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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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I experienced a stunning revelation today. I'm not sure yet how to assess the impact of what has been brought to light. At this point I will just relate the story.
In a conversation thread on goggle.sci astro with Roland Christen, he brought to light that many binoculars are internally vignetted and the exit pupil is not fully illuminated in the outer edges. The conversation went back and forth a bit just over the last two days, but this is what I found.
It occured to me that, assuming (for example sake) the baffles cut off 30% of the objective from providing light to the exit pupil,
and
every point in the exit pupil is lit from every point (unobstructed) in the objective,
then
the light fall off at any point in the exit pupil is only that of the obstructed aperture on the same side.
If viewed from the objective end the baffle seems to be cutting off 30% of the objective light from hitting the same side of the prism, then the exit pupil towards the edges is still 70% illuminated. These drop offs in light towards the edges of the exit pupil may not be as easy to see as I thought.
This may begin to correlate very well with the drop off of limiting magnitude towards the outer edges. A good test will need a binocular that is very well corrected for distortions as far out as possible. Oberwerk 15x70 and 20x80 standards, Fujinon 16x70 and Pentax 16x60 are all pretty well corrected at least to 80%-90% out from center, out past (what I thought would be) the expected area of light drop off in the image if baffles cut in at 30%. With distortions nearly eliminated, most of the limiting magnitude drop may be attributed to light cut off. Does that seem a reasonable presumption?
Roland Christen suggests vignetting is prevalent in binoculars. By viewing from the very edge of the objective end into the binocular, I was able to easily observe the light path is obstructed by the baffles in every binocular I own. I measured each binocular by how much of the light path is obstructed. For example, if the light path were obstructed (vignetted) 50%, I would be able to line up my line of sight from the edge of the objective past the edge of the baffle and see exactly to the middle of the exit pupil. If it were more than 50% vignetted, as I found, less than half the exit pupil will be seen.
These are the OBSTRUCTED measurements I observed.
Minolta Standard 7x35 60%
Minoltas Activa 7x35 50%
Swift Ultralite 8x42 80%
Pentax PCF III 12x50 70%
Pentax PCF V 16x60 70%
Oberwerk 15x70 80%
Fujinon FMT SX 10x70 70%
Fujinon FMT SX 16x70 70%
Oberwerk Deluxe 20x80 70%
Oberwerk Standard 20x80 80%
Oberwerk BT100 100% (edge clipped by aperture stop, see BT100 post)
At first I was skeptical of this claim. However I now cannot see any way that a ray diagram would prove this to be false. I believe he is right. My only question now is what impact is this causing?
What is the incidence of this same type of vignetting in scopes, if any?
Why should this be so prevalent in binoculars?
Are binocular exit pupils really truly lacking full illumination to this extent?
Needless to say, I’m stunned. I need to sit back and digest this before I make any other comments on this. I am so curious to see what types of readings anyone else is getting.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (04/05/04 09:25 AM)
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rboe
Numbfinger
Reged: 03/16/02
Posts: 39735
Loc: Phx, AZ
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This may begin to explain why performance index's, in real life, be off. But if they have been like this all along we need to ask why it's a problem now. Will users demand non-vignetting bino's? If someone made a non-vignetting binocular would it perform as well as we would think it or would be worse - an ugly trade off and we'll find out why manufactures build them that way.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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KennyJ
Reged: 04/27/03
Posts: 10155
Loc: Lancashire UK
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Ed,
Quite an earth shattering discovery !
As a result I am taking all ALL my binoculars to the pawnbrokers this morning to get what I can for them before the news breaks out into the wider world.
It has however been most enjoyable discussing these instruments over the years.
I suspect what we need now is a MONOCULAR forum :-)
Regards , Kenny.
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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KennyJ
Reged: 04/27/03
Posts: 10155
Loc: Lancashire UK
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On a less jocular note than my last post , by co-incidence over the past couple of days I have spent a LOT of time looking at things through , within and without my binoculars , as a result of my problems with trying to collimate my 15 x 70s, details of which are scattered between this and the Cloudy Days forum.
I too noticed that what appear to be flat -sided "baffles" obstruct the light path to the parameters of the eye-piece lenses , and upon closer inspection , when looking at the exit -pupil from the "normal" side , I see an "inner ring" of perhaps 1.5 - 2mm diameter which is of a different , strangely more opaque , appearance , than the rest of the exit -pupil area.
This is exactly the same with my Swift 10 x 50 Kestrels too.
