Sorry for this long post, but this is one case where I believe a single observer can provide reliable and useful evaluations using a single pair of eyeglasses and reasonable performance criteria. Physical measurement requires a lab setup with calibrated equipment.
In this case the observer is me and the performance criteria are: (1) ability to see a crisp circular field stop, and (2) ability to maintain acceptable (uniform, constant) pressure on my spectacles. The second criterion is necessary because there is a range of eyeglass movements that can produce marginally useful views when applying enough pressure, but this is hard to maintain and very annoying on a prolonged basis. In such cases I would declare that the glass is unsuitable for use with spectacles, or marginal at best.
I have done this evaluation with several of my own binoculars. Some were acquired when I didn't use spectacles, others only later when the need arose. I will post only the manufacturers' advertised eye relief and eyeglass suitability (for me!). The AFOVs of these five glasses are very similar (61-63 deg) so that is not a critical variable in meeting the two criteria. The eyeglasses and viewing situation were identical in all cases.
Swaro 10x42 SLC. 15mm. Unsuitable
Swaro 8x30 SLC 15mm. Unsuitable
Swaro 8x42 SLC HD 18.5mm. Suitable
Zeiss 7x42 BGATP 18mm. Suitable
Nikon 8x32 LX L 17mm. Marginally suitable
The 10x42 and 8x30 SLC are unsuitable because I can't produce a crisp circular field without applying considerable pressure. The SLC HD and Zeiss BGATP are virtually identical and easy to use. The Nikon LX L (note the 32 has a shorter ER than the 42) is marginally suitable using somewhat more pressure and alignment effort. (I've also done this with several Porros, none of which are suitable for eyeglass use, so I haven't mentioned them.)
Comments/Conclusions:
1. The physical meaning of eye relief is well know in the industry, i.e., the distance between the eye lens and the eyepoint (or Ramsden disk).
2. It strains credulity that Zeiss would uniformly underestimate advertised ER by using a non-technical meaning. See:
http://www.zeiss.com/C1256BCF0020BE5F/Contents-Frame/C2F7D329F790A3188525756900599FEC
3. Personal observation is consistent with advertised ER across three manufacturers, including Zeiss. (More, actually, including Swift and Bushnell.)
4. Personal opinions may differ about eye relief adequacy for several reasons that come to mind:
a. Eyeglass fit.
b. Personal criteria used for assessment.
c. Non eyeglass users opining about the subject.
Anyway, ... when there is a chance to evaluate a new Zeiss with 16mm ER I'll see if this holds up. As mentioned earlier, my brief exposure to FLs in the past was that they were marginal at best (er, for me).|>|
Ed