Vespobuteo
Well-known member
Why would it appear "brighter"?
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7x42 (2 Viewers)
- Thread starter dries1
- Start date
Why would it appear "brighter"?
dwatsonbirder
Well-known member
I would like to try the Meopta and Zeiss FL. No plans for a 7x42 EDG to join your enviable Nikon collection then?
A very competent binocular IMO. My first serious pair was the 8x42HGL which I bought with my first proper paycheck when I was 17. Excellent bins, but the rubber wore quickly and sold them to get something lighter - but I ended up with a heavier pair of Swaros!
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looksharp65
Well-known member
I find this reasoning strange, counterintuitive and downright incorrect.
The actual objective size per se is irrelevant, only the exit pupil size and the transmission rate determine the final light output.
If your pupils can't dilate more than 5.25 mm, the 7x42 will be identically bright as the 8x42, provided the transmission rate is the same.
The 8x42 will not appear brighter than the 7x42 since binoculars are afocal instruments. Neither will the AFOV have impact on the light output.
So it is not as you seem to suggest, that a 7x42, when used with a 5.25 mm human pupil, will behave as if it were stopped down.
Edit: If stopped down to have a 5.25 mm exit pupil, the light lost outside your pupil won't be there like before stopping it down.
But the light intensity within the beam of parallell rays exiting the binocular will not change. It is always the same as the intensity of the incident light minus the usual losses.
//L
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looksharp65
Well-known member
With a 5.25 mm max dilatation for baseline, the following will happen:
Exit pupil 6 mm (7x42)
(100% = 100%)
90% = 90%
80% = 80%
70% = 70%
Above, we again see the light loss bear a linear relationship to the exit pupil size. Being larger than the user's pupil, the full width of the beam pencil can not enter the eye.
The light intensity within the beam pencil is, as always with afocal systems, solely determined by the transmission rate and not the width of the beam.
Exit pupil 5,25 mm (8x42)
(100% = 100%)
90% = 90%
80% = 80%
70% = 70%
Just as with the 7x42, the transmission rate decides how bright the beam pencil will be. No constricted pupil yet to cause further light loss.
Exit pupil 4 mm (8x32)
(100% = 58%)
90% = 52%
80% = 46%
70% = 41%
It is very apparent that 8x32, 10x40 and the like are suitable mostly for daytime viewing.
A very good 90% transmission drops considerably to 58% if you can dilate to 5.25 mm when night falls. If your pupil is no wider than 4 mm, you can enjoy all the light output the binocular can deliver.
Exit pupil 3,2 mm (10x32)
(100% = 37%)
90% = 33%
80% = 30%
70% = 26%
Definitely daytime binoculars
//L
Exit pupil 6 mm (7x42)
(100% = 100%)
90% = 90%
80% = 80%
70% = 70%
Above, we again see the light loss bear a linear relationship to the exit pupil size. Being larger than the user's pupil, the full width of the beam pencil can not enter the eye.
The light intensity within the beam pencil is, as always with afocal systems, solely determined by the transmission rate and not the width of the beam.
Exit pupil 5,25 mm (8x42)
(100% = 100%)
90% = 90%
80% = 80%
70% = 70%
Just as with the 7x42, the transmission rate decides how bright the beam pencil will be. No constricted pupil yet to cause further light loss.
Exit pupil 4 mm (8x32)
(100% = 58%)
90% = 52%
80% = 46%
70% = 41%
It is very apparent that 8x32, 10x40 and the like are suitable mostly for daytime viewing.
A very good 90% transmission drops considerably to 58% if you can dilate to 5.25 mm when night falls. If your pupil is no wider than 4 mm, you can enjoy all the light output the binocular can deliver.
Exit pupil 3,2 mm (10x32)
(100% = 37%)
90% = 33%
80% = 30%
70% = 26%
Definitely daytime binoculars
//L
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looksharp65
Well-known member
Admittedly, you explain it well and make me uncertain what to respond, but I'm absolutely confident that you must be wrong.
The objective forms a real image, just as when you use magnifying lenses to collect the sunlight to start a fire, or with photo lenses that project the image onto the sensor or film. The eye can't see a real image, it must be projected on the retina by the eye's optical system, and the eye can only handle diverging or parallel beams.
The exit pupil is not a converging projection, it is a flow of mainly parallel and diverging beams in a beam pencil.
Apart from the losses and the magnification, the binocular receives and emits identically parallel beams. It cuts out a slice of all the incident light that surrounds us. Very little is changed.
