cease to see the distortion and/or be bothered by it, sometimes even when they specifically try to see it. It is quite common when two birders try out each others' binoculars that both complain about exessive "rolling ball" in the others' binocular. The Zeiss Classic 10x40 was rather famous for having a lot of the "rolling ball" (we called it "carousell effect"), but many birders still use them happily and certainly don't consider it ever when viewing birds. Indeed, these types of distortions might well be more meaningfull to binoholics and collectors who use several different types of binoculars intermittently and do not give their brains time to solidly accommodate to any one image.

Secondly, all else being more or less equal, I personally prefer, like Henry does, binoculars that have the least amount of astigmatism in the largest possible area of the field. This is because when the eye/brain system perceives an image as unsharp, the eye will try to focus it, and when the unsharpness is caused by field curvature, the accommodation actually helps, at least partially. However, with astigmatism it doesn't help, as while the tangential focus might be improved, sagittal becomes more defocused and vice versa. My experience is that this tires my eyes significantly more than dealing with field curvature. This also relates to sweet spots. I am happier with a sweet spot which fades away into defocus than an equally large sweet spot which fades away into astigmatism.

I have compared two binoculars with field-flatteners, the Nikon SE 10x42 with a single-element flattener, and the Canon 10x42 L IS with a doublet flattener. The Nikon has a 60 degree apparent field, while the Canon has 65 degrees. Despite its wider field, the Canon has less astigmatism and field curvature towards the edges. Both binoculars have very large sweet spots, and allow my eyes to roam around the image without fatigue, but the Canon manages to maintain higher image quality further towards the edges.

So, like Henry, I can only see benefits from using field-flatteners, and based on the two examples quoted above, feel that a doublet can be better than a singlet. Back in the darker ages loss of transmission and contrast and increased scatter would strongly have argued in favor of simpler optical designs, but modern coating technologies seem to have changed that.

Kimmo

 

[Pileatus]

A couple of comments, nothing new here.

Firstly, concerning the "rolling ball" effect and pincushion/barrel distortion (and I may note here that I do not own any binoculars that have barrel distortion), it seems that at least for most idividuals, their brains adjust to the type of distortion there is in the binocular they use regularly, and they cease to see the distortion and/or be bothered by it, sometimes even when they specifically try to see it. It is quite common when two birders try out each others' binoculars that both complain about exessive "rolling ball" in the others' binocular. The Zeiss Classic 10x40 was rather famous for having a lot of the "rolling ball" (we called it "carousell effect"), but many birders still use them happily and certainly don't consider it ever when viewing birds. Indeed, these types of distortions might well be more meaningfull to binoholics and collectors who use several different types of binoculars intermittently and do not give their brains time to solidly accommodate to any one image.

Secondly, all else being more or less equal, I personally prefer, like Henry does, binoculars that have the least amount of astigmatism in the largest possible area of the field. This is because when the eye/brain system perceives an image as unsharp, the eye will try to focus it, and when the unsharpness is caused by field curvature, the accommodation actually helps, at least partially. However, with astigmatism it doesn't help, as while the tangential focus might be improved, sagittal becomes more defocused and vice versa. My experience is that this tires my eyes significantly more than dealing with field curvature. This also relates to sweet spots. I am happier with a sweet spot which fades away into defocus than an equally large sweet spot which fades away into astigmatism.

I have compared two binoculars with field-flatteners, the Nikon SE 10x42 with a single-element flattener, and the Canon 10x42 L IS with a doublet flattener. The Nikon has a 60 degree apparent field, while the Canon has 65 degrees. Despite its wider field, the Canon has less astigmatism and field curvature towards the edges. Both binoculars have very large sweet spots, and allow my eyes to roam around the image without fatigue, but the Canon manages to maintain higher image quality further towards the edges.

So, like Henry, I can only see benefits from using field-flatteners, and based on the two examples quoted above, feel that a doublet can be better than a singlet. Back in the darker ages loss of transmission and contrast and increased scatter would strongly have argued in favor of simpler optical designs, but modern coating technologies seem to have changed that.

Kimmo

Kimmo,

Bravo!
This is one of the BEST posts I've ever read.

John