I also noticed that if I look through the binoculars BACKWARDS at printed card held very close to the eye-piece end , the card takes on a distinct CONVEX appearance , as if it is formed into a slight curve with the centre closer to you than the edges , which is what I would describe as being representative of pin -cushion distortion as seen in reverse.
Perhaps , amongst other things , these revelations go some way to explaining why it is that I can see improved brightness with larger exit pupils EVEN WHEN my entrance pupils are apparantly smaller than the "official " exit -pupil diameter ? ( which has been a "gripe" of mine for a very long time )
Regards , Kenny.
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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The entire exit pupil is visible because some portion of the field stop is illuminated by some portion of the objective. But not every portion has the same amount of light. The baffles are there to help control the lens, or in essence correct for any number of things that might be seen if the objective was unobstructed. One huge implication of this is the the prism is not fully lit by the entire objective. The center of the exit pupil may be 100% illuminated, but the outer edges of the exit pupil are only 30% illuminated, or whatever it may be for your sample. In some cases I tested, not even the center of the exit pupil was fully illuminated.
When you look in from the objective side, you can see in most cases that
some part of the objective can see some part of the prism field stop.
But you can also see that not every part of the objective can see every part of the prism.
In some cases it may be found that some parts of the objective cannot see any part of the prism. In these cases the objective is stopped down, period. Roland describes this. In fact, I think Barry has described this before. If you have a 100mm lens and the edge of the objective cannot see the opposite side of the prism, the amount of the objective in use could be determined by placing progressively smaller masks over the objective until a point is reach where the eye can be placed on the outer rim of the objective mask and see the opposite edge of the prism stop. In my samples, my 100mm lens is operating more like a 80mm to 85mm lens.
In order for the exit pupil to be 100% illuminated across the entire width, every point on the objective lens must see every point inside the prism field stop. If the objective can only see a portion of the exit prism field stop, that portion not seen is not illuminated by that point from the objective. Of course, to some extent this is done to eliminate aberrations at the edges of the objective lens.
As Kenny suggests, this should go a long way towards helping to identify why one binocular may appear brighter than another. The binocular that has a greater percentage of the objective illuminating the most of the field stop should be the brighter of two with equal size.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Anonymous
Unregistered
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This is very interesting. Following is what I've been able to find in my binoculars with a cursory inspection. The outer edges of the exit pupils (right side of the right exit pupil and left side of the left exit pupil) of my 15x70mm Oberwerk binoculars seems to be nearly fully illuminated- perhaps 90%. However, the left edge of the right exit pupil seems to be only about 70% illuminated. And the right side of the left exit pupil seems to be about 70% illuminated. I think the fact that I haven't noticed a much dimmer/darker image towards the outer part of the field of view shows that the field of view in my 15x70mm Oberwerk binoculars is fairly evenly illuminated.
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 7444
Loc: Hanover, Ohio
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All I can say is, wow, this is scary. Brings to mid a question though(no trolling here either): Could it be that the optics in the middle, and higher end binos are so fine that the degree of vignetting doesn't really matter? I'm scared to really look at my Skymasters now(for vignetting), but I'll get bored, and brave later, and do so. Will let you know. Clear Dark Skies!! Pete
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Pete
6" Apogee/LXD55 - "The Beast"
Starhopper 6" Dob - "Shiva"
Spaceprobe 130 EQ - "Spacey"
Bushnell Fatboy
The Abomination
Sun Pak Pro 7500 Platinum Edition
10X25 Bushnell Camo Roofies
7X35 Tasco Classic Plastic (good views though)
7X42 Tasco Rare Bird
10X50 Nikon Actions (Type 7)
15X70 Skymasters - "DroolMeisters"
One ratty old IBM 600E LapTop
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craig_oz_land
sage
Reged: 01/21/04
Posts: 343
Loc: Sydney, Australia
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None in my Fuji 7x50s.
The way I am looking at it is in between the edge of the objective and the "field stop" of the prism housing there is no cut off of the light path.
I suppose the next question is does the prism housing field stop vignet the light cone?
Am I missing something here?
Craig (cross eyed from looking into the wrong end of my binos).
I told my cat not to do it. It would not listen. 
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Takahashi FS-102
Fujinon FMT-SX2 7x50
Takahashi Astronomer 22x60
Edited by craig_oz_land (03/31/04 09:28 AM)
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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Craig, that picture is hilarious.
