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Size of sweet spot (1 Viewer)
- Thread starter BobinKy
- Start date
Kevin Purcell
Well-known member
The optical design of the system (mostly the oculars) and a trade off between astigmatism, field curvature and all other parameters of interest.
The "sweet spot" is limited by how much the astigmatism and field curvature increase as you move sagittally (i.e. way from the axis) to the edge of field.
Different design make different trade offs: some (Zeiss) go for very low field curvature but allow the astigmatism to increase. A more conventional trade off is to allow some field curvature and some astigmatism. Some prefer more of one (usually field curvature that you accommodation, if you have any, can deal with).
The optics design programs provide a plot of field curvature and astigmatism (in both tangential and sagittal direction) to show how the design trades these off in a qualitative (and quantitive) way. These also interact with the (desired) pincushion distortion.
The usual way it's done here with some estimating some "percentage of sweet spot" is not very accurate and often doesn't distinguish between the three components. How fuzzy does a bin have to be to se the edge of the "sweet spot"?
Add to that difference peoples sensitivities two the three components and you have a problem with communicating the differences. The good accommodation of a young viewer might not find field curvature much of an issue and some even like it to add "depth of field"). How much do the two astigmatisms have to be balanced so the view doesn't look too "odd". How much do you really like use edge sharpness (some really look around the view others centre on movement in the field). There's a lot of variation between bins users.
One way of reducing the effect (say on the Nikon SE, EDG and even more so on the Swaro SV) is to add a field flattener (removing the field curvature component) so that the ocular design can spent time optimizing the other components.
The "sweet spot" is limited by how much the astigmatism and field curvature increase as you move sagittally (i.e. way from the axis) to the edge of field.
Different design make different trade offs: some (Zeiss) go for very low field curvature but allow the astigmatism to increase. A more conventional trade off is to allow some field curvature and some astigmatism. Some prefer more of one (usually field curvature that you accommodation, if you have any, can deal with).
The optics design programs provide a plot of field curvature and astigmatism (in both tangential and sagittal direction) to show how the design trades these off in a qualitative (and quantitive) way. These also interact with the (desired) pincushion distortion.
The usual way it's done here with some estimating some "percentage of sweet spot" is not very accurate and often doesn't distinguish between the three components. How fuzzy does a bin have to be to se the edge of the "sweet spot"?
Add to that difference peoples sensitivities two the three components and you have a problem with communicating the differences. The good accommodation of a young viewer might not find field curvature much of an issue and some even like it to add "depth of field"). How much do the two astigmatisms have to be balanced so the view doesn't look too "odd". How much do you really like use edge sharpness (some really look around the view others centre on movement in the field). There's a lot of variation between bins users.
One way of reducing the effect (say on the Nikon SE, EDG and even more so on the Swaro SV) is to add a field flattener (removing the field curvature component) so that the ocular design can spent time optimizing the other components.
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It's all those little clear glass thingies in there.
I enjoy unique viewing pleasure with the nearly pure field curvature in the Fujinon 7x50 and 6x30 FMTs. If my eyes were young again and had merely 3 diopters of focusing ability, the field could appear essentially flat and sharp all over. (Unfortunately, I didn't develop the taste nor budget for such stuff until it was too late.) But even with my eyes' sorry aging state, this field curvature can be used as a kind of emergency focus control: infinity in the center, 30 yds at the edge. In a pinch, for example if you are forced for some reason to one-hand the binocular, this mitigates the stodgy IF focusing, and helps turn the easily grippable 6x30, already blessed by great depth of field, into a, no kidding, acceptable birder.
My point is not that FMT is best, only that, in agreement with Kevin, you can "measure" sweet spot any way you like, but in practice sweet spot depends on the nature of the aberrations, your eyes, and how you use the binocular.
Ron
I enjoy unique viewing pleasure with the nearly pure field curvature in the Fujinon 7x50 and 6x30 FMTs. If my eyes were young again and had merely 3 diopters of focusing ability, the field could appear essentially flat and sharp all over. (Unfortunately, I didn't develop the taste nor budget for such stuff until it was too late.) But even with my eyes' sorry aging state, this field curvature can be used as a kind of emergency focus control: infinity in the center, 30 yds at the edge. In a pinch, for example if you are forced for some reason to one-hand the binocular, this mitigates the stodgy IF focusing, and helps turn the easily grippable 6x30, already blessed by great depth of field, into a, no kidding, acceptable birder.
My point is not that FMT is best, only that, in agreement with Kevin, you can "measure" sweet spot any way you like, but in practice sweet spot depends on the nature of the aberrations, your eyes, and how you use the binocular.
Ron
BobinKy
Well-known member
Ron...
Thanks to the field curvature, like you, I often use the emergency focus on my Fujinon 7x50 that you described above. I can only imagine how wonderful the 6x30 does this maneuver.
...Bob
brocknroller
porromaniac
Kevin and Ron have answered the technical aspects quite well, and Kevin touched upon the subjective aspect too, that is, focus accommodation.
A closely related question that should be asked is what is one's tolerance for fuzzy edges? I've always disliked fuzzy edges on bins, perhaps because I started out using bins for astronomy.
When panning, my eyes dart ahead and look into the fuzzy ring, and I find that distracting, because instead of looking for the bird, I'm looking at a blur.
This is the main reason why most of my bins have been Nikons (Jap. made) since they tend to have very ample sweet spots and gradual fall off at the edges.
Attitudes seem to have changed since I first registered on this site. I remember discussing edge performance, and getting answers like, Duh! my good man, you simply move the bird to the center.
But today, more birders seem to be paying attention to edge performance in binoculars.
It could be because of the popular models with field flatteners or ample sweet spots such as the Trinovid, SE, EL, SLC, Meopta, Kowa Genesis/Prominar, Premier LX L, and EDG have attracted their attention, and they have finally become converts.
I hope that this trend will continue and that edge performance will be valued as an important design criterion by bin manufacturers, though I would like to see a some trade off in introducing pincushion for more smooth panning and the elimination or at least reduction of "rolling ball".
In particular, it will be interesting to see if Zeiss changes its design philosophy now that the SV EL has shown that you can have both a razor sharp centerfield and good edge performance.
