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"Sweet Spot"

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#1 polariez

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Posted 10 March 2016 - 10:52 PM

What is the technical term for "Sweet spot"? 

 

It's like you have to look only at the center of the binocular to get the full view.. when you move your eyes a bit to the sides or up/down, you see black veil.

 

For example you are watching the skies.. then you aim your binocular at a nearly object say the lighted garden nearby you. After you adjusted the IPD to be closer and watching at center, it's like the sides are more dark. Does it mean you mustn't adjust your IPD when viewing closer objects?

 

Does the Swarovski 8x30W have large sweet spots? Because when I move a little off center.. there is dark veil. I'm planning whether to get the Zeiss HT or Swarovski 8.5x42 EL. Do these two have larger sweet spot? Again what is the technical terms so I can gauge if the triple prices of these are because of larger sweet spots?


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#2 Erik Bakker

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Posted 11 March 2016 - 02:27 AM

I think sweet spot often refers to how big the sharp part of the image is. The Swaro EL's excel in that. Both from side to side (edge to edge) and from near to far (depth of field).

 

What you are describing looks more like the ease of eyeplacement to take in the (whole) view. That improves with a larger exit pupil of the instrument and is best in the 6-7mm exit pupil range but is also influenced by eyepiece design. The Germans have a unique word for this: "Einblickverhalten". Which might be translated into: " the ease to take the views in" or "behavior of the views in how they present themselves to the observer".

With your example of darkening when looking at objects at closer distances that can be overcome by adjusting the inter pupillary distance is likely a matter of the exit pupil of the instrument no longer lining up perfectly with the entrance pupils of your eyes. This is easiest with the largest exit pupils fitting in the smallest entrance pupils, hence the 7x50 binoculars being a favorite for marine use.

 

Another thing to consider is that to an extend, all binoculars have uneven illumination of the field. Illumination is best in the center of the field and reduces towards the edge of the field and is caused by vignetting.

 

Random black-outs dancing through the field of view and letting large parts of the views disappear, also known as kidney beans, originate in spherical aberration of the exit pupil. This phenomenon, which plagued the larger of the TeleVue Nagler type 1 eyepieces, is described in more detail in this great optical reference work.

 

These are but a few of many characteristics of an instrument considered by manufacturers when designing and manufacturing their binoculars and influence size, shape, weight and cost of that instrument.

 

Hope this helps.


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#3 MartinPond

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Posted 11 March 2016 - 06:53 AM

'Blackouts' and position sensitivity are different than 'sweet spot'.

 

I have seen sweet-spot described as the range where the field is in focus

to its sharpest resolution.  That's silly from a metrology point of view,

since most binoculars deliver past your acuity, so your standard would be

less than you can detect. It's an unusable definition.

 

Something much easier to see and accurate to talk about is when

things are obviously losing focus, say...twice your acuity.

That's when you notice.  Once your acuity is like saying

+/- .1 volt  with a meter that measures to +/- .1 volt.

You standard must exceed your accuracy.



#4 Jon Isaacs

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Posted 11 March 2016 - 08:28 AM

'Blackouts' and position sensitivity are different than 'sweet spot'.

 

I have seen sweet-spot described as the range where the field is in focus

to its sharpest resolution.  That's silly from a metrology point of view,

since most binoculars deliver past your acuity, so your standard would be

less than you can detect. It's an unusable definition.

 

Something much easier to see and accurate to talk about is when

things are obviously losing focus, say...twice your acuity.

That's when you notice.  Once your acuity is like saying

+/- .1 volt  with a meter that measures to +/- .1 volt.

You standard must exceed your accuracy.

 

The term sweet spot is one that is used for both binoculars and telescopes but I don't believe there is a concise definition. Rather I think it is simply the region that appears to the observer to give the sharpest possible view.  That does take into account the observers acuity.

 

But in this thread is not about that, the sweet spot is a general term that generally refers to some sort of optimization. For example:  the "10 inch Dob is a sweet spot, it combines enough aperture to be reasonably capable but is compact enough that it is easy to setup and transport."

 

I think Erik addressed the question of a sweet spot in terms of eye placement. I suspect that eye relief also plays a role since short eye relief makes eye placement more critical.  With longer eye relief adjustment to the IPD may be possible just by adjusting the distance.

 

Jon 



#5 polariez

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Posted 11 March 2016 - 11:09 AM

I think sweet spot often refers to how big the sharp part of the image is. The Swaro EL's excel in that. Both from side to side (edge to edge) and from near to far (depth of field).

 

What you are describing looks more like the ease of eyeplacement to take in the (whole) view. That improves with a larger exit pupil of the instrument and is best in the 6-7mm exit pupil range but is also influenced by eyepiece design. The Germans have a unique word for this: "Einblickverhalten". Which might be translated into: " the ease to take the views in" or "behavior of the views in how they present themselves to the observer".

With your example of darkening when looking at objects at closer distances that can be overcome by adjusting the inter pupillary distance is likely a matter of the exit pupil of the instrument no longer lining up perfectly with the entrance pupils of your eyes. This is easiest with the largest exit pupils fitting in the smallest entrance pupils, hence the 7x50 binoculars being a favorite for marine use.

 

Another thing to consider is that to an extend, all binoculars have uneven illumination of the field. Illumination is best in the center of the field and reduces towards the edge of the field and is caused by vignetting.

 

Random black-outs dancing through the field of view and letting large parts of the views disappear, also known as kidney beans, originate in spherical aberration of the exit pupil. This phenomenon, which plagued the larger of the TeleVue Nagler type 1 eyepieces, is described in more detail in this great optical reference work.

 

These are but a few of many characteristics of an instrument considered by manufacturers when designing and manufacturing their binoculars and influence size, shape, weight and cost of that instrument.

 

Hope this helps.

 

My Swarovski individual focus Habicht 8x30W has only 12mm eye relief and consider it has only exit pupil of 30/8 = 3.75mm then there is only a small spot where the exit pupils can line up with the eyes entrance pupil. Is 12mm eye relief and 3.75mm bino pupil on the small side in your experience? Is this indicator one must not adjust the IPD of the binocular for this values?

 

Let's say my eyes IPD is 63mm.. would it make sense to fix the binocular at 63mm at all views?   Also, do people really have to keep adjusting the IPD of binoculars when focusing at different distances (let's say daytime use)? Do you?

 

I'm justifying keeping the IPD at 63mm.. because if so.. then I can make a belt system to engage the individual focus because it's a pain to keep adjusting each eyepiece for different focus.


Edited by polariez, 11 March 2016 - 11:09 AM.


#6 drollere

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Posted 11 March 2016 - 11:25 AM

as pointed out already, the blackout described has to do with the alignment of the eye pupil and the exit pupil of the binocular, not the quality of the image. binoculars generally have a large exit pupil, which affects (especially in daylight) how the image can be sampled by the eye.

 

because binoculars are handheld or tripod mounted, they can easily be aligned so that the target is centered in the field. to select a different target, you usually move the binocular but hold attention in the same position. this makes the optics of the exit pupil less noticeable.

 

a fundamental point about astronomical optics is that they are nearly all designed to optimize some kinds of optical performance or image parameters more than others, which determines how they should be used.




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