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Image Sharpness Across the Field of View

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#1 EdZ

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Posted 04 May 2007 - 03:38 PM

This is an excerpt from an article where a lot of data was presented and some discussion of the method of measurement is written. There are several previous articles where some or all of this explanation resides. This first excerpt is from one of them. You will see results posted for a variety of binoculars. (Note: I use this same method for testing eyepieces.) The results are image fidelity of double stars at various locations in the field of view. I have previously measured the actual True Field of View of all the binoculars (or eyepieces) used in these tests. To get complete results, not only must you test at various positions, but you must use a variety of double stars with differing separations. In addition you must have a way to measure accurately where the reading is being taken in the field of view. I discussed that process in this review.

Sharpness Across the Field
The following measurements are recorded by observing various double stars while placing another charted field star on the very edge of the field. The field star used for the distance measurement is noted. Then later, the sketches are compared to Charts and the distance from edge to the double star resolved is easily determined. In this way I can get fairly accurate measurements of resolution along the axis from the edge. I have a table of measurements that I have collected.


The following text was first reported in this article;
100mm Binoculars - What Can You See?

Sharpness Across the Field of View
We know what the edge sharpness test seems to show. Not surprising, but some of the best binoculars show significantly better sharpness towards the outer edges of the field of view. The Fujinon FMT-SX 16x70, the Nikon SE 12x50, Pentax PCF WP 16x60 and the Oberwerk Standard 20x80 display excellent sharpness across the field. The Oberwerk BT100 at f6.2, with 20mm TV plossls at 31x is sharp right up to the very edges of the field stop. You can put a 7" double right up to the edge of the field stop and still observe the two components cleanly separated.

A binocular with a sharper image still sees fainter stars further out in the view. One with a less sharp image spreads the star images out over such a large area that it has already lost the faint stars from view by the time the sharpness has become only fair and has lost many more than just the faint stars at the point where sharpness is poor.

In the Celestron 25x100 sharpness of image is still good at 70% out from center, but then it drops off pretty quickly beyond 70%. By 80% out it gets rather poor. The Celestron has a problem with one side of the binoc that won't allow it to reach pinpoint focus as well as the other side.

The Celestron 25x100, when observing a 14" double, was very clear at 50%, still good at 60% but poor at 70%, all the way around. Doubles of 21” and 22” could still be seen at 70-75% out from center. A 29” double was the limit at 80% out.

In my sample of the Oberwerk 22x100, an unusual result is found. The best image sharpness is not centered in the lens. In order to see it, sharpness needs to be tested across the lens at various hour lines on a clock. Then it becomes obvious. Along one hour line, sharpness is OK to 70%, poor by 80%. But on the opposite side, it's sharp to only 40-50% and poor by 60%. Testing other hour lines shows the sharp central point is off-axis.

The other side of the binocular is mirror image. That tells me the optical axis of these binocular barrels is out of alignment in these 22x100s. The images are merged, but the optical axis of the two barrels does not center the optimum light in the middle of the exit pupil.

In the Oberwerk 22x100 the 14” double, on one side was good to 60-70% and poor at 80, but on the opposite side of the same barrel was good to only 40% out and poor by 50-60% out. The 21” double was seen out to 70%. Best with a 22” double was 70-75% out, less on the poor side. The 29” double could be seen at 80% out.

In the Pentax 16x60, the same 22” double can be seen as two components within 10% of the edge.

In the Fujinon 16x70, Mizar, at 14.4”, can be seen at 60% out from center and a 22” double can be seen at 80% out. A faint mag 7.0-7.2 double at 16.7” can be seen at 50% out. Another faint double Struve485 in NGC1502 7.0-7.1 at 18” is seen out to 65-70%. The BD components of the Trapezium, 19.2” are still seen at 70% out from center. A 36” double can be seen out within 10% of the edge.

The Nikon 12x50 SE can see 14.2” out to 50% and can see 22” at 70% out handheld. Faint mag7 Struve2690 at 16.7” is seen out to 45%. Faint mag 7 Struve485 at 18” is out to 60-65%. Alberio at 34” is still seen at 80% out.

