William Optics 10x50 ED review
Posted 28 February 2008 - 07:12 PM
II - Advertised Features
III - Advertisement Accuracy
IV - General Impressions
a - Mechanical Construction
b - Eyepieces
c - Objectives
d - Coatings
e - Internal Construction
V - Optical Performance
a - Chromatic Aberration
b - Coma
c - Field Curvature
d - Distortion
e - Spherical Aberration
f - Astigmatism
g - Exit Pupil Curvature
h - Brightness and Contrast
i - Internal Reflections
j - Prism Light Cut-off
VI - Resolving Power
VII - Collimation
VIII - Notes on Focusing
IX - Conclusion
I - Introduction
Binoculars were my first Astronomy equipment and I'll always remember the views of Hyakutake and Hale-Bopp I had through my old 7x50. Long ago dismantled, I can't remember actually how good they were. I just remember those great (in my memories they were great!) views. I don't remember why I "compulsively retired" them, but I'm sure I wasn't completely satisfied with them.
My second pair, a 15-20x50 Tasco, lasted for about 2 weeks. I let them inside the car (not at direct sunlight) in a hot Portuguese summer day and they melted. Well, sort of... When I took them outside, I looked through them and saw a twin traffic sign down the road. I instantly realized they were lost.
So instead of beating the dead horse, I forgot about the subject until a year ago, when I found cloudynights and discovered that quality plays a definitive role in this kind of equipment. Any auto manufacturer will produce cars that actually move, but not all optical manufacturers will produce binoculars that you can actually see through them. That's why prices vary from 20 to $2000 (a factor of 100) in the same category. In the car industry that would translate into one's being able to buy small hatchbacks between 10 000 and a $1 000 000!
I won't go into why I bought a 10x50 or this particular model, but I'll explain why I decided to buy from this particular vendor. Theses binoculars are of the "Kunming series 8" breed and as everybody knows now, there are plenty of vendors offering them in the West. Three things made me choose this version in particular, a bit of precipitation, the (this is a bit stupid) advertised ED glass and (completely irrational) the color. I don't regret anything but right now, I don't know if I'd make the same choice. There are other vendors that sell, for a bit less, very similar products and are known for very good support and warranty.
Just a note before we cut to the chase. Since my other binoculars are dismantled and were in a bad condition, I have no means of making comparisons. So, I'll be as discriminating and objective as I can (when possible) and will try to avoid saying things like "The view is awesome, I never saw anything like this!". I will not go into details discussing how I identified optical aberrations, checked for collimation errors or this sort of details. There's a huge amount of info here on cloudynights, search for it.
If you have any doubt, want to make a correction or have a suggestion please send me a message.
II - Advertised Features
Focus system: IF
Prism type: Porro
Exit pupil: 5mm
Eye relief: 18mm
Minimum focus distance: 4m
Focus adjustment: +5 to -5
- Enhanced FMC on all surfaces
- Nitrogen filling for fog and water proof
- Shock proof and anti-slip
- Extra-low dispersion glass objectives
- Semi-hard case
- 2 year warranty
Price tag: 299.0 USD
III - Advertisement Accuracy
There are some inaccuracies. I won't discuss everything in detail, I didn't even measured IPD, eye relief or FOV:
- The focus adjustment is labeled between +5 and -5. However, you can go past this a full diopter. That's a bit more adjustment that you may need in certain situations, the effective range is -6 to +6.
- The close focus distance is advertised has being 4m but I found it to be more like 9m. Maybe you can focus a bit closer, say 8 or even 7m, but at cost of eye strain.
- Although not advertised, the tripod adapter came included. I don't know if this was a mistake but I appreciate it!
IV - General Impressions (warning, subjective evaluation)
Very large and very heavy! The eyepieces are huge.
They look very well built and give a nice quality feel. The touch of the rubber is nice and soft. The massive prism housings are nice to grab, they provide a good grip while looking through. To me, and this is something that varies with the person, they are very good ergonomically.
IV a - Mechanical Construction
Very solidly build. Nothing moves without offering a good amount of resistance, you can focus and adjust them for your IPD, use them every day for a whole month and they won't lose the settings. The focus mechanism and center hinge work smoothly. The body is all metal, covered with soft rubber, very solid and every component fits perfectly.
