This thread continues a discussion of field curvature and astigmatism that began with a question on another thread about the correct terms to use in describing softness at the edge of the field.
Today I tried an experiment to see if I could crudely measure field curvature and astigmatism in a group of binoculars. This is all very preliminary, so I’m hoping for some feedback and ideas from Alan and Alexis and anyone else who is interesred, maybe our resident Australian optometrist ?
As a target I used some ordinary lined paper from a memo pad. I taped two pieces on the wall, one with the lines horizontal and the other vertical. The horizontal lines were taped directly above the vertical lines. In each binocular I placed the target at 3 o’clock on the field edge and observed it, using only one eye, from about 25’. I used two pairs of 1.25 diopter reading glasses as my very crude measuring tools.
In binoculars with astigmatism the two sets of lines cannot be brought to best focus simultaneously. In the worst cases one set of lines will look focused while the other set will be spread so wide by astigmatism as to become invisible. I focused the horizontal lines without glasses, then used the glasses to change the best focus to the vertical lines. My only choices for correction were 1.25 diopters or 2.5 diopters, if I wore both pairs at the same time. Clearly I am going to need some 1.5, 1.75 and 2 diopter glasses to make this more accurate.
I used a similar technique to measure field curvature. First, without glasses, I found the best, least astigmatic focus at the edge where the two sets of lines were about equally unfocused. Then, without changing the focus I used the glasses (if necessary) to bring the center of the field into focus.
Here are the crude and preliminary estimates. The first number is astigmatism and the second number is field curvature, both in diopters:
Zeiss 8x42 FL - 2.5, <1
Swarovski 8.5x42 EL - 1.5, 1.25
Nikon 8x32 SE - <1, 2
Nikon 8x32 EII - 1.25, 2
Nikon 7x50 Prostar - almost none, <1
Kern 8x30 - 5, <1
I included the Nikon Prostar because I thought it would be unusually good and I thought the Kern would be particularly bad, although I didn’t realize its problem is almost all astigmatism. Keep in mind that the apparent fields of this group vary from 51 degrees for the Prostar to 70 degrees for the EII and the Kern so they are not exactly comparable unless the fields are close ( EL, FL, SE).
I should also mention that I am 58 and don’t have much focus accomodation, but I don’t know how much. I think the field curvature figures should be the amount of field curvature above my ability to accomodate, but I am unsure about how the astigmatism measurement is effected by accomodation. An ophthalmologist told me recently that she had encountered young people with over 12 diopters of accomodation ( I hope I remember that correctly). For someone like that field curvature in binoculars should never even be noticed.
Alan, was your experiment similar to this (without the glasses)?
Zeiss 8x42 FL - 2.5, <1
Swarovski 8.5x42 EL - 1.5, 1.25
Nikon 8x32 SE - <1, 2
Nikon 8x32 EII - 1.25, 2
Nikon 7x50 Prostar - almost none, <1
Kern 8x30 - 5, <1
I included the Nikon Prostar because I thought it would be unusually good and I thought the Kern would be particularly bad, although I didn’t realize its problem is almost all astigmatism. Keep in mind that the apparent fields of this group vary from 51 degrees for the Prostar to 70 degrees for the EII and the Kern so they are not exactly comparable unless the fields are close ( EL, FL, SE).
Henry
Whilst i can just about understand the tests and what is being examined i find it particularly interesting that the Nikon SE and E11 are very close in this measurement
Forgive my naivety but does this result actually place the E11 in a better light so to speak due to its greater fov ?
Regards
Rich
Rich,
Using numbers probably makes this look more precise than it really is. 0.5 to 1 diopter differences are probably real, 0.25 maybe not. I need to do this a few times to see how repeatable the results are. It's not quite ready for a peer reviewed journal of binocularology ;-).
I just compared the SE and EII more closely. Moving the target in a little from the edge in the EII to try to match the field width of the SE seems to improve the astigmatism a liittle, maybe to 1 diopter. Looking at the SE again I think its astigmatism is not just less but probably much less than 1 diopter. It's really almost as good as the Prostar even with the wider field.
For anyone who is mystified by these numbers, the very best result would be a low number for each, but I think astigmatism is more important. People with wide focus accomodation probably won't see field curvature, but refocusing the eye doesn't correct astigmatism.
Henry