In the Oberwerk 20x80 Standards, the 14” double is resolved perfectly at 60% out from center and can be seen out to nearly 70%. The 22” double can be seen to 80%.

In the BT100 with a 26mm TV plossl at 24x, the 14” double can be put right on the edge of the field stop and still be seen as two. At 25x with the standard 24.5mm wide-angle eyepiece supplied with the BT100, a 7.4” double can be seen out to 60-70% before it becomes distorted.

Method - Apparent Resolution
This method of checking doubles with various binoculars at points along the radius allows us to determine the loss of sharpness by referring to the apparent resolution at the points measured. For this, the magnification comes into play. The separation of the double multiplied by the magnification gives apparent separation as seen at that magnification thru that binocular. It should be easily understood even a fairly close double will get easier with higher magnification. However, a considerable degree of sharpness is still required to see close doubles split. Therefore, various doubles are used to test sharpness and a lower apparent separation # is better.

Based on the above, the sharpness as measured by apparent resolution (in arcseconds) of various binoculars is given. When comparing one binocular to another, at any given % out from center, lower is better.



The following excerpt is from an article on small binoculars. No table data is included in this article, but I was reporting values in this article from data that I had collected in the table
Nikon Action Extremes 8x40, 10x50 and 12x50 compared to Orion Ultraview 10x50 and Pentax PCF WP 10x50, Nikon SE 12x50 and Oberwerk LER 15x70

Sharpness Across the Field
The following measurements are recorded by observing various double stars while placing another charted field star on the very edge of the field. The field star used for the distance measurement is noted. Then later, the sketches are compared to Charts and the distance from edge to the double star resolved is easily determined. In this way I can get fairly accurate measurements of resolution along the axis from the edge. I have a table of measurements that I have collected.

The excellent Nikon 12x50 SE has one of the best measures towards the edges for sharpness resolution of any fixed power binocular I own. It can clearly see 14” out to 50% for an apparent resolution (sharpness of image) equal to 12x14.2= 170 arcseconds. An 18” double is seen at 65% out and I can see 22” at 80% out (70% handheld) for an apparent resolution of 12x22= 264 arcseconds at 80% out. Of the other 20-25 binoculars I’ve owned only the Pentax 16x60 came close and only the Oberwerk BT100 exceeded that performance.

In the 10x50 AE model, 50% out from center the image is good, stars are slightly enlarged, a 22" double can still be seen as double. By 60% out from center the distortion begins. Stars don't turn into slightly unfocused enlarged blobs, they elongate into curved streaks as if you were seeing a half-hour long time exposure photo pointed at the north star. At 60% out, the image is fair, OK for wide-field background, but not clear enough to see the 22" double split, it's now an elongated line. 70% out from center, the image is poor, the double is an elongated blob, all stars have a radial elongation. The further out from center the longer the streaks get. Between 70% to 80% out from center, the curved streaks that are supposed to be stars are really distracting. Stars near the edge of the FOV are extended lines several arcminutes long.

As far as sharpness out from center, the Nikon AE 10x50 and the Orion Ultraview 10x50 were nearly identical. If the distortion can be noted on star fields, it is present in whatever you choose to look at. The Pentax PCF WP 10x50 has pinpoint sharpness. Unlike these other two mentioned it does not drop off quickly, but sharpness holds fairly well out towards the edges.

The 12x50 AE did a little better than the 10x50. I would say it was decent out to 70% from center. But the 10x50 has a wider field of view. So really both the 10x50 and the 12x50 are producing approx. the same undistorted field of view of about 3.3°. The 8x40 model suppresses this distortion a little more than the others. The lower magnification makes it less apparent.

For detailed viewing, only the central 50%, possibly to 60% in the Orion, is effective. For wide angled finding, you could use the field out to 70%. The net effective field out to 70% provides a 4.25° usable field of view in both models of 10x50. Beyond that, everything is distorted.