IV b - Eyepieces
These are very large. In fact, they measure about 50mm in diameter. If you recall, this is a 50mm instrument, so that means that the eyepieces are almost as large as the objective lenses. That may become a problem to some people with narrow IPD. My IPD is in the lower 60's, my nose is "normal" and I can use them comfortably but almost at the limit. In fact, my nose doesn't let the eyepieces get completely glued to my face but I found that to be a good thing because it keeps my eyes at the right distance in order to see the whole field and avoid blackouts.
The exit pupils are perfectly round, unobstructed, and measure between 5 and 5.2mm, depending on the focus setting. This is normal because when the focus mechanism is rotated, the distance between the eyepiece and the objective varies, then the magnification also varies and then the exit pupil diameter also changes.
Other vendors (WO doesn't provide this information) say they are of a 5 element, 3 group design. The external face of the eye lens is flat and very large. After I tested everything for aberrations, I researched a bit and I'm pretty sure they are of the Erfle type. Everything matches. If they really are Erfles and knowing that these eyepieces work better with longer focal length, I reckon that the 7x version of this model might show less pronounced aberrations at the edge of the field.
IV c - Objectives
Well, this is a selling point. There's been some debate about the type of glass being used here, some really think Extra-Low Dispersion glass is being used due to what they consider to be a very good color correction, others argue that the price of the ED glass would make these binoculars much more expensive. So, are they really using ED glass? Good question, if you know the answer, please mail me.
As many already know, in the real world (not the advertising world) ED glass isn't for itself synonym of absence of chromatic aberration. ED glass is a glass with some interesting properties for the optical designer. If used properly, and this is a big if, it can eliminate a big deal of chromatic aberration. But for that to happen, the optical designer of the instrument must take that into account, mate those ED elements with other elements with appropriate refraction indexes, with the right lens curvature, etc. You can't just start using ED glass and make everything to the exact same specifications. Ok, you can but the results wouldn't be good.
So, the only way to know for sure is to send a glass sample to a laboratory for analysis and obviously I'm not going to do it. The easiest way of being relatively sure, is to compare these to a similar competitor model from other vendor. Since they apparently are the same, if this one reveals a better color correction, it would very likely use ED glass (and a design taking advantage of it).
IV d - Coatings
Coatings look really good, it's very cool sometimes look at the objective lenses at certain angles and see nothing but a black hole. On the other hand, it's hard to evaluate this without some terms of comparison or a fairly complicated and accurate device.
A fairly good coating may look great isolated but if you put an extremely high quality coating next to it you will notice a big difference. Other than that, I noticed that the right objective is a bit more reflective than the left one. The reflections are also a bit more purple tinted. This is just cosmetic since the lens are sorted by the manufacturers to match their appearance and there's no detectable difference in brightness and contrast between the two telescopes.
IV e - Internal Construction
I didn't open them obviously but this is what I found looking through the objectives: They are well baffled; almost everything is painted with dark paint (with exception of some not so dark screws). The prisms have a black (metal, I suppose) cover in the polished faces and the lateral faces are painted in dark grey. Some very thin, shinny, unpainted metal edges are still visible in the prism's shelves. Not really a Zeiss Nobilem shielding (or price tag) but looks good.
V - Optical Performance
Overall they perform very well within 70% of the central part of the field and poorly in the remaining 30%. The worst aberrations are astigmatism and field curvature, by far. The image starts to deteriorate between 65 and 75% out from the center. There is a small eccentricity in the usable field (it's not centered in the FOV). During the day, the outer FOV performs noticeably worse, but the view is acceptable. For Astronomy it's unusable.
V a - Chromatic Aberration
With the eyes well placed in the eyepieces and the observed object right in the center of the field, you won't be able to see any colored fringes at all under any circumstances, day or night.
Starting at the center of the field and moving the Moon to the periphery, some colored glow stars to appear in the limb of the Moon. At 30% from center it's just a faint glow, barely noticeable and absolutely not intrusive, at 70% it's definitely a color fringe. However, due to the other problems that arise at approximately 70% from center, you won't be able to use the remaining field anyway, so, in general, color correction across the usable field varies between excellent (absent) and not bad (visible, but not distracting).
