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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1404719" data-attributes="member: 68323">I suspect the EP design does help CA a bit. It doesn't eliminate it (there is still CA from the objective). If you want very low CA (i.e. I can't really see it CA) the ZR ED is the way to go.By field flatness you mean "edge to edge" sharpness? You can have a flat field that isn't sharp to the edge (i.e. field curvature isn't the problem defocusing the edge but coma or some other aberration is).One issue about field curvature is how much accommodation you have to deal with it. For a 47 year old like me with about 2D of accommodation (I've measured it) I will deal with (and probably see more field curvature) than a much younger person (say a 20 year with perhaps 10D accommodation). So this can be a minefield.On my 8x ZRS I can focus a target at 10m or so then move it to the edge where it is less sharp then refocus a little to sharpen it up i.e. there is some field curvature. This doesn't seem to be the case on the ZR ED.These 8x ZRS have a decent sweet spot (perhaps 70% by eyeball, I hate estimating this stuff ... how sharp is the sweet spot edge ... everyone has a different idea) but it's not "edge to edge" sharp. When I compare the ZRS to my 8x ZR ED then the ZRS is sharper right at the edge of the FOV than the ED. But they have different sized FOVs (7 versus 8 degree). If I sort of compensate for that they are perhaps about the same with the ED having less field curvature.This sort of estimate is all very approximate. I don't have the kit (or the time) to do this "properly". So take these comments with the appropriate amount of salt. But for me, someone who doesn't worry about this sort of thing (I put the birds I'm interested in in the center of the field) they're sharp enough in a large enough portion of the center to work well.I'd like FrankD to comment on this as he as a Meopta that is supposed to be edge to edge sharp (and has a yellow bias!).One other comment is Steve and I are exchanging emails to see why we differ in our views. One point may be I have only the 8x and Steve (I think) likes the 10x. Again more on this as we figure it out.</blockquote>

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