None of these three Action Extreme binoculars rival the field of sharpness out from center seen in the Oberwerk 15x70, Pentax PCF WP 10x50, Pentax PCF III 12x50, or the Swift Ultralite 8x42. These other four binoculars have good image quality to between 70% and 80% out from center.

The Pentax PCF WP 10x50 have good image quality to between 70% and 80% out from center. The Pentax has a narrower field of view than both the Nikon AE and the Orion Ultraview. The Pentax is 5.0°, both the others have 6.1°. BUT, in favor of the Pentax, at 75% out in the Pentax you can still see double stars as stars with decent resolution. The Pentax PCF WP 10x50 has a sharper wider-field image than either of the above. At 80% to 85% out in the Pentax, you are still able to use 4.25° of the field for observing. You can use the entire 5.0° field for wide-angle finding.

The Pentax PCF III 12x50 has a TFOV of only 4.1°, but it is so sharp across the field, it is fine for detailed views out to 70% and the view is acceptable for wide-field views all the way to 90% out. That gives a good 3.7° usable field of view.

The Nikon AE 12x50 actually measures Tfov of 5.2°. The 12x50 has good resolution sharpness out to 60% of the field and a usable field of view of 70%. That would give 3.6°, just slightly less than the Pentax. Both the Pentax PCF III 12x50 and the Pentax PCF WP 10x50 exceed the resolution ratings for the Nikon AE 12x50, by going about 10% wider.




This is the excerpt from the article where one section of the Sharpness Table appeared. I have been posting results from data collected and reported in this table for about 5 years, and there are several previous articles where some or all of this explanation resides. This table (linked thru my gallery) covers the results of all binoculars tested.

Oberwerk Ultra 15x70
See also this article for complete explanation of the aberrations that cause lack of sharpness.



SHARPNESS ACROSS THE FIELD OF VIEW
This is basically a phrase or term used to define how sharp or how poor the image gets in the outer field of view, without any attempt to define which aberrations are the cause of the distortions. The use of the word distortion here is not meant as displaced star image, but distorted size or shape of the point image; bloating, elongated, curved, by various aberrations, mostly due to curvature, spherical aberration, astigmatism and coma. This term distortion is what helps define the measure of field sharpness.

Distortion
The Ultra 15x70, GO20x70 and Obie v2003 15x70 were all compared to see the distortion of the image of a star in the outer edge of the field. Using a double star of known separation in the same field of view, or at least near the field of view, so I could pan back to it as a reference scale, I measured the size of the distortion. In this case I used the stars in the head of Draco, choosing a star that would end up near the edge of field of view while at the same time I could also see Nu Draco which is a 62 arcsec double. The spread on the distorted star images was almost exactly the same as the spread in Nu Draco. At approximately 80% out in the field, a bright star was elongated to about a half arcminute in length. At near 90-95% out, the star was elongated to one arcminute in length. There was not a significant difference among the three binoculars.

For comparison, a new pair of Meade 5000 26mm plossls in the BT100 shows approximately the same amount of 60 arcseconds distortion in the image near the edge. The Fujinon 16x70 has about 40 arcseconds distortion at the very edges. A pair of 26mm TV plossls in the BT100 shows about 15 arcseconds of distortion at the extreme edge of field. A pair of 14mm TV Radians in the BT100 shows less than 6 arcseconds distortion at the extreme edges.

Field Sharpness
Five out of my 7 top binoculars (BT100 excluded) with the sharpest pinpoint images in the outer field of view have 52, 51, 51, 50, and 45° AFOV. The only binoculars with a wider field of view that make it into the top seven are the Fujinon 16x70 and the Nikon SE 12x50.

Generally, the wider the Afov of the eyepieces, the more quickly aberrations show up in the outer portions of the fov. This Oberwerk Ultra 15x70 has an Afov of 65°. So, unless it is in a class with the Fujinon FMT-SX 16x70 (65° Afov) or the Nikon SE (60° Afov), it might not be expected to achieve the rank of having some of the sharpest pinpoint images in the outer edges of the field.