V b - Coma
I didn't detect any, but may be present in a very small amount and masked out by the other aberrations.
V c - Field Curvature
Present and very strong at the edge of the field. Starts to show up at about 70% away from the center and gets really bad towards the edge. In practice, if you focus a star in the center of the field and then move it to the very edge, it will be very blurry. To get it in focus again you need to rotate the focus mechanism about 3,5 diopters.
V d - Distortion
I looked for pincushion (negative distortion) and angular distortion. I used a distortion target with parallel and perpendicular lines and also used the good old traditional targets, power lines and fences.
The degree of pincushion distortion varies with the distance to the target. At close focus there's a somewhat strong amount of pincushion but at 800m it is almost undetectable. Pincushion isn't noticeable during normal observation during the day (unless you look for it) and at night, since all the objects in the sky are so far away, the distortion is undetectable.
This is to be expected, and there's a reason for that. When designing a binocular, the designers have to make choices, and one of them is flat field vs. pincushion. A flat-field, although desirable for astronomy, introduces the rolling-ball effect while panning during the day. Pincushion on the other hand, makes the view much more natural while moving the binoculars, so the majority of binoculars made have some pincushion distortion by design.
Angular distortion was detected, but very weak.
V e - Spherical Aberration
Detectable and it is annoying but not a big problem. It's rather weak but the annoying part is focusing the image. A very bright star looks almost focused throughout half a diopter, so it's somewhat hard (or even impossible if you are holding them with your hands) to find the best focus position (circle of least confusion). You can see that the brightest stars (mag 1 or brighter) hardly focus to an exact pin point but the others look amazingly razor sharp.
V f - Astigmatism
Detectable but very weak until 70% out from the center of the field. Very strong progression from that point outwards. At the edge, due to the combination of field curvature and astigmatism, bright stars become small lines and the fainter ones completely disappear. Like field curvature, this can be almost eliminated just by refocusing the binoculars. The focusing difference between the periphery of the field and the center is about 3.5 diopters.
V g - Exit Pupil Curvature
Well known as kidney-bean effect blackout effect. This happens more in some eyepiece designs than others (very well known in wide-angle eyepieces such as the nagler) and although it is present here, only to a small extent. On the first couple of days, I experienced that several times but as soon as I found the sweet spot, hardly happened again. It's really a matter of getting used to it and then you won't even notice that since you intuitively put your eyes in the right place.
This usually happens when one tries to look at the edge of the field, moving the eyes around. If you want to look at the edge you must reposition the binocular in relation to your head and look through them obliquely. This might seem a problem but really isn't because instinctively we just move the binocular until the observed object reaches the center of the field.
V h - Brightness and Contrast
Warning, subjective evaluation:
The image brightness and contrast is very good. During the day, I find it almost addictive looking through the binoculars because of this. Sometimes I don't have anything interesting to look at yet I find myself looking through them just because it's really a pleasure.
V i - Internal Reflections
There are some, but only when looking at very bright objects. If you look to the full Moon or a street light at night you'll see some ghosting, but nothing too bad or annoying. That only happens under certain angles, with very bright objects (didn't see in any other occasion other that those) and the reflections are fairly dim. Not distracting, not frequent, not a problem.
V j - Prism Light Cut-off
There's a very small dent in the exit pupils caused by the second prisms in the light path obstructing a really small amount of light that passes through the periphery of the objective. This is known as prism light cut-off. The obstructions are symmetrical to the hinge, obviously. Can be seen between the 1 and 2 o'clock position in the left telescope. This is completely innocuous, the light loss is certainly less than 1%, however I think this is a minor design issue.
VI - Resolving Power
I only tested the resolving power in day light. For that I used a 1951 USAF resolution test chart, measured the thickness of the lines on each set and measured the distance from the target. Then the resolving power was calculated in arc seconds. I didn't use the standard lines per millimeter measuring system because I feel that the resolving power in angular distances is more meaningful for amateur astronomers (at least to me is).
Keep in mind that this is a test in daylight, with an high contrast target. The eye's resolving power depends on the illumination and decreases very quickly in low light conditions.
