Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Food for thought. For years people have been discussing binoculars and referring to that with a larger exit pupil as being brighter. Also for years, I've been saying, that's way to simplified and there is far more to determining brightness than just the size of the exit pupil.
Probably one of the most important factors is degree of illumination, how much of the exit pupil is fully illuminated by all rays on-axis and off axis. They all contribute to the total light, so they all contribute to brightness. While the best sample binocular I've ever seen is illuminated 100% across the entire exit pupil from all on/off axis rays out thru 80% of the objective, even other better binoculars show this same extent to only 40-50% and far more common are binoculars that show this extent thru only 20-35% of the objective diameter. There are also binoculars that show full illumination from only about 10-15% of the objective. Almost all roofs I've seen fall into this group.
I think this little collection of test data shows something similar to brightness. This test is comprised of a constant light source, shining thru a tube about 30 inches long, the binocular objective placed at the opposite end, room lights turned off, and the resultant reading is taken at the eyepiece with a SPER Scientific brand light meter. Meter readings are listed for each binocular. Actual measured aperture and actual measured exit pupil are listed. Each binocular was measured several times, some 4-5 times. In most cases, the readings clustered closely for a particular binocular. The best reading is listed.
I have more thoughts on this, but not enough time, so this is a short excerpt of the data set. As you can see, image brightness is far from being consistent across exit pupil sizes. One might expect as exit pupil gets larger, the lightmeter reading would move in tandem. If that held true, I thought it would also be reasonable to divide meter reading by % increase in exit pupil area to see if all factored meter readings come out the same, indicating some dependany on exit pupil area. That does not seem to be the case. All it did was move a few sampes around a little bit, but the highs stayed high and the lows stayed low. Of course, I didn't expect that to show a clear trend as there is more invloved. Perhaps what this does show is that there are far more influences on brightness than just exit pupil. I do expect that illumination and also aperture will come into play, but that will come later.
You might note some surprises about the low brightness of roof prism binoculars.
When time allows, I'll post more on the illumination of each of these binoculars. For now, just a fairly simple data set that shows some similar sized exit pupils obtain a reading as much as twice as high as some others. Not a definitive analytical result yet, but obviously there is much more involved than just exit pupil.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (09/14/09 11:48 AM)
|
BobinKy
Carpal Tunnel
Reged: 04/27/07
Posts: 1682
|
|
EdZ...
Very interesting. Is the last column, Meter/EP, the measure of image brightness that is received by the observer's eye pupil?
--------------------
Bob
38°N
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
No, It is simply a mathematically factored meter reading. If anything, I would expect the direct meter reading to represent brightness at the eye.
Assuming exit pupil is the only controlling factor for brightness, then I made the further assumption that the constant unchanging light source entering the objectives might show output at the meter that is either brighter or dimmer in lock-step with larger or smaller exit pupils. If that were in fact the case, then I should be able to divide the meter reading by the exit pupil and get a factored (meter reading/ep area) result for all samples that is near exactly the same across the board.
For example, say two binoculars are 10x40 and 8x40. Area of exit pupils (nominal) are 4squared and 5squared = 16 and 25, one having an increase of 25/16 = 1.56x the area over the other. And lets say the meter reads 10x42 = 125 and the 8x42 = 200. Well, we would expect the 8x40 to be have a brighter exit pupil and therefore the meter to read higher. If the higher reading is entirely due to exit pupil then we should be able to check all the results by dividing the readings by the increase in the area of exit pupil. By doing so, all values should come out equal. So here we divide 200/1.56 and get 128. If enough samples could be divided like that and have all the results come up to close to the same value, it would be a certain indicator that area of exit pupil is the only determinant of brightness.
That's not what I got. Actually, I didn't expect to get the above balanced result, but if everything were balanced on exit pupil ONLY, I'm pretty certain what I suggested above should occur.
Perhaps what this really shows, as I suggested in my first post, is that there are other factors that need to be looked at to explain differences in brightness. That should not really come as a surprise, but as far as I know, no one correlates a measure of other factors with brightness. Of course, at this point, in my brief look at the initial data, no other factors are yet taken into consideration, and that might be why it looks so out-of-balance.
