Thank you Ed, Kevin, Alexis & Bob. You have provided the info I was looking for! 18mm of eye relief works well for me. I was very comfortable with the eye relief on my 8x42 Nikon Premier LX's. I replaced them 6 months ago with 8x42 Zeiss Victory FL's. After my initial love affair with the Victory's I began to notice some discomfort and eye strain after prolonged use. I never experienced these symptoms with the Nikon's. I am not an optics expert so I attributed my problems with the Victory's to the reduced eye relief (16mm vs. 18 or was it 20mm for the Nikons?). Am I barking up the wrong tree? Would going from the 8x42 Victory's to the 7x42 Classics help (theoreticaly)? I have experimented with the diopter and the IPD but that didn't help. I have been "lured" by the discriptions of the 7x42 Classic view as "Natural", "Easy", "Comfortable", etc. I use my bins for birding and I bird 300+ days a year. Your comments would be instructive.
I don't think your eyestrain has anything to do with the lower ER
unless you are using the bins with eyeglasses and are pushing your eyeglasses hard against your face so you can see the entire FOV. That can give you a headache and create a deep ridge on your forehead that will make you look like a Klingon.
If you don't wear glasses with bins, then you have to consider other possibilities such as the binoculars being out of collimation - not enough to give double images but enough to cause eyestrain -- this is particularly true if the vertical alignment is off since there's less tolerance for vertical misalignment than horizontal.
Or perhaps after using the 8x42 LX, which is sharp to the edge, your eyes are used to scanning the entire FOV, and with the FLs, you have to concentrate on the centerfield because the edges are out of focus (Zeiss's design scheme to increase sharpness at the center rather than spread the sharp field at a lesser resolution over a wider portion of the FOV).
The three bins I use the most have large sweet spots with gradual fall off at the edges - the Nikons SE, EII, and LX.
I also use my 804 MC Audubon, which has a decent sweet spot and fuzzy edges that I have to move my eyes to see. The 804 Audubon has a very relaxed image.
However, if I use a bin with a small "sweet spot", my eyes tend to scan the entire FOV, particularly the leading edge when I pan, and my eyes will try to focus the out of focus areas, and I end up with eyestrain if not a headache.
Brock