First, I tested my eye's resolving power:
Well resolved - 45"
Barely resolved - 40"
Only a guess - 35"
A 20/20 vision equals 60" resolving power (angular distance between two points in order to be seen as separated) and is used as a screening cut off, if you see 20/20 it's good enough. The human maximum acuity is between 20/16 (48") and 20/12 (36"). I only searched for this after I done the measurements to avoid any biased judgment and it matched perfectly. Now I can do the same tests with the binoculars and use the results obtained for my unaided vision as comparison.
Center resolution (hand held) - 14" (140" apparent)
Peripheral resolution (mounted) - 14.4" (144" apparent)
Center resolution (mounted) - 3.8" (38" apparent)
There are several interesting things here. The first one is, when holding the binoculars with bare hands, the resolving power at the center is as bad as at the edge when using them mounted. Hand held the resolving power is almost 4 times worse, that's how bad it is.
The peripheral resolution (at 90% from the center) was measured with the binoculars perfectly focused at the center of the field. Since the two major aberrations present at the edge are astigmatism and pincushion, you can refocus at the edge reaching a much better resolution of about 60" (I didn't actually measured this, it's an estimate). Nevertheless no one will do this under practical use.
The resolving power of a lens increases with its diameter. A 50mm lens has a much greater theoretical resolving power than the eye lens. However, added to the theoretical resolving power, all optical aberrations must also have to be considered. That way and remember that the image that reaches our eyes is amplified by 10, a 50mm lens plagued by some severe aberrations can perform really bad, not reaching the eye's resolving power even at this low amplification. If you reach your eye's maximum resolution using binoculars, you know that they aren't preventing you to see anything even if not being perfect.
That's exactly what happens here. Although several optical aberrations were detected, from the center of the field to approximately 70% out, I can reach my eyes resolving power.
The resolving power at the center of the field, when mounted, is about 3.8". That's seeing a 0.3mm gap between 0.3mm thickness parallel lines at 16.3m! The apparent separation (at the eyepieces) is about 38". I say "about" because the magnification isn't exactly 10x, but the results accurately match (with some residual error) those found when not using the binoculars.
So I must conclude that what can be seen within the usable field is in fact limited by visual acuity. Please note that if the eyepieces were interchangeable and/or you could vary the magnification, the story would be different!
The values presented here relate to the angular distance separating two consecutive lines (a single line width). Some measurements in other texts and reviews are related to "line pairs" which effectively represent two times these angular distances. So, a 3.8" resolution here translates to a 7.6" resolution when talking in "line pairs". In those cases, all values must be doubled.
VII - Collimation
Well, I used an unusual target for this purpose, the Moon. I have an annoying amount of light pollution where I live and the moon at the first quarter was washing out the sky even more. So I just went on and used the moon. I used a technique called "staring at the infinity" that I perfected in the boring high school Philosophy classes. My teacher always told me that her classes would be useful one day, turns out she was right. I doubt she was thinking about checking collimation though.
Anyway, with the Moon at the first quarter in the field, the two images (from each telescope) don't become completely superimposed, they begin to start closing together till they stop merging and stay separated by a fifth of the Moon's diameter. So, we have a horizontal divergence error that prevents the two fields of being completely superimposed. Vertically, I could barely see any mismatch, so I used a bright star just to be sure. I didn't bother to measure it because the separation was really small.
By Bill Cook from the "Best of" thread:
"Alignment that is within the original JTII standards can be considered to function as "perfect" in that the eyes can easily accommodate the error without introducing noticeable eyestrain. For example, an 8 power instrument should have errors equal to or less than 4" Step, 10" Divergence and 6" convergence, with a 16 power having 2", 6" and 3" respectively."
So, by JTII standards, for a 10 power binocular:
Horizontal error (divergence)
Allowed error - 4.8'
Measured error - 6'
Vertical error (step)
Allowed error - 3.2'
Measured error - < 1'
The most important error is step (that's why the tolerance is smaller). The step error is only a fraction of the allowed by the standards.