Sharpness in the outer edges of the field of view is reduced by a combination of aberrations from the objective lens and the eyepieces. In the Ultra, stars begin to show minor but acceptable elongation parallel to field edge at 55% to 60% out from center. It begins to get unacceptable about 70-80% out. Significant elongation really distorts stars at 80-85% out.

The 15x70 Ultra can see theta Ser, a 22” double star, at 65-70% out from center. This means at 70% out it has a sharpness rating of 330. The Ultra can still see 16 Cyg at 80% out, therefore the Ultra, at 80% out, has a sharpness rating of 585.

By 70% out the Ultra is still keeping up with the sharpness of the Fujinon. By 80% out the Fujinon pulls away. Between 65-70% out the GO 20x80 has an average sharpness of about 500-600, not as good as the Ultra. By 75-80% out the GO 20x80 drops to 750.

100 Herc 14.2" seen clear out to 55% = 213 arcsec at 55% out
Stf 485 Cam 18" seen clear out to 60% = 270 arcsec at 60% out
Theta Ser 22" seen clear out to 70% = 330 arcsec at 65% out
resolution appears even all the way around, no off center sharpness

Compared to the Ultra 15x70, in all cases the Fujinon 16x70 was able to resolve the pairs 15% to 20% out further towards the edge of field. The Fujinon could resolve both 15 Aql (38") and 16 Cyg (39") right at the very edge of the field stop, 100% out. The Fujinon could resolve theta Serpens (22") at 80-85% out from center. The Ultra could only see theta Ser to 70% out from center. In most cases the Garrett Gemini 20x70, using the average of uneven readings, sharp field closely matches the Ultra out to about 65%. Beyond that it drops off.

What I see is this Ultra binocular seems to have a fov sharpness that is not pin-point sharp all across the field, or as some people might say, sharp edge-to-edge. Stars begin to show minor acceptable distortion at 55% to 60% out from center. A 22" double at 65-70% out is a pretty stringent test. At 75% out I could not see a 22 arcsec double as a double star. By 85-90% out that same 22" double is distorted to a size of approx 1 arcmin and cannot be recognized as stars. I would put the usable field at about 75-85%, not as good as the best, but better than many.


Another article were a section of this table appeared and a brief explanation of how the data was collected and compared resides in this article:
Binocular Tests and Comparisons

This test is really checking the outer field of view for aberrations such as coma or spherical aberration. Not surprising, but some of the best binoculars show significantly better sharpness towards the outer edges of the field of view. The Fujinon FMT-SX 16x70, the Nikon SE 12x50, Pentax PCF WP 16x60 and the Oberwerk Standard 20x80 display excellent sharpness across the field.

A binocular with a sharper image still sees fainter stars and objects further out in the view. One with a less sharp view spreads the star images out over such a large area that it has already lost the faint stars from view by the time the sharpness has become only fair and has lost much more than just the faint stars at the point where sharpness is poor.

Sharpness as Measured by Apparent Resolution (in arcseconds)
Checking doubles at various points along the radius allows us to determine the loss of sharpness by referring to the apparent resolution at the points measured. For this, the magnification comes into play. The separation of the double multiplied by the magnification gives apparent separation as seen at that magnification thru that binocular.

It should be noted here, the placement of the test star is measured accurately by placing a chartable field star either in the center of the fov or right on the edge of the field stop and then measuring a large scale chart distance from the test star to the chart star.

It should be easily understood even a fairly close double will get easier with higher magnification. However, a considerable degree of sharpness is still required to see close doubles split. So a higher magnification binocular must be able to split a closer double to get at same apparent resolution as a lower magnification binocular. As an example, a 12 arcsecond double in a 20x binocular would be equivalent to a 9.6 arcsecond double in a 25x binocular. Therefore, various doubles are used to test sharpness. Based on the above, the sharpness as measured by apparent resolution (in arcseconds) of various binoculars is given. When comparing one binocular to another, at any given % out from center lower is better.