Suppose you have a 4 mm wide pupil and you look at a huge wall (those have gained an unexpected raise in popularity recently
), and that's the brightness benchmark. The observation distance is irrelevant, but I'm thinking a big, fat freaking grey wall covering the entire visual field.If then you take a 20" telescope, choose an eyepiece yielding a 4 mm exit pupil with the scope and look at the wall, will it appear immensely bright?
Really? Or if I take my new 12x50's to look at the overcast sky tomorrow, will it appear considerably brighter than with the naked eye? I don't think so.
Now, if you punch a 4 mm wide hole through a black paper, thoroughly centering it in front of your eye, look at the wall. The brightness of the wall will obviously not change. It's just a hole, no optics. But there's an infinitely large imaginary "front aperture" involved, which would make the system melt down, not?
Finally: What's your explanation to the brightness formula which entirely depends on the (squared) exit pupil size, i.e. the exit pupil's area?
It does not favourise front aperture at all. A too big exit pupil does not mean light lost. A too small one does.
//L
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I own two!
I do go back and forth on liking 7x or 8x better.
Think it comes down to: if general viewing I prefer the 7x, if trying to see more detail I prefer the slight more magnification of the 8x.
Unfortunately I do both types of viewing equally as much as the other... so its a toss-up or it depends on the particular day.
CG
I do go back and forth on liking 7x or 8x better.
Think it comes down to: if general viewing I prefer the 7x, if trying to see more detail I prefer the slight more magnification of the 8x.
Unfortunately I do both types of viewing equally as much as the other... so its a toss-up or it depends on the particular day.
CG
Kevin Conville
yardbirder
Pileatus and Looksharp65,
I'm really enjoying this dialog between you two.
In my amateurish, but dangerously marginally informed opinion, I side with Pileatus.
I've been hoping Henry Link will chime in here because, as I remember, his favorite bin are 8x56s. Paraphrasing, he said something like: During daylight, because of a smaller pupil diameter, these are effectively stopped down, increasing focal length, therefore improving the short focal ratio common to binoculars and improving the aberrations we're used to. Or, something like that.
Stopped down, in photographic terms (among other things) means less light. It also means more depth of field. So if there is effectively a "stopped down" effect when using a binocular that provides a larger exit pupil than one's own pupil can match, how does the brightness (actual photons) not be reduced?
I'm really enjoying this dialog between you two.
In my amateurish, but dangerously marginally informed opinion, I side with Pileatus.
I've been hoping Henry Link will chime in here because, as I remember, his favorite bin are 8x56s. Paraphrasing, he said something like: During daylight, because of a smaller pupil diameter, these are effectively stopped down, increasing focal length, therefore improving the short focal ratio common to binoculars and improving the aberrations we're used to. Or, something like that.
Stopped down, in photographic terms (among other things) means less light. It also means more depth of field. So if there is effectively a "stopped down" effect when using a binocular that provides a larger exit pupil than one's own pupil can match, how does the brightness (actual photons) not be reduced?
I once had a Fujinon 7x50 FMTRC-SX and a Swarovski Habicht 8x30 W at the same time. 99% of the time the Habicht would give me just as bright and good of view to my eyes as the huge, heavy Fujinon. I sold the Fujinon. Lesson learned "Bigger is not always better."
Kevin Conville
yardbirder
So, why are you biting your tongue? Let's hear it.
black crow
Well-known member
I'm 65 and I've had my 7x36 Zens for several years now. I read somewhere before I got them that 7 was really the preferred size for most people in some test they did where the people didn't know what the power was when viewing and most preferred 7x no matter what they said they liked before the test. So I was really into the idea but the reality is I wish they were 8x or even 9x over the 7x. What I do like however is the 477 ft FOV.
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Pileatus,
I'm with Lars on this one.
It's a topic I've found challenging to get my head round myself, and have found even more difficult to explain. I'll have another go.
I think it's best to go back to defining magnification. It's a change in the apparent angular size of an object. If a tree is 1 degree tall to the eye then it will be 8 degrees tall with an 8x binocular. It's the light level for the angle that translates to brightness, not total light flux. The angle of the 'light beam' at the exit pupil in this case will be 8x wider than the objective. This means the exit pupil will consequently be 8x smaller. You could imagine the magnification causes a 8 fold reduction in light intensity and the smaller EP an 8 fold increase, cancelling each other out exactly. Together it means that the light level per given angle of view from any point in that exit pupil will be exactly the same as that for the unaided eye. The target will appear equally as bright with or without a binocular (ignoring internal light loss).