The second worst problem is when images diverge (some say that axes converge, depends on how you look at it). The second worst error has the second strictest tolerance, 4.8'. In the transcription the convergence and divergence are related to the axes (they cross behind the eyepiece); in the image you see the opposite error. The error measured is the maximum allowed for an 8x instrument, so we're a bit out of the standards in this department. This is not to say that images don't merge properly, they do. Some people use binoculars that have 3 times the allowed errors here and don't complain. My eyes merge the images completely without any strain, so it's within my eye's standards.
Mind you, this is only an approximate measure of the errors at my IPD, not all the available range. Collimation is the alignment of the optical axes in relation to the hinge, to be collimated a binocular must remain aligned throughout the entire IPD range. Therefore these errors usually vary with the IPD, so at a smaller or larger IPD the error could be greater or smaller.
VIII - Notes on Focusing
There is some important information here in the forum regarding this matter. I recommend reading it. With some experience I learned that the "normal focusing method" isn't the best way of doing it at night, at least with this particular binocular. In the beginning I used to look with one eye, focus in till the smallest pin point was reached, and repeat it with the other eye. This is probably the best way to focus the binocular during the day but that really isn't the best way of doing it when skygazing.
Let me remind you that a small amount of spherical aberration and astigmatism are present. Usually I would follow the "normal procedure" of focusing in and as soon as I reach the smallest pin point, stop and leave it alone. In this case however, one has to focus in and out trying to find the position where both the smallest amount of spherical aberration and astigmatism are present. That, in addition to the eye's ability to compensate small focusing errors, allows a star to seem focused within almost a full diopter. Frequently, when I finish focusing using that method, I look at it with both my eyes, instead of having a perfectly sharp star, I get a significantly defocused image almost always to a different degree on each telescope.
To precisely focus, should be used a medium brightness star. Just focus one eye, then the other as described. However, after that, you should fine tune the focus with both eyes looking at the target. It's much quicker and precise. This way you can focus to the greatest possible accuracy, where just a slight rotation on the focusing mechanism will deteriorate the image. This happens because both eyes are looking at the star, so you can make the two superimposed images match perfectly and at the same time it prevents one of the eyes compensating the focus error.
IX - Conclusion
Adding up everything what do we get? A very well built binocular with a very good optical performance within the usable field. The build quality is up to my expectations, optically, I expected them to be better corrected in the periphery of the field.
The image is very good to 70% out from the center. That's what I call the "usable field". Within this area, some aberrations are detectable but only on a very small degree and overall the image is very good. From that point outwards all hell breaks loose and the remaining field is plagued by rapidly growing field curvature and astigmatism. These aberrations can be focused out to a great extent, but no one will ever do that during observation. The instinctive and logical thing to do, mounted or hand held is to move the instrument until the object is in the center of the field.
Although the last 30% of the field isn't usable for Astronomy, it isn't useless either. If you asked me if I'd prefer to have a smaller field stop to hide these problems like some manufacturers do in some models, I wouldn't. I know many people prefer the opposite, a small but sharp FOV. I prefer a wider field, even if not very sharp, than looking through a straw. Humans have almost a 180Âº FOV (horizontally) and our eyes can only focus on an area the size of a coin at an arm's length. The periphery is there so we can identify the objects we really want to observe. Of course, the greatest thing would be a wide and sharp FOV, but I don't think that's available for less than 300 USD.
Posted 28 February 2008 - 10:11 PM
Posted 29 February 2008 - 12:44 AM
I am surprised at the amount of peripheral distortion that you have described - 'all hell breaks loose and the remaining field is plagued by rapidly growing field curvature and astigmatism'
I wonder if the binocular you have is representative of the 10x50 ED series ? .
I have the Kunming 'series 8' 15x70 MX [non ED ] versions and the edge distortion is not nearly as severe as you describe in the smaller versions.
Thanks for the comprehensive review.
Posted 29 February 2008 - 03:50 AM
Thank you for this excellent review.
Well, It's what I was expecting: very good binos but not as Fujis which are 2X more expensive.
I've owned 2 binos since lasts years:
20x90 chineese (entry level): this one allowed me to enjoy astronomy, but unusuable without tripod;
20 euro's 10x50 BRESSER (lidl!): I think the WO will make a great difference!.
I will post when I receive it.