Binoculars with very high percent illumination may be showing considerably higher brightness than expected. Binoculars with very low percent illumination may be showing considerably lower brightness than expected.
This table shows the same output as the first table, but also included here is the % central diamter of objective that provides 100% illumination. Not a perfect match, but certainly some correlation.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (09/14/09 01:28 PM)
|
milt
professor emeritus
Reged: 09/13/04
Posts: 551
Loc: Arizona
|
|
Quote:
Assuming exit pupil is the only controlling factor for brightness
Hi Ed,
Well, I know you don't mean that.... 
Getting to the test methodology, even with a tube in front of the objective you are still going to capture significant off-axis rays. For example, assuming a 50mm objective and 30" tube of the same diameter you could capture rays up to 3.8 deg. off-axis.
So in addition to aperture and the other factors you already mentioned, field stop diameter will also figure into the total exit pupil brightness. IMO, the laser illumination tests you have done are more precise.
Most of all, it's important for folks to remember that for the majority of astronomical objects aperture dominates. The brightness of a star, planet, globular cluster, galaxy or other small DSO depends not on exit pupil diameter but on unobstructed aperture. That's why your illumination tests are so important because they reveal when the entire aperture isn't being used.
Milt
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
Quote:
Assuming exit pupil is the only controlling factor for brightness
Hi Ed,
Well, I know you don't mean that....
Getting to the test methodology, even with a tube in front of the objective you are still going to capture significant off-axis rays. For example, assuming a 50mm objective and 30" tube of the same diameter you could capture rays up to 3.8 deg. off-axis.
So in addition to aperture and the other factors you already mentioned, field stop diameter will also figure into the total exit pupil brightness. IMO, the laser illumination tests you have done are more precise.
Most of all, it's important for folks to remember that for the majority of astronomical objects aperture dominates. The brightness of a star, planet, globular cluster, galaxy or other small DSO depends not on exit pupil diameter but on unobstructed aperture. That's why your illumination tests are so important because they reveal when the entire aperture isn't being used.
Milt
Well first, a better choice of words might have been, "If one were to assume exit pupil is the only controlling factor, which we know it isn't, ..." Sometimes, setting a false assumption allows one to see the implications (or lack of) in the results.
The laser illumination test is how I got to the values for 100% illum. Of course the data from those laser tests goes well beyond just that one value. I agree, it is valuable. That does tell me when there is a lack of full illumination, but that doesn't tell me how bright the exit pupil is.
Seems to me the inclusion of all rays, on and off axis, in other words all the light coming thru the tube, will mimic total light entering if the binocular were pointed out in the open in daylight. I think that would pretty closely resemble realistic use for total light in the exit pupil. In fact, the inside of the tube is white, allowing rays to bounce all over inside the tube. The only reason I'm using a tube with a light source at the end is to insure that every binocular in the test gets the exact same light. BTW, the light meter reading outside the tube is always about 2 or 3 or 4, so the only light in each binocular in the test is the tube light.
I hadn't thought about field stop being a factor. I'm still trying to figure out what all has implications and by how much, so I'll consider that too. For instance, transmission has an affect, as well as aperture.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1250
Loc: Ottawa, Ontario, Canada
|
|
I'd like to see the specifics regarding the metering instrument. I suspect there is some overloked detail that is causing these very discrepant values. Some factors which can have an impact:
- Whether the meter is designed to accept light from a wide range of angles, or employs optics to image a fixed angular field.
- The size of the sensor, if it's un-lensed and itself is placed at the exit pupil. For example, is it larger than the largest exit pupils you test, or somwehere in between largest/smallest, or smaller than approx. 2mm?
- The size of the imaging aperture, if optics are employed to restrict the angular field.
There are probably other concerns related to the light source/tube, but that will depend on the meter's design.
I'm planning to make my own meter for doing these kinds of tests. My preliminary criteria:
-The light source will smoothly and evenly illuminate an angle larger than the true FOV of the instrument under test. Probably 30+ degrees, to include the ultra low-power glasses.
