Field Flattener lens..MEopta, Swaro, NIKON (1 Viewer)

[308CAL]

I am a sweet spot junkie and love a wide edge to edge sweet flat image..

I have some Meopta Meostar 8x42's that I have compared with Swarovski SCL neu and EL (not SV), Zeiss FL, and some others...

While the Meopta has a slight warm color bias, in low light testing I cant appreciate any additional brightness from the Binos above.

I have seen the sweet spot and edge resolution to be the equal of the swarovski and much more preferable to me than the FL.

WHat other Binos have these field flattener lenses? I wish the FL would add them. The FL and EL were much more neutral in color than the Meoptas.

I think Nikon uses these but in a different location than Meopta.

By the way, im struggling with wanting an SLC HD 8x42 but when I look through my Meoptas Im having trouble justifying it!!!!!!


IS anyone else addicted binos that use these lenses?

 

[Kevin Purcell]

The Nikon SE porros have a field flattener in the EP.

The Nikon EDG roofs (which you allude to but don't mention the model)

Some of the Fujinon bins (FMTX 6x30 and 8x30, I think) also have a field flattener.

The Canon IS bins. The IS system needs a flat field otherwise the image would distort as the IS system moved to compensate for movement. The side effect is both flat field and IS in the 10x30 for $350ish.

The original Zen Ray ZRS (not the current 2010 design) also had an LaK field flattener in the design. It would be the "obvious" feature to add to the current batch of positive focuser Chinese ED bins. Though as ZR found that LaK element did add a reddish color bias that some found objectionable (and others didn't have a problem with) so life is more complicated than just adding another element.

The Zeiss FL bins actually have a flat field already but they trade off the flat field for other aberrations (astigmatism?) at the edge of field (hence the blurriness). Remember that a flat-field doesn't imply "sharp to the edge" though marketers like to make that connection. A field flattener does reduce the constraints on the eyepiece design (as it no longer has to flatten the field) though with modern EPs with multiple elements unless they really do put an element in to flatten the field they're more likely to add an extra element then optimize on flat field and astigmatism and coma at the field edge.

I suspect the next Zeiss top-of-the-range bin will also follow this trend and be "sharp to the edge". I suspect they'll have a price to match the other three. Same with Leica.

 

[308CAL]

Thanks Kevin

 

[dannat]

i have the canon 12x36 & the field is nice & flat and good almost edge to edge (atleast to 90%) i was pleasantly surprised when i got these how well they show star fields at night

 

[henry link]

The so called called "field flatteners" don't all do the same thing. I made a crude drawing to illustrate how the main off-axis aberrations (field curvature and astigmatism) interact.

The vertical line represents the plane of focus at the center of the field (bottom) extended to the edge of a 60 degree field (top). The horizontal line at the bottom indicates how far (in diopters) the the focus of the curved lines depart from that plane. When astigmatism is present (as it is in every binocular) there are two foci, tangential and sagittal. Essentially, that means that at the edge of the field you cannot focus on horizontal and vertical lines at the same time. The solid curved line represents tangential focus and the dotted curved line is sagittal focus. The red line represents the "best" focus which is midway between the two.

The condition "a" on the left has both high astigmatism and high field curvature. The wide separation between the dotted and solid curves indicates high astigmatism and the large departure of the red line away from the vertical indicates high field curvature. This is what you get with something like a simple Kellner eyepiece.

Condition "b" has high astigmatism, but low field curvature. There is wide separation between the curved lines, but the red "best" focus lies along the plane of center focus. The problem here is that the "best" focus is not very good because of the astigmatism. The Zeiss FL uses this approach.

Condition "c" has high field curvature and low astigmatism. Some binoculars with so called "field flatteners" actually correct astigmatism, but not field curvature. In this case the edge of the field is not in focus at the same time as the center, but can be re-focused (or accommodated) to a sharp image because the astigmatism is low. The Nikon SE, Fujinon FMT and Pentax PIF take this approach.

Condition "d" is what we all really want. Both astigmatism and field curvature are corrected. The Swarovski SV, Nikon Prostar, Astroluxe, LX-L and EDG and (I am told) the Canon 10x42 L IS manage to do both, but not with completely equal success. In my experience the SV comes the closest to the ideal.

BTW, none of these corrections have anything to do with distortion. The Swarovski and Nikons do it with minimal pincushion while some eyepieces, like TeleVue Panoptics and Naglers do the same thing with huge pincushion.

 

[ceasar]

Thanks Henry. Even I can understand that!:t:
Bob

 

[ronh]

Henry,
Nicely said and drawn. Dang Fujinon for lying to us and calling it a field flattener, when it's really a tangential and sagittal surface normalizer. But we don't really need the STSNMTCR-SX, which sounds like it would weigh not three pounds but five.

You do justice to the big marine Fujinon and Pentax, but how about the vaunted Nikon Prostar, the former gold standard for field edge correction? Do you really think the SV beats it in this regard?
Ron

 

[henry link]

Ron,

Yeah, even "Astigmatism Corrector" doesn't quite cut it as a marketing term.