Of course that assumes the EP isn't limiting for the eye. If the light level for a particular target means your pupil is 2.5mm unaided, then the light level for a given angle of view will be the same regardless of whether you are using a 5x12.5, a 8x42 or 16x70. On the other hand, in low light with a 5mm pupil diameter, the light level per given angle will be 25% and 76% of the 8x42 levels for the 5x12.5 and 16x70 respectively. However, increased magnification make targets more visible, and it is quite likely that the 16x70 will give the illusion of being brighter than the other two.
Hope that helps.
David
I'm with Lars on this one.
It's a topic I've found challenging to get my head round myself, and have found even more difficult to explain. I'll have another go.
I think it's best to go back to defining magnification. It's a change in the apparent angular size of an object. If a tree is 1 degree tall to the eye then it will be 8 degrees tall with an 8x binocular. It's the light level for the angle that translates to brightness, not total light flux. The angle of the 'light beam' at the exit pupil in this case will be 8x wider than the objective. This means the exit pupil will consequently be 8x smaller. You could imagine the magnification causes a 8 fold reduction in light intensity and the smaller EP an 8 fold increase, cancelling each other out exactly. Together it means that the light level per given angle of view from any point in that exit pupil will be exactly the same as that for the unaided eye. The target will appear equally as bright with or without a binocular (ignoring internal light loss).
Of course that assumes the EP isn't limiting for the eye. If the light level for a particular target means your pupil is 2.5mm unaided, then the light level for a given angle of view will be the same regardless of whether you are using a 5x12.5, a 8x42 or 16x70. On the other hand, in low light with a 5mm pupil diameter, the light level per given angle will be 25% and 76% of the 8x42 levels for the 5x12.5 and 16x70 respectively. However, increased magnification make targets more visible, and it is quite likely that the 16x70 will give the illusion of being brighter than the other two.
Hope that helps.
David
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looksharp65
Well-known member
Reality
Yup, I definitely think you have something there, David.
In the meantime, I arranged my ED82A 30x (2.78 mm exit pupil) directed towards the overcast sky to have a brightness measurement with the help of my phone. The phone in question has a 4.6 mm f/2.0 lens, so its aperture is 2.3 mm.
The other instrument used was my Nikon 6x15. Its front aperture area is 3.3% of the scope's, so the scope should be about 30 times brighter if the other theory would be correct.
I locked the AF to infinity, tried but failed to lock the ISO value (the difference was negligible, though), and set the camera to spot metering of the exposure.
The camera has no variable diaphragm and operates at full stop regardless of brightness. To avoid getting too large image angles, I used the digital zoom to crop away trees and such when using the camera without other magnification and with the small Nikon.
The result is shown below. Remember that the exit pupils of the scope and the small binocular is bigger than the aperture of the camera lens, with a small margin. The weather is about as dull and boring as the images suggest. The images are cropped to fit into Birdforum's size regulations.
The image taken without other magnification than the digital zoom has ISO 50 and a 1/195 s.
The image through the scope has also ISO 50 and a 1/141 s exposure.
Finally, the miniature binocular's image has ISO 52 and a 1/120 s exposure. All numbers appear quite feasible, although I had hoped that the ED82A would have had a better transmission rate. It takes more meterings to establish a more reliable number.
While things could be done more accurately, I guess I've proved my point.
//L
Yup, I definitely think you have something there, David.
In the meantime, I arranged my ED82A 30x (2.78 mm exit pupil) directed towards the overcast sky to have a brightness measurement with the help of my phone. The phone in question has a 4.6 mm f/2.0 lens, so its aperture is 2.3 mm.
The other instrument used was my Nikon 6x15. Its front aperture area is 3.3% of the scope's, so the scope should be about 30 times brighter if the other theory would be correct.
I locked the AF to infinity, tried but failed to lock the ISO value (the difference was negligible, though), and set the camera to spot metering of the exposure.
The camera has no variable diaphragm and operates at full stop regardless of brightness. To avoid getting too large image angles, I used the digital zoom to crop away trees and such when using the camera without other magnification and with the small Nikon.
The result is shown below. Remember that the exit pupils of the scope and the small binocular is bigger than the aperture of the camera lens, with a small margin. The weather is about as dull and boring as the images suggest. The images are cropped to fit into Birdforum's size regulations.
The image taken without other magnification than the digital zoom has ISO 50 and a 1/195 s.
The image through the scope has also ISO 50 and a 1/141 s exposure.
Finally, the miniature binocular's image has ISO 52 and a 1/120 s exposure. All numbers appear quite feasible, although I had hoped that the ED82A would have had a better transmission rate. It takes more meterings to establish a more reliable number.