- It will employ an objective lens in order to image an angular field no larger than about 1/20 the apparent field angle (approx. 2.5 - 5 degrees, hopefully rather smaller.) This allows to sample small enough portions of the image in order to map illumination fall-of with field angle.
- This lens will have an aperture smaller than the exit pupils under test. In this way I will be certain to always test *surface brightness* of the image only.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Well what I can tell you is this,
the meter has a sensor diameter of 10mm, wider than any of my sources.
SPER Scientific Pocket Light Meter 840010
purchased from Edmund Scientific
It is extremely sensitive to being placed at the exact exit pupil distance and centered on the exit pupil. At this scale, slight variation will cause misreading of 20-30, 2-4mm of variation from optimum postition will cause a misreading of 100
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1250
Loc: Ottawa, Ontario, Canada
|
|
This suggests that differing apparent FOVs will impact readings. Say we are examining two otherwise identical binos, but one has a larger AFoV. Each bino will deliver quite similar surface brightness of the view. But the meter will "see" the larger circle and integrate a larger amount of light, yielding an artifically brighter reading.
This is why I want to use an imaging objective so as to have full control over the sensor's view angle (as stated before, no larger than 5 deg., tops.)
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
This suggests that differing apparent FOVs will impact readings. Say we are examining two otherwise identical binos, but one has a larger AFoV. Each bino will deliver quite similar surface brightness of the view. But the meter will "see" the larger circle and integrate a larger amount of light, yielding an artifically brighter reading.
that agrees with what Milt said up above. thanks, I had not considered that. I wonder if I can correlate that and normalize?
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
milt
professor emeritus
Reged: 09/13/04
Posts: 551
Loc: Arizona
|
|
Quote:
Each bino will deliver quite similar surface brightness of the view. But the meter will "see" the larger circle and integrate a larger amount of light, yielding an artifically brighter reading
Glenn, Ed's previous laser test predicts how fully illuminated the central part of the field will be for a given aperture, and that's more important for most objects. However, it still doesn't capture transmissivity losses like coatings and I think that's what you guys are after.
To measure relative transmissivity near field center, perhaps one could measure the exit pupil brightness of a green laser (near the peak of our eyes' sensitivity) used as a single paraxial ray. This would even be aperture independent, but a filter might be required to prevent saturating the meter. In any case I predict those stylish green coatings wouldn't do so well. 
Milt
Edited by milt (09/14/09 10:01 PM)
|
GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1250
Loc: Ottawa, Ontario, Canada
|
|
EdZ,
If you can measure the AFoV to reasonable accuracy, you can calculate its solid angle (steradians) and normalize from there. But there will still be a limitation to this method, in that from the start you're integrating all light across the full field.
This can lead to what I would consider artifically-poor results for some binos. For example, if two 10X50s have the same AFoV, but one suffers worse vignetting well *off axis*, it could 'score' lower because of off-axis light loss even though it MAY have a somewhat brighter on-axis view (better coatings, more of the objective aperture being used, etc., etc.)
Personally, I can tolerate lots of illumination drop-off toward the field edge, especially if it's gradual. What matters most is that at least the center of the field have good illumination. Hence my reticence to rate illumination on the *integrated* light over the full field. Better to sample field brightness across tiny angles that are a significantly small fraction of the total field angle. A graph/map of light drop-off then can be constructed, much like the output of software such as NEWT (used for newtonian 'scope design.)
Because you're sampling the full field, do ensure that your illumination source be truly uniform across an angle at least as large as the true field of the instrument under test.
And there are other details which bear consideration, mainly because your light source is uncollimated:
I'll have to think this one over more carefully, but my first instinct is to suspect the fact that the optical path length between illuminator and your meter will *vary* by size of bino. Bigger binos force the exit pupil farther back, and this *could* cause a diminution in surface brightness. Again, let's put our thinking caps on with this one...
