The word "flattener" in the term "field-flattener" would seem to imply that the field is levelled, the edges being improved at the expense of some degradation of the centre. The Nikon SE, being a high quality porro, was presumably so sharp in the centre that its designers felt they had room to sacrifice some of that sharpness for a flat field. And maybe, having the porro's inherent advantage, it was able to sacrifice centre sharpness and still be on a par with top roofs. As someone who doesn't care about the flatness of fields, I wonder what it would be like to look through an SE without the field-flattener.
Michael
A field flattner doesn't imply any such thing.
A (perfect) field flattner lens converts a spherical field at the focus (from a spherical lens) to a flat (planar) field. It's not redistributing any resolution from the center to the edge.
It's changing the shape of the field so the occular that designed to expect a flat field can show the whole image in focus at the same.
Of course depending on how accurately the field flattner is constructed it may or may not affect the resolution anywhere in the field. It also affects how flat the field is and how the distortion changes across the field. But you can build a field flattner with a doublet close to the objective focus. But where you place it is problematic (the closer to focus the more likely imprefections in the lens are to mess up the image) so it's placed a little way toward the objective which means you need to make the objective and the flattner lens a little stronger which increases spherical abberation and longitudinal CA.
So for Nikon and Zeiss (and the others) I suspect they may use a aspherical lens in the field flattner to compensate for some of these issues.
The redistribution argument applies to how you design the ocular to match the field presented by the objective and which trade-offs you make with the lenses you can manufacture (spherical or aspherical) across a whole host of parameters. Like all curve fitting you can decide how to constrain the fit: do you want the best fit (in a least squares sense) over the whole field or do you want the sharpest possible fit in the center and a reduced fit at the edge. How do you trade of distortion across the field? And CA? And then you do that for all foci of interest (or according to SI Zeiss only do it for infinity but others may tweak for closer focus too).
Plus there are the complications in manufacture (another doublet or aspherical lens to make and install and keep aligned). Another two glass/air transitions to loose a little bit of light from.
Binocular design (all optics design!) is all about trade-offs and I'm not so sure I believe SI's view that this is the "best possible result (because it's the best Zeiss bin
). The measured results may actually be true (at that time) but is it "the best"?
I think sharpness at the edge of field is often "overrated" because we point the bins to the target and ours eyes have a limited high resolution field of view. But there are edge of field distortions I really dislike too (that aren't sharpness related). But then again people paying almost $2000 for their bin want "perfection" even though it can't actually be done. At least until they have to pony up for the next "best" set of bins in a few years.
Different manufacturers like to make this trade off in different ways too e.g. the Canon IS bins al have a doublet field flattner lens (to help make the IS system work, I think, by having a flat field at the plate they use to shift the image). This gives them edge to edge sharpness but a narrower field of view. That's their trade off but they need for the IS to work and perhaps also to help the perception of sharpness overall of the bin when IS is on (another design goal).
It's all trade offs ... even Steve says this about the FLs in another earlier post:
http://www.zbirding.info/zbirders/forums/thread/54.aspx