I need to get home and do some researching on this, but the first thing that comes to mind is that if the prisms in the binoculars were fully illuminated, there might be all sorts of really nasty artifacts from the prism edges. I think that part of the reason the baffles cut down on the light transmission is to cover up inherent defects in the prism system. Just a thought.
Mike Swaim
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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others
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Anonymous
Unregistered
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This is an excellent topic to find EdZ. Barry may be a good person to ask...any expert in binocular construction/engineering would be good too. Unfortunately I can't search around now, but maybe someone else can.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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Daniel,
I own two pair of Oberwerk 15x70s. Both are obstructed by 80%, not illuminated by 90%. What the obstruction clearly indicates is that nearly every binocular is ONLY FULLY ILLUMINATED by no more than 30% to 40%, and in some cases 20%.
edz
Actually I need to revise this. what is indicated when the field stop is anything more than 50% obstructed when viewed from the edge of the objective is that not even the center of the exit pupil is fully illuminated. That still does not yet give me a clear indication of the % illumination.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (03/31/04 04:31 PM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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Mike,
I commented above, we know the edges of binocular lenses are baffled to eliminate aberrations. But this is indicating that most binoculars have baffles that prevent the edge of objective from providing light to 70% or 80% of the prism.
edit
Exit pupils are NOT 100% illuminated at all, even in the center, if 70% of the prism is hidden when viewed from the edge of the objective.
Craig,
I'd like to take a look down a Fujinon 7x50. Based on what I've seen, that bothe the Fujinon 10x70 and the 16x70 are obstructed by 70%, I'm skeptical that the 7x50 has 0% obstruction.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (03/31/04 04:49 PM)
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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EdZ:
Took a look at the back end of my LX 8x42s and saw the same thing you're describing -- maybe 70-80% obstruction from the edge of the objective.
Without understanding all the optical details, I have to say there must be a good reason why they're built this way. Nikon pretty much pulled out all the stops on their Premier binoculars. There is no way they cut corners on the prisms in these suckers.
In ATM circles decisions about secondary mirror size are based on the percentage of illumination desired around the outer area of the field of view. For planetary observation, you want a smaller secondary to reduce diffraction effects that lower contrast. The trade-off is that the outer area of the field is illuminated to a much lower degree.
Even when using larger secondaries for faster scopes primarly intended for DSO observation and wide-field work, the outer area of the field is never 100% illuminated. Off the top of my head, I can't think what is considered to be generally acceptable (and people argue about this, needless to say  ), but it's nowhere near 100%.
When I get home tonight, I'll root around in a couple of my ATM books to see what they say on the topic. I'll also set up my TV-101 and check to see what the exit pupil looks like from the business end of the critter.
Craig:
That was a hilarious picture. Have you tried whacking the feline on the back of its head to see if that uncrosses its eyes?
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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EdZ:
You know, I seem to also recall illumination issues being discussed in a couple of my astrophotography books. That might be another area of investigation that could shed some light on these questions.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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Quote:
EdZ:
Took a look at the back end of my LX 8x42s and saw the same thing you're describing -- maybe 70-80% obstruction from the edge of the objective.
Without understanding all the optical details, I have to say there must be a good reason why they're built this way. Nikon pretty much pulled out all the stops on their Premier binoculars. There is no way they cut corners on the prisms in these suckers.
This conversation is going in three forums, so excuse me if I haven't said this here. I'll repeat, this is not an issue with the prisms being too small. Increasing the size of the prisms would have no affect on this condition. Think about it, a larger prism would still be centered on exactly the same optical axis. Nearly just as much a percentage will still be hidden, no matter how large the prism gets. Yes slightly more will be visible. But once the obstruction goes past the center, you will never see more than 50%.
Here's an important piece of the equation.
If you view into the objective from the very edge and see 70% of the prism field stop circle is obstructed, that means even the center of the exit pupil is NOT fully illuminated. In order for the center of the exit pupil to be 100% illuminated by every light ray from the objective, the center of the filed stop must be visible from every point on the objective lens. Only one out of the twelve binoculars I looked at met that test, and it just barely.
What I have not yet determined is this. I do not know the effective aperture of 100% illumination. Roland points out, and this seems so clear now, it can be found by placing progressively smaller masks over the objective and repeating the test until the entire prism field stop can be seen all the way around from every edge.
Rather than make so many different sized masks, I believe it could also be found by securing a measuring scale across the center of the objective end and noting with a piece of sticky tape the point at which 100% of the field stop can be seen. Check both sides and the distance between the two pieces of tape would be the effctive aperture of 100% illumination.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (03/31/04 02:59 PM)
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Anonymous
Unregistered
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Ed, I used the method Roland Christen mentioned. I assumed it could be used to obtain a good approximation for the amount of illumination at any part of the field of view. Here’s what I did.