You should ensure that no significant amount of non-image-forming light is illuminating the internals outside of the image-forming train. And so you should dispense with the white-walled tube, as it makes for a most excellent out-of-field source of unwanted light splashing about inside the bino.
Because you're testing a wide range of true FOVs AND a range of apertures, you may well have to vary the size of the illuminator and/or the separation between it and the objective so as to obtain proper illumination geometry. But this is not simple, mainly because the light source lies so close to the objective and hence its edge is imaged by the bino as hugely out-of-focus. This then requires a somewhat larger-than-ideal illuminator so as to ensure that the fuzzy zone of decreased surface brightness is placed outside of the field.
The foregoing is of more importance if instrumental baffling can't do a good job of supressing internal scatter. Perhaps the setup could simply be adjusted to a point just shy of causing gross internal illumination. That is, bring the illuminator closer until a bit too much inner wall, etc., illumination occurs and then back off a bit. However, this could be rendered inadvisable IF the issue of changing optical path length is indeed significant....
The best method is to make a collimated light source. Any old refractor having an objective a tad larger than the biggest bino objectives will serve. It suffices to then place an evenly illuminated target at the position of infinity focus which covers an angle of, say, 12 degrees. It's easy to calculate its diameter from the 'scope's focal length. Because this quite angularly large target will require removing the focuser and possibly any baffle rings within the main tube, better to make a crude tube and attach the objective to it.
The advantages of a collimated light source are considerable, because you effectively have an infinitely distant target. You can then place the bino objective near to or far from the collimator, within the limits imposed by illumination geometry, of course. Up close is best at any rate.
And if you wish to control off-axis light which doesn't contribute to the image, simply place a suitably-sized circular mask against the target so that it forms an image just a little bigger than the bino's true field (the target's edge will be imaged as perfectly sharp, so it's now dead easy to tell when it's just big enough for the test.)
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3455
Loc: Cambridge, MA, USA
|
|
Quote:
Personally, I can tolerate lots of illumination drop-off toward the field edge, especially if it's gradual. What matters most is that at least the center of the field have good illumination.
Quite so. This subject comes up frequently among people who design Newtonians. For any scope with a secondary mirror, there are two conflicting desires. You want to make the secondary small to minimize diffraction effects at high magnifications, but you also want to make it large to minimize vignetting the edge of your lowest-power, widest-angle field.
It's generally agreed that for visual use, a 50% light falloff at the edge of the FOV is barely detectable, and even a 70% falloff (30% throughput) is acceptable. Of course, this is a different situation. For a telescope, low power is only one of many choices, and it's not usually considered the most important -- otherwise people would use bigger secondaries.
As Glenn says, by far the most important statistic is the effective aperture at the center of the field. Then, ideally, one would like a contour map of the FOV showing precisely how the effective aperture falls off as one approaches the edge.
For a Newt, this is easy to do from first principles, because Newts have very simple geometry that can be characterized with high precision. I doubt that it would be possible to do the same with binoculars.
In theory, one could trace the rays directly by shining a laser pointer into the binocular's objective at different points and at different angles. But I estimate that it would take at least a thousand measurements to make a satisfactorily detailed map, and this is hardly practical.
Glenn's suggestion of shining a well-collimated beam into the objective at various different angles is an attractive one. Certainly worth trying.
The most direct approach would be to bring the light from the binoculars to a focus, just as the eye does, and meaasure at the focal plane. I've wondered if this could be done with a point-and-shoot digital camera.
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
If you can measure the AFoV to reasonable accuracy, you can calculate its solid angle (steradians) and normalize from there. But there will still be a limitation to this method, in that from the start you're integrating all light across the full field.
This can lead to what I would consider artifically-poor results for some binos. For example, if two 10X50s have the same AFoV, but one suffers worse vignetting well *off axis*, it could 'score' lower because of off-axis light loss even though it MAY have a somewhat brighter on-axis view (better coatings, more of the objective aperture being used, etc., etc.)
What matters most is that at least the center of the field have good illumination. Hence my reticence to rate illumination on the *integrated* light over the full field. Better to sample field brightness across tiny angles that are a significantly small fraction of the total field angle. A graph/map of light drop-off then can be constructed, much like the output of software such as NEWT (used for newtonian 'scope design.)