Nicolas, from France
Posted 29 February 2008 - 08:36 AM
I believe this is a representative sample of the series, there's no evidence or anything that leads me o suspect otherwise. However, keep in mind that the ONLY complain I have regarding this binos is the edge of the field. On every other single aspect, it is a very good instrument.
There's one particular vendor that claims that these are "flat field". Although I didn't tested that particular model, if these are actually the same and coming from the same place as everything suggests, that claim is far from the truth.
I can't talk about the 15x70, I never used one. However, even if they make use of the same type of eyepieces, just by varying the focal length, the amount of field curvature (for instance) would also vary.
I also never used the Fujinon, but judging by what others have reported, I have the same feeling though.
20 euro's 10x50 BRESSER (lidl!): I think the WO will make a great difference!
Trust me on this, you will notice a world of difference!
Posted 29 February 2008 - 09:16 AM
first very nice thorough review, well written and well organized. thank you
now, just a few comments
Field curvature does not contribute to distortion. However, I agree that the use of a field flattener to reduce field curvature, therefore resulting in a flat field which is extremely beneficial for astronomy, contributes to the undesirable rolling ball effect for terrestrial use. Pincushion, or rectilinear distortion, is employed to releive that affect. So, if you see pincushion, it should help relieve rooling ball effect.
A resolution value of 3.8" is something that might be expected from a 10 power binocular if boosted to 60 power, but certainly NOT at 10x. In fact most 10 power binoculars I've tested cannot achieve 3.8 arcseconds of resolution even when tested at 60x. Typical normal power resolution for a 10x binocular ranges from 8-10 arcseconds. Typically, resolution is measured across "line pairs". That in itself would double all your values.
You can refer to this Table of Values for USAF 1951 Res Charts for the actual measurements of line pairs and the forumla for determining resolution. Also while it is true increasing aperture contributes to increasing resolution, that does not come into play in the extreme low power use of binoculars. That can be seen in a table of values where 32mm and 40mm binoculars equal or exceed the resolution values for 50 and even some 70mm binoculars. Lens quality and Magnification are the primary contributors to resolution.
Posted 29 February 2008 - 08:54 PM
I appreciate your comments.
After some careful reading about the Seidel aberrations, I believe you are right. I don't know how I got that misconception.
Indeed it is possible to have an image corrected for field curvature, exhibiting distortion at the same time. I will edit the review, thank you for that.
About the resolution:
As I explained, I measured the resolving power by angular distances because I'm more comfortable that way. Something like "X lines/mm" is meaningless to me.
The values you get really depend on the testing conditions. I tested it in day light, with the USAF chart, black on white. If I were using a 50% grey target or testing at night using double stars, I would certainly not reach the 3.8" separation. The eye acuity really falls in low light conditions and you also have to take into account the seeing conditions, brightness difference between to stars, etc.
My 3.8" correspond to a block of horizontal and vertical lines, 0.3mm in thickness and separated by the same 0.3mm, with 5 times that length (1.5mm) at 16.3m. That's smaller than a house fly. Since this is a 10 power binocular, the separation in the field of view is 10 times greater, 38". That's the limit of human vision and I could see the separation through the binoculars, so the minor aberrations across the field aren't preventing me to see anything.
Posted 01 March 2008 - 06:14 AM
I tested and reported resolution obtained using the 1951 USAF line pairs chart (a common target of various sizes goups of line pairs) simply by using the mounted binocular just as I would for normal viewing. My results were based on the standard LINE PAIRS resolution formula upon which this chart is based.
The 4.5 arcsecond result is obtained with the same type of test, but the tester's result is reported based on SINGLE LINE resolution formula.
The 3 arcsecond result was based on line pairs formula, BUT the readings were obtained by putting a SUPPLEMENTAL POWER scope behind the binocular eyepiece to increase the power for the test up to approx a magnification of 40x to 50x. The supplemental power allows you to see a much smaller target on the resolution test chart, Commonly referred to as Boosted Resolution.
Obviously these three very different results are measured to three different criteria. They cannot directly be compared to each other. And yet there are many tables of resolution data results published all around the internet that do not explain how the results were obtained. This leads to considerable confusion.