With the Oberwerk 25x100, I was able to observe a 7.3 arcsecond double (11 Mon) still cleanly separated at about 50% to 60% out from center for an apparent resolution of 183 arcseconds. That's really very good central area resolution performance and at the 60% out mark exceeds the capabilities of the Celestron 25x100 by nearly a factor of 2. It also exceeds the capabilities of the Oberwerk 22x100 which at very best achieved a resolution of 310 arcseconds at 60% out. The Oberwerk 25x100 is still able to resolve doubles of 18” and 19” at 75% to 80% out from center.

In the Celestron 25x100 sharpness of image is fair at 70% out from center, but then it drops off very quickly beyond 70%. By 80% out it gets rather poor. The Celestron has a problem with one barrel of the binocular that won't allow it to reach pinpoint focus as well as the other side. The Celestron 25x100, when observing a 14" double, was very clear at 50%, still good at 60% but poor at 70%, all the way around. Doubles of 21” and 22” could still be seen at 70-75% out from center. A 29” double was the limit at 80%.

The Oberwerk BT100 at f/6.2, with 20mm TV plossls at 31x is sharp right up to the very edges of the field stop. You can put a 7" double right up to the edge of the field stop and still observe the two components cleanly separated. Why? Because it is using a well corrected TV 20mm plossl.

With the BT100, superior sharpness is seen with the TV 26mm plossl 2.0° fov and with Televue 20mm plossl 1.5° fov eyepieces. View is quite a bit narrower than the stock WA 2.5° fov eyepieces, but you can literally put objects right out to the edge of the field stop and still see a near perfect view with almost no distortion present. In the BT100 with a 26mm TV plossl at 24x, a 14” double can be put right on the edge of the field stop and still be seen as two. At 25x with the standard 24.5mm wide-angle eyepiece supplied with the BT100, a 7.4” double can be seen only out to 60-70% before it becomes distorted.

I compared Meade 26mm SP LP to Televue 26mm plossls and also to the stock Oberwerk 24.8mm WA that came with the BT100. Also I compared stock WA 25x eyepieces to Obie 25x100 IF. Both 26mm plossls are much sharper towards the edges than the stock WA eyepiece. There is very little loss of image sharpness. Stars are nice round sharp stars right up to the edge. The BT100 stock WA 25x eyepieces are equivalent to the Oberwerk 25x100 IF out to 60%. But by 80% to 90% out the 25x100IF holds up better and the stock WA eyepieces really lose sharpness of image.


Table of Field Sharpness Ratings for 30 Binoculars

In addition to field sharpness, these same type tests are used for on-axis rersolution. Also, you can see from some of the discussion in the articles above (some of which you need to refer back to the complete article to fully understand) that you can find out a lot more about a binocular by field resolution tests than just sharpness across the field. Examples include identification of the aberrations causing the lack of sharpness and identifying off-center optical axis.

edz

#2 BobinKy

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Posted 04 May 2007 - 04:06 PM

EdZ:

WOW!

Thank you for making this available. It looks fantastic. I look forward to digging into this important topic.

Again, thank you very much.

:band:

#3 SaberScorpX

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Posted 04 May 2007 - 05:23 PM

While I'm sure this is all relatively accurate information, it is specimen-specific, i.e. among ten of the same model one may find substantially varying results. Just something to remember.

Was also wondering how much testing binos while wearing glasses might skew the data, especially between instruments with and without sufficient eye relief to even take in the whole field. Viewing field edges off-axis at an angle thru ones personal prescription seems very subjective.


Stephen

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http://www.astronomy...er/saberscorpx/


#4 EdZ

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Posted 04 May 2007 - 07:39 PM

Some of the most relavant notes culled out of these articles:

Five out of my 7 top binoculars (BT100 excluded) with the sharpest pinpoint images in the outer field of view have 52, 51, 51, 50, and 45° AFOV. Generally, the wider the Afov of the eyepieces, the more quickly aberrations show up in the outer portions of the fov.