I haven't compared the Prostar to the SV directly, but remember the Prostar AFOV is only about 51 degrees compared to almost 65 for the SV. The really remarkable thing about the SV is that such good corrections for astigmatism and field curvature have been achieved at the edge of such a wide apparent field.

Henry

 

[OPTIC_NUT]

I am a sweet spot junkie and love a wide edge to edge sweet flat image..

I have some Meopta Meostar 8x42's that I have compared with Swarovski SCL neu and EL (not SV), Zeiss FL, and some others...

While the Meopta has a slight warm color bias, in low light testing I cant appreciate any additional brightness from the Binos above.

I have seen the sweet spot and edge resolution to be the equal of the swarovski and much more preferable to me than the FL.

WHat other Binos have these field flattener lenses? I wish the FL would add them. The FL and EL were much more neutral in color than the Meoptas.

I think Nikon uses these but in a different location than Meopta.

By the way, im struggling with wanting an SLC HD 8x42 but when I look through my Meoptas Im having trouble justifying it!!!!!!


IS anyone else addicted binos that use these lenses?

I love a flat clean wide field. There are some old units
that have "field flatteners", however.
They did it with the "Plossl" design for the ocular.
I have only one pair that's this flat and sharp,
the AMC 603. Plossl eyepieces are a lot of work to clean
and reassemble. Multiple focal planes means multiple places
you focus a microscope on debris. Putting the flattener in the middle
avoids that.

I'm surprised there aren't more super-telescope technologies
in binoculars, like apochromatic objectives. It's probably a matter
of size and weight.

 

[james holdsworth]

Every binocular I have used with a field flattener has not been truly sharp throughout the FOV - most have areas just outside of the sweetspot [maybe 70% out] that go soft, then sharpen again towards the edge. Brock called this the ''Absam Ring.''

I find this distracting, as the zone of unsharpness is kind of ''hanging'' out, surrounded by sharpness on either side, which makes it more noticeable.

 

[OPTIC_NUT]

Ah, a fuzzy donut.
Either the focus fades a bit or the brightness fades. Or the shape, of course, for
3-element oculars. They have to pick a trade-off. The idea of ultrawides was that
even if the outer 10-20% is fuzzy you can detect presence or motion, and your peak
eye resolution is a much narrower chunk. Right, but that's annoying to some people.

Oddly enough, I find the flatness itself more disturbing when I'm sweeping fast.
Sort of like the woods turn into wallpaper. The focus on the AMC's holds on until
the last 5-10% but that can be weird when scanning. A little barrel distortion is
OK with me if the edges brighten up.


About 8 degrees in an 8x30 helps a lot without getting strange.
10 degrees in a 7x35, if it's nice and bright.

The 6x30s usually have 8 degrees, which is not really wide for 6x,
but you can buy some models that are truly sharp and bright
right to the edge. I am finding that very "relaxing" on sharp models.
You can scan but the image isn't too much to get used to when it moves,
between lower power and no visible distortion. More 'real'.

 

[brocknroller]

Whatever you want a call it, I'm pretty sure the entire FMT line has a flat field, not just the defunct 6x and 8x30s. But the IF EPs make them limited for birding. In fact, with my present focus accommodation, even the 6x30, with its great depth of field, still required frequent focus tweaking at different distances. My first pair, bought about 10 years earlier, wasn't exactly "set and forget," but my eyes certainly accommodated better than they do now.

The other problem with the FMT series is that they are heavy. Even the 6x30 weighed 30 oz., which was heavier than the 8x30, because of the huge prisms.

I do like the 8x32 SE because of its flat field, but in dense woods and brush, I still prefer the 8x30 EII for its wider FOV, and the fall off at the edges isn't steep and the field is so wide (8.8*) that I have to take an object to the edges to get it to blur.

Then there's the issue of "rolling ball" with the Nikon Premier and Swaro SV ELs, and the image blackouts with the SEs (not specifically related to the field flatteners, but to the EP design). So even bins with field flatteners aren't necessarily ideal for everyone, even those who do like flat fields.

If someone made a bin with a "tangential and sagittal surface normalizer" that would produce a reasonably sharp image even at 80%, and also had an extra wide FOV (70* AFOV) w/out "side effects" such as RB, that would be the ideal bin for me. But that seems to be a rather tall order since I'm not aware of any bin that has that combination of characteristics.

Brock

 

[FrankD]

Every binocular I have used with a field flattener has not been truly sharp throughout the FOV - most have areas just outside of the sweetspot [maybe 70% out] that go soft, then sharpen again towards the edge. Brock called this the ''Absam Ring.''

I find this distracting, as the zone of unsharpness is kind of ''hanging'' out, surrounded by sharpness on either side, which makes it more noticeable.

Ah yes, the Fuzzy Donut.

 

[OPTIC_NUT]

More fun to say than "Tchebycheff phase ripple", although that's fun too.