Well, I already do all the above.
I report the fov for all these binoculars. Included it here for info
I agree if one would score lower because of off-axis light loss, but that's exactly the point. the total light in the exit pupil.
I've already given the metric for center field illumination, in percent area providing 100% illumination. In fact, I've already reported previosly on mapping illumination across the exit pupil. That's not what I'm trying to show here.
RE: off axis light in the tube. But, wouldn't this mimic exactly a daylight scene with bright sky all around. If not, what's different about it? Remeber, I'm not trying to measure light at a specific point (the central 10% vs the outer 10%), Iv'e done that eslewhere. I'm purposely trying to measure all the light that can get in, and how much gets into the exit pupil.
Here's a table showing the weight of fov on the meter readings
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
and here's a table showing influence of exit pupil and FOV combined.
In these above two tables, some of the positions move but the general groupings remain about in the same locations.
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
Steve Darden
sage
Reged: 07/29/07
Posts: 213
Loc: Woodstock, GA
|
|
EdZ, can you go back and run these test on all the binoculars you've tested in the past.
Just kidding.
Very interesting. Thanks for your effort.
--------------------
Orion XT10 with Moonlite focuser
Orion Starmax 127 Mak
Orion 80ed with Moonlite focuser
Too little free time
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Quote:
EdZ, can you go back and run these test on all the binoculars you've tested in the past.
Just kidding.
Very interesting. Thanks for your effort.
I actually have run these tests on probably 30-40 binoculars. I've just never reported it like this before.
The raw data reading (maximum lux) is reported for every binoculars in my Small Binocular Series of CN Reports.
I'm still trying to figure out the best way to present it.
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1250
Loc: Ottawa, Ontario, Canada
|
|
Ed,
Indeed, if the desire is to include ALL light emerging from the eyepiece through the exit pupil, non-image-forming included, then mimicking the effect of 'skylight' is perfectly acceptable.
However, this would be more meaningful if the meter's sensor diameter is of the same size or barely larger than that expected of the observer's iris during use. If we're talking astronomical observation, then a safe upper limit for the majority would be the usual 7mm.
If the sensor is large (you said 10mm for yours, which samples an area fully twice that of a 7mm pupil) and the exit pupil is small, then one can pick up a fair bit of extraneous light well *outside* the exit pupil. Especially bad would be to include those nasty, clipped pupil ghosts. But even internal scatter off the near-to-eyepiece prism walls which would not otherwise enter the observer's eye can be a not inconsiderable source of additional light.
Perhaps you could make a 7mm diameter mask for your meter?
Tony,
Just to be clear, the method I was advocating involved a stationary, full-field illuminated target. It's not the light source which one aims into the front, but instead is the small-FOV meter which is pointed about through the exit pupil and into the field (while hopefully keeping it well centered in the exit pupil.)
Your idea of projecting the illuminated field has some merit. The most immediate benefit is the ease of seeing and sampling the part of the field desired.
There is one small fly in the ointment, however. By racking the eyepiece farther back in order to project the image at a nearer distance, the illumination characteristics will change slightly. The effect will probably be small enough to be largely ignored. But for the sake of a full understanding, I'll expand on it.
A minority of binos are barely able to fully field the on-axis light cone when at infinity focus, and as we know most actually have their apertures already reduced. By moving the eyepiece farther back, the circle of full illumination decreases because as seen from the eyepiece, the front prism aperture appears to shrink (angularly) more rapidly than does the objective aperture. If there already exists aperture restriction, then the percentage of on-axis illumination decreases still more.
The foregoing effect will be made more severe when one tries to focus the eyepiece field on an ever nearer screen, because the eyepiece is racked ever farther back.
Moreover, a flat screen is not ideal because of the cosine four law which describes illumination when a relatively small aperture images onto a large flat surface. The best bet would be to construct a narrow strip of a screen curved so that its center of curvature is located at the exit pupil. I suggest a strip instead of a circular section of a sphere because one can get by with measurements along just one diameter bisecting the field. (And it's a lot easier to make!)