Sharpness in the outer edges of the field of view is reduced by a combination of aberrations from the objective lens and the eyepieces.

This test is really checking the outer field of view for aberrations such as curvature, coma, astigmatism, and spherical aberration. Not surprising, but some of the best binoculars show significantly better sharpness towards the outer edges of the field of view. The Fujinon FMT-SX 16x70, the Nikon SE 12x50, Pentax PCF WP 16x60, the Oberwerk Ultra 15x70 and the Oberwerk Standard 20x80 display excellent sharpness across the field.

A binocular with a sharper image still sees fainter stars and objects further out in the view. One with a less sharp view in the outer field area spreads the star images out over such a large area that it has already lost the faint stars from view by the time the sharpness has become only fair and has lost much more than just the faint stars at the point where sharpness is poor.

The same tests show the same types of results on eyepieces tested in telescopes. The significance is that probably most of the aberrations are coming from the eyepieces. A correlation is that it is eyepiece field stop diameter that determines field of view.

edz

#5 Ortho2000

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Posted 05 May 2007 - 12:12 AM

What an excellent approach for evaluating binoculars. I wonder if it would be worthwhile to develop a rough " checklist" that could act as a guide for readers who wish to submit a binocular review. It would serve as a guide, not a mandatory format.

Understandably, when people write a review, each person may have their own way of reviewing performance. With an easy to use checklist guide, future binocular evaluations might be improved, become a tad more standardized, and therefore allow more meaningful comparisons to be made by readers scanning the reviews.

Just a thought... :grin: Again....GREAT job EdZ..Kudos

#6 EdZ

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Posted 05 May 2007 - 05:35 AM

That's sort of been done already, actually about two years ago. If you follow the "Best Of" threads thru "Edge Performance - Sharpness of Field" you come to threads where the process is described. This is nothing new here, it is just new to some of you who haven't been looking for it. In fact if you follow thru the Best Of threads on the topic "How to Write a Review" you will come to a page that points to all the aspects that you might want to measure. From there it points back to the Field of View Sharpness discussions. So really there are several pointers out there to this process. You will see thru those threads that some users prefer data reported in different formats.

edz

#7 Mark9473

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Posted 03 June 2008 - 02:22 PM

EdZ, your recent thread about the Vixen Foresta 7x50 made me go back to this older thread about measuring image sharpness across the field of view. I hope it's appropriate that I post a question here.

Having followed most of your writings on this topic over the last couple of years, there is one thing which I still do not grasp:
What does it mean to see a double star as double for the purpose of these tests?

Does it count as a double star as soon as it looks different than a single star?

Example: I was recently looking at epsilon Lyrae through a pair of 10x50 with pretty poor edge sharpness. At the very edge of the FOV, the two components had swollen to about 2.5 arcminutes large blobs which were overlapping about 3/4 their diameter. Yet it still very obviously was a double star...

#8 EdZ

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Posted 03 June 2008 - 02:49 PM

At the very edge of the FOV, the two components had swollen to about 2.5 arcminutes large blobs which were overlapping about 3/4 their diameter. Yet it still very obviously was a double star...


what you were very clearly able to see from this is:

that a 207 arcsecond pair is not so totally distorted at the edge of view as to have totally dissappered as a double;

but that it also appeared to be swollen to about 2.5 arcminutes distortion;

it therefore showed about 150 arcseconds distortion at the edge.


Depending on the power of the binocular and the width of the double used and the location of the observation within the fov, you can find a point where the pairs used for the observation just blend together and can no longer be seen as pairs. If you work carefully and watch for the point of disappearance and mark the position by some means, orientation to other stars and then reference to large scale detailed charts, you can scale the view in your binocular and record the measurements.

At the point a 60" double disappears from view as two, you are observing 60 arcseconds of total distortion. What you are really using doubles for in this test is to watch for the point where the double can no longer be seen as a double. Best to use several different sizes of doubles to test acrooss the entire fov.