High end birding binocular users may recognize that to which I refer. There are tables of data published on resolution of birding binoculars with not a clue given as to how the data values were obtained. Are they direct line pair readings, direct single line readings or are they line pair reading using supplemental magnification? Below I explain the accepted standard for measuring resolution in optics is either two point sources or line pairs. Perhaps reading the detailed discussions linked below will help understand why. But here are short excerpts from those discussions.
Discussion of Different Types of resolution measurements
The Snellen formula for resolution crops up whenever someone's resolution figures are related to the ubiquitous assertion that 20/20 vision equals one arcminute resolution. The letters on a Snellen chart for 20/20 vision measure 9mm high. Each letter gives an angular measure of 300 arcseconds at a distance of 20 feet. That is, each letter is 300 arcseconds or 5 arcminutes high with each line that makes up the letter 1 arcminute in thickness. So for instance, the capital letter E from top to bottom has 3 lines and 2 spaces all 60 arcseconds or 1 arcminute in width. This is what represents 60 arcseconds resolution. Typical human vision has 1 arcmin detection when observing an object letter that is 5 arcminutes long with 1 arcmin thickness. This is for high contrast black lettering on a brightly illuminated white background. Very high contrast, not achievabale under almost any other circumstances, including the observation of USAF 1951 resolution charts.
This reading of resolution from Snellen charts is much different than the accepted way we measure resolution for optics and visual astronomy. For instance, in the Snellen measurements above, the resolution is recorded based on single line thickness, or if you prefer single space thickness. For the measurement of optical instruments the accepted practice is more common to use line pairs. That is the distance from center of one line to center of another, with an equal sized space between. This is the well known line pairs measure used in the modulation (MTF) or contrast transfer function. As you can see, since this in now based on line pairs, this would double all the values from above. So, for instance measuring line pairs using a 1951 USAF Resolution Chart of exactly the same sizes lines as mentioned above would not result in 60 arcseconds (for 20/20 vision), but it would be 120 arcseconds.
Binocular Resolution Testing w/USAF Charts
There are several major differences between resolution readings from these charts in daylight (photoptic light) and night time (scotopic) point source resolution. They are (1) these charts are viewed in broad daylight, photoptic viewing, under which human eye resoltion is considerably better than in darkness, as much as 2x to 3x better; (2) They are very high contrast line pairs, black on white, as opposed to point sources, tiny round points of white on black. Resolution of black lines on a white ground can be observed at 3x to 5x normal point source resolution.
Many indivuals using these charts do so without regard for comparing optical resolution to accepted MTF standards. Often we see realistic results do not agree with what is posted on binocular sites all over the internet. Therefore it may help to give a brief explanation of resolution in optics.
The resolution of a lens is determined by the Rayleigh Limit. .... It is Rayleigh Limit that relies on the laws of physics to give fixed values for the resolution of a lens and even that must be measured under specific criteria. The long explanation and derivation of Rayleigh Limit can be found in CN Technical Reports and it need not be explained here. For any lens, the Rayleigh resolution, based on observing two point sources, is 138.4/D, where D= diameter in milimeters.
Based on the above, the point source resolution limit of various lenses is:
100mm = 1.38 arcseconds, 80mm = 1.73 arcsec, 70mm = 1.97 arcsec, 60mm = 2.3 arcsec, 50mm = 2.76 arcsec, 40mm = 3.45 arcsec. These resolution limits can only be achieved with very high magnifications, generally in a range that will produce a 1mm to 0.8mm exit pupil, or magnifications in the range of 25x to 30x per inch of aperture.
A Williams Optics Megrez 80 SD II scope was used on the same chart in the same type of daylight conditions. For binoculars, normal resolution values were obtained in a range of 95 arcsec apparent down to 80 arcsec or so.
Using a Megrez 80 at
20x80 I could see 4.83 arcsec for 92 arcsec apparent res.
40x80 I could see 2.71 arcsec for 108 arcsec apparent res.
50x80 I could see 2.15 arcsec for 108 arcsec apparent res.
These results were very similar to all the binoculars tested.
Below these res values, I began to approach the Rayleigh Limit, and as expected it takes higher magnification to see the rersolution.
In all cases of resolution testing, first, not all lenses can reach the diffraction limit, and second, as you approach the difraction limit, you will find it requires much higher magnification to see the resolution that normal acuity would indicate.