While things could be done more accurately, I guess I've proved my point.
//L
Attachments
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looksharp65
Well-known member
What falls on the iris, is wasted, but does not take away from the brightness within the beam pencil.
Professional photographers rather use a 300 mm f/2,8 than a f/5,6. This isn't exclusively to obtain shorter exposure times, it's because a f/2,8 stopped down to f/5,6 with the diaphragm is optically a lot better than the f/5.6 at full stop.
Pileatus wrote "Aperture rules". But he unfortunately forgot the fundamental difference between actual aperture and relative aperture.
//L
Kevin Conville
yardbirder
So all of the light entering a 42mm objective (normal losses excluded) is brought to a 6mm circle in the case of the 7x and 4.2 in a 10x. Your pupil is dilated to, say 4.2 mm. Are you saying that somehow your eye captures the same amount of light in both despite about 50% (roughly 14 vs 28mm sq.) of the 7x42 isn't reaching the pupil?
That doesn't make much sense to me.
It gets better, I think.
Say you're looking at a boat from shore through 8x35s and 16x70s. Both have 4.375mm exit pupil diameters. Through the 16's the boat fills the image circle. The 8's fill half of that circle. The value of the light through the 16s is better because it's the light of your subject. If the boat is brighter than it's background your pupils will constrict a bit more increasing the effective focal ratio, somewhat improving the optics. If the boat is darker than it's background your pupils will dilate a bit more and allow more light to your eye.
This might also help to explain the mysterious Twilight Factor that used to get a lot of attention.
With the added resolution of magnification comes less noise. Everything else being equal it means less flare, glare, and wasted photons.
That doesn't make much sense to me.
It gets better, I think.
Say you're looking at a boat from shore through 8x35s and 16x70s. Both have 4.375mm exit pupil diameters. Through the 16's the boat fills the image circle. The 8's fill half of that circle. The value of the light through the 16s is better because it's the light of your subject. If the boat is brighter than it's background your pupils will constrict a bit more increasing the effective focal ratio, somewhat improving the optics. If the boat is darker than it's background your pupils will dilate a bit more and allow more light to your eye.
This might also help to explain the mysterious Twilight Factor that used to get a lot of attention.
With the added resolution of magnification comes less noise. Everything else being equal it means less flare, glare, and wasted photons.
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looksharp65
Well-known member
That's also true, but does not take away from the analogy. It wasn't me who introduced camera lenses and photography into the discussion.
Nope, there is no focal plane there, since binoculars are afocal.
//L
looksharp65
Well-known member
Makes no difference whether or not it makes sense to you, it's the reality.
How would you explain that the image taken through my scope with a 30 times bigger aperture than the 6x15 only has a marginally shorter exposure time, that can be attributed to better transmission rate? Now, that shouldn't make sense to you.
The measurements are empirical data rebuting your theory.
//L
Kevin Conville
yardbirder
I deleted my last post, Lars, because I realized I misread what you wrote. You quoted me just before it evaporated. Apologies.
Yes lenses do perform better when a bit stopped down, but you're not allowing that a faster lens, F2.8 vs say an f5.6 makes for a brighter viewfinder and allows more accurate and quicker focus for both manual and auto focus systems.
Just as with a camera lens, the pupil "stopped down" a bit makes for a longer focal ratio through a given aperture.
To your last point: OK binoculars are afocal, that's why I said effectively.
Look, the light has to fall on something, right? In a camera it's the focal plane, film or image sensor. Using field optics the light fall on the pupil.
The real point is that the focal plane of the camera doesn't change, the pupil does.
Yes lenses do perform better when a bit stopped down, but you're not allowing that a faster lens, F2.8 vs say an f5.6 makes for a brighter viewfinder and allows more accurate and quicker focus for both manual and auto focus systems.
Just as with a camera lens, the pupil "stopped down" a bit makes for a longer focal ratio through a given aperture.
To your last point: OK binoculars are afocal, that's why I said effectively.
Look, the light has to fall on something, right? In a camera it's the focal plane, film or image sensor. Using field optics the light fall on the pupil.
The real point is that the focal plane of the camera doesn't change, the pupil does.
Kevin Conville
yardbirder
You're missing the fact that with a camera attached all of the light is being focused on the image sensor. With a binocular with a 7mm exit pupil and an eye pupil of 4mm much of it is wasted. Though you didn't bring cameras into the discussion you nevertheless aren't making good use of them in this discussion.
I never argued about differences in throughput, just usable light.
I'm going to bed...