Some closer pairs of doubles would help you close in on the total distortion at various positions out from center. For instance, the 207" pair you used is pretty much beyond the total distortion for about half of all binoculars. Perhaps a pair at 2 arcminutes or about 2.5 arcmin is good for the extreme edge. But at about 50%-75% out from center, you'll need to use some doubles as close as 20" to 60" to get a better measure.

Typical binoculars in the power range from 7x to 12x have 60 arcseconds distortion somewhere between 50% and 90% out. The higher the power binocular, the smaller the doubles you need to use to test. Observing a 30" double in a 20x binocular is the same as observing a 60" double in a 10x binocular. That's also the same as observing a 24" double in a 25x100.

edz

#9 Mark9473

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Posted 03 June 2008 - 02:56 PM

Thank you EdZ! So had epsilon Lyrae merged to a single blob, the distortion would have been 207 arcseconds. Now that I understand it, it's actually pretty simple...

#10 jrw11

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Posted 03 June 2008 - 05:28 PM

I have noticed that with some bino's at least. The image seems to be sharper if I put near the bottom FOV of the bino.

#11 EdZ

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Posted 04 June 2008 - 04:19 AM

There are two different possibilities.

Distortion near the edge of view is almost always in the form of stretched out lines parallel to the edge, That's a combination of curvature and astigmatism causing that. If you align the pair of stars parallel with the edge of view, as opposed to having the pair of stars perpendicular to the edge of view, the distortion in the stars can be seen to stretch between the pair and will merge together. If you align the stars perpendicular to the edge, the stretched out view of each star will never come into contact with the other star. Yes, you will be able to still see the two stars, but they will look like lines. The intent here is to measure the distortion, and that requires the distorted line to stretch between the known dimension of the pair.

Try placing nu Draco (62 arcseconds) so that they are parallel to the edge and then so that they are perpendicular to the edge. You will see the same amount of distortion in the stars, but in the parallel position, the distortion will be lined up between the stars and you can measure it.

Now, that being said, I've tested a number of binoculars that have considerable imbalance in the centered sharp image, some being as much shifted as 20% of the field of view. In other words they are much more distorted on one side of the fov than on the other side. The sharp fov may appear to go from 80% on one side to only 40% on the other side of the view, as opposed to a balanced 60% each side of the center of fov.

In almost all cases, this shifted imbalance in field sharpness correlates with a shifted imbalance of light beam transmission (another test). In other words, this is showing the optical elements are tilted and the optical axis is not precisely aligned with the mechanical axis. Adjustments are made such that the light beams exit parallel to each other and you have a merged image, but the parallelism of the binocular view is either moderately or severely off-center.

I see this often, and have seen it to a degree as much as 20% out of balance. In any higher quality optics, I have never seen this more than a very small few percent.

edz

#12 jrw11

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Posted 04 June 2008 - 10:34 AM

Thanks, Edz! I noticed it when looking at the craters of the moon.

#13 GlennLeDrew

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Posted 18 June 2008 - 10:23 PM

For myself, a large apparent field of view is extremely important. I observe Milky Way structure, and particularly dark nebulae. A large apparent field allows more of the sensitive outer retina to be utilized, and also allows to take in larger objects in their entirety.

Therefore I'm willing to sacrifice edge sharpness to an almost heretical degree, according to my friends. In smaller instruments, particularly those hand-held, it's so easy to swing the instrument so as to center an object of interest and hence extract maximum detail. Why bother swiveling one's eyes toward the field edge anyway, where almost always the sharpness is inferior?

Moreover, the parts of the retina at any significant angular distance from the fovea sample the image at lower spatial frequency. As long as your gaze is directed toward the field center, even surprisingly bad aberrations are sensed by peripheral vision as good enough.

#14 DJB

DJB

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Posted 19 June 2008 - 01:12 AM

Hi Glenn,

I agree with you 110%. That's the reason I love my VWA, smaller binoculars. I will forgive the optical deficiencies with these units just to get what most binoculars do not offer. Just my thoughts on this.

Best regards,
Dave.


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