I looked at the shape of the exit pupil a couple of inches away from the eyepiece. It appeared to be circular. I then moved my eye to the side of the eyepiece, so that the part of the field of view I could see through the exit pupil was right at the field stop. The exit pupil appeared to be crescent shaped, I guessed at the area of the crescent shaped exit pupil compared to the fully illuminated exit pupil I saw near the center of the field of view.
This is how I obtained my 70% illuminated approximation. I don’t know very much about how optics work, so I fully realize that I may have made a huge mistake and misunderstood the use of this test. My “common sense”(using my very small amount of knowledge ) tells me that the ratio of the area of the exit pupil at the very edge of the field of view and the area of a fully illuminated exit pupil is the same as the ratio of illumination at that part of the field of view.
Is this right, and our binoculars are just different, or am I making a mistake?
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12592
Loc: Cumberland, R I , USA42N71.4W
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Roland's description of how to view this, in all the discussions we have been having, is by looking into the objective end of the binoculars. Rather than re-explain it all here, please go back and read the first post and subsequent posts.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Anonymous
Unregistered
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This is what I'm talking about.
"A person can see if the edges of the field are vignetted by viewing the exit pupil at the eyepiece end at an extreme angle. If the edge is vignetted, then the exit pupil will look D shaped." -Roland Christen message 23 in thread
If this can be done to see if it is vignetted or not, shouldn't it also be possible to use this method for a more exact measurement?
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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I'm having some trouble fully understanding why I should care about this. On the one hand, you've got my curiousity up. But, on the other hand, this strikes me a lot like looking for CA in binoculars that have pleased one up until the time that CA is purposely found. I'm still peeved at the joker that first had me purposely look for CA in the daytime with my Fujinon 16x70s. Seeing it really changed the regard that I previously had for those binoculars.
So, I'm debating on whether I'll even dare to invert the Fujis and the Nikon SEs to see how little of the exit pupil is illuminated. If I like them as is, will anything gained by this experiment help me use them better in the field?
I'll definitely do it with some others, just to see what I see. But, why go looking for faults if none are noted at the eyepiece end of things? On the other hand, I can see the value of it if trying to decide between two optics, or if trying to find out why a particular optic seems a lesser performer.
Mike Swaim
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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Quote:
this is not an issue with the prisms being too small. Increasing the size of the prisms would have no affect on this condition. Think about it, a larger prism would still be centered on exactly the same optical axis. Nearly just as much a percentage will still be hidden, no matter how large the prism gets. Yes slightly more will be visible. But once the obstruction goes past the center, you will never see more than 50%.
Ed:
Maybe I don't understand what you're saying correctly.
What you seem to be saying is that since the prism is centered on the optical axis of the objective, a percentage of that optical axis is blocked from the eyepiece because the EP is not on the same optical axis.
However, the prism is not only centered on the optical axis of the objective lens but of the eyepiece as well. It has two optical axes. The whole point of the prism is to redirect the optical axis of the objective lens, which it does with multiple relection/refractions (4?).
Since the focal length between any surface in the prism is so short, it stands to reason that you would not be able to view the entire exit pupil from the object end of the bino (at least if your eye is immediately next to the edge of the objective).
The position of your eye is what is causing the vignetting. It's the same thing if you don't get your pupil centered on the binocular's exit pupil -- that's what causes blackouts in the field of view. If you have a binocular with long eye-relief, don't wear glasses, and try to use the instrument without extending the eyecups, it is extremely difficult to maintain a fully illuminated view because the angle at which your eye intersects the exit pupil is too acute. Actually, if your eye is close enough to the eyepiece, you won't be able to see anything at all. As you move the eyepiece back, you see a small, crescent-shaped vignette of the fov, which gradually waxes to a gibeous phase before becoming a full disk. (Bet the lunar observers would like that description. )
While viewing through the objective lens of the binocular, if you move the bino directly away from your eye you see pretty much the same thing.
Actually, the whole problem can be simplified by thinking of a periscope instead of a porro or roof prism. If you position your eye closely enough to the edge of the upper mirror, you will only be able to see a small percentage of the lower mirror (and presumably of the eyeball on the other end). If you move your eye directly back from the edge of the mirror, you will see more and more of the opposite mirror until you see the entire thing.
--------------------
Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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