But then, how to get enough signal from a necessarily dim image in projection? Better in the end to simply aim your meter (which must sample a suitably small field of its own) into the exit pupil in a controlled fashion that ensures proper sampling.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
Well not to ignore latest suggestions, but the mapping of illumination by projecting a laser thru the objective is the method I documented in this forum starting back about 5-6 years ago. See the current GO-WO-Tak22x thread for a set of about 6-8 links to all of that data and discussion. There are hundreds of posts and numerous maps store both in this forum and in my gallery. But that is another topic and that is not what I've been doing here. So I'll come back to this topic.
Here, at the request from above, I've gathered data from several years of measurments. I had the raw meter readings in a table. Here it is presented in binocular sets grouped and sorted by AFOV. I wanted to see if there was any pattern that would show up giving some indication that fov might be affecting the readings. Initial glance at the sorted results seems to show no pattern related to fov.
I arbitrarily broke the groups into fov sets that do not vary by more than 3%+/- from mean. BTW,the difference in meter reading on all these from those in my posts above is simply due to a different intensity of light behind the source screen. These are all the same intensity, but unfortunately cannot be mixed with those in the posts above.
meter = light meter reading in lux (higher value set is using a 60 watt bulb, lower value set is using a "Lenser" brand 5 LED pocket light).
aperture = actual measured effective aperture
ep = actual measured exit pupil
ep area = sq mm area of exit pupil
ep area % = ep area compared to ep area minimum (used to normalize)
Afov = calculated Afov based on actual measure tfov (Afov without distortion)
full illum = % central diam of aperture providing 100% illimination to exit pupil
EP% = ep area % value from above converted to factor
M/EP = meter reading divided by EP factor, test to normalize for differences in exit pupil area
Afov% = factor for Afov divided by minimum Afov (used to normalize)
M/Afov = meter reading divided by Afov factor, test to normalize for differences in Afov
method of measurement for Lux, aperture, illumination, ep, and Afov are all explained in expanded detail in "Best Of" threads linked to Measuring and Testing Aspects of Binoculars
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (09/18/09 06:21 AM)
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14713
Loc: Cumberland, R I , USA42N71.4W
|
|
the rest of the data set broken into groups and sorted by AFOV
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3455
Loc: Cambridge, MA, USA
|
|
Quote:
Tony,
Just to be clear, the method I was advocating involved a stationary, full-field illuminated target. It's not the light source which one aims into the front, but instead is the small-FOV meter which is pointed about through the exit pupil and into the field (while hopefully keeping it well centered in the exit pupil.)
Your idea of projecting the illuminated field has some merit.
Actually, I wasn't thinking of doing eyepiece projection with the binoculars' own eyepiece, but rather of focusing the bino at infinity and using an auxiliary lens to focus the light from the exit pupil -- precisely mimicking the way that the lens of the human eye forms an image on the retina.
But you're right that the wide FOV creates all kinds of problems with this idea. In particular, my original thought of simply using a camera with its lens at the exit pupil won't work at all. A wide-angle camera lens uses different parts of the front objective to form different parts of the image, and an eyepiece's exit pupil won't illuminate the camera lens properly. To put it another way, the camera will introduce additional vignetting that will completely ruin the experiment.
But your idea requires a light meter with a very narrow FOV, yet one that's wide enough to accept the entire 7-mm exit pupil of typical binoculars. Basically, this is going to come down to the same thing as my solution -- you're going to need a lens to bring the light to focus on the sensor. But now, instead of trying to capture the image as a whole, you'll tilt the lens to sample different parts of the image.
All of this is confounded by the fact that in real life, the exit pupil is somewhat of a legal fiction. The light cylinders from an eyepiece -- especially a wide-field eyepiece -- never converge on a neat circle in one plane, as shown in textbooks. A real-life exit pupil is a messy, three-dimensional affair.
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
|
|
5 registered and 9 anonymous users are browsing this forum.
Moderator: EdZ
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 1089
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 