This 80mm scope has a resolution limit of 1.7 arcsec. As I approached this limit:
at 71x80, I suspected 1.91 arcsec for an apparent separation of 136 arcsec.
at 83x80, I could see 1.71 arcsec barely split for an apparent 142 arcsec.
at 91x80, I could see 1.71 arcsec barely split for an apparent 156 arcsec.
As I got closer to the resolution limit, it took more magnification to see the split. That can be seen by the apparent size of the resolved objects, which at all sizes of resolution well above the resolution limit only required that I magnifiy it to an apparent size of 90 arcsec to 120 arcsec to see it.
The USAF Lens resoluion Chart has a stepped series of patterns, each three lines with two spaces. The width of the lines is equal to the separation between the lines and the lines are 5 times as long as the width. The chart must be observed at a known distance to determine the resoluion value. A formula is given to perform the calculation. The resolution is measured from center to center of two lines, seperated by an equal space.
For binoculars, apparent normal resolution falls in a range that varies slightly between approximately 80 and 100 arcseconds, regardless of normal magnification or aperture.
For comparison to point source resolution, the closest pair of stars I have ever cleanly separated with the 16x70s was 9.6 arcseconds, for an apparent resolution of 154 arcseconds. With nearly every instrument listed here, I have cleanly separated stars that result in apparent separations between 154 and 172 arcseconds. Rather than reading a table of results that shows incrementally finer resolution for higher powered binoculars, showing the data by apparent resolution allows the comparison of any one binocular to another binocular, regardless of magnification.
Posted 01 March 2008 - 02:27 PM
From the Online Medical Dictionary
"The resolution of an optical system defines the closest proximity of two objects that can be seen as two distinct regions of the image. This limit depends upon the Numerical Aperture of the optical system, the contrast step between objects and background and the shape of the objects. The often quoted Airy limit applies only to self luminous discs."
That was precisely what I measured, the angular distance separating two consecutive lines.
I'm not seeing the point of measuring from the center of one line to the center of the next one (a line pair) but I'm sure there must be a reason for that.
This means that my results can be easily converted (just by doubling the values) to be in conformity with that measuring method.
Now I see why you were saying that a 3.8" resolution could not be reached under 10 power. I totally agree with you if we're talking about line pairs, which wasn't the case. However, that's my fault for not clarifying exactly how I measured in the text. I will do that right now.
Thanks for bringing this to my attention.
Posted 01 March 2008 - 05:10 PM
Posted 01 March 2008 - 07:13 PM
If you need more, refer to the "Best Of" links to Resolution.
What you are really measuring is the peak to peak of a ContrastTransfer Function between two types of objects, a black line and a white space. You've actually observed this, you just haven't given the approprite measure of what you observed. The peaks are in the center of the white space. The valleys are in the center of the black space. You've actually tried to give a measure from mid-slope rising to mid-slope falling on the same peak point, or in essence, the sides of a white space. Since a black provides a valley and a white provides a peak, you really need to give the measure across two whites to get the peak to peak distance.
When the peaks are so close that they converge, where they minimum intensity of light between the two peaks cannot be discerned from the peaks themselves, where the lines can no longer be identified as separate, then you have reached the point where the lines cannot be resolved. That is the limit of resolution.
Actually, the definition you referred to above stated exactly that, only it did not explain it for you.
Posted 01 March 2008 - 10:10 PM
I think I see what you mean. I’ll read more on that...
I’ve seen your results, including the recent Nikon Action 10x50 result and they’re about 1” above what I measured (mine 7,6” vs. yours 8,6”). Although these cannot be directly compared due to a lot of variables that influence the test (illumination, contrast, criteria used, physiological aspects, etc), in this test, 1” is a significant difference.
I’ll repeat the test over the next week just to be completely sure.
Posted 30 November 2008 - 10:39 AM
When you say 30% of the field is distorted, does that mean the outer 30% of the area of the field, or the outer 30% of the radius of the field?
Posted 30 November 2008 - 03:00 PM
Ed already answered your question, I just wanted to add that that's not necessarily a bad thing.
During the day you almost won't notice it and I much prefer a wider, soft on the edge FOV, than looking trough a sharp straw.