Nikon 8x20 Lx Vs.10x25?? (2 Viewers)

[elkcub]

Hello elk and Bawko,

Happily, I think we are in complete agreement about magnification in binoculars. More magnification allows the eye to see more detail under any lighting conditions. However, the above quote about FOV is simply not correct and I think it can only cause confusion. I haven't checked, but for the moment lets assume that the Nikon 8X20 and 10X25 have the same AFOV. This could be true even if it isn't since many sibling bins like this use the same eyepiece. If the AFOV's are identical that means the area of retinal projection is identical for both. AFOV determines the size of the retinal projection, not real FOV. But really that hardly matters as far as brightness is concerned, because the size of the retinal projection has no effect on the brightness of the light within it. In this particular case the larger size of an object in the image of the 10X25 precisely matches the increase in objective size so the surface brightness of the object and the "intensity per square millimeter of light energy" on the retina is identical in the 10X25 and the 8X20, no matter what the lighting conditions. Its very likely that you will see the object more clearly through the 10X25 in low light, but only because it's larger, not because it's brighter.

I'm sure elkcub remembers that he, Ikka and I endured a long thread on the subject of FOV and brightness last year. I don't think I've got the stamina for doing that again.

Best Regards,

Henry

Henry,

Elkcub remembers well how you guys "endured" while he became educated, but the remnants of his modeling faux pas are still lurking out there. That line of reasoning is incorrect, Bawko, — but it is a captivating idea one has to admit. In reality, the field stop determines the area of coverage, and light per unit area remains constant.

Better to have tried and failed ... etc., etc, :gn:
-elk

 

[elkcub]

Bawko,

My last comment was really a reply to Post #19, but I am also interested in Henry's response to your contention — if he has the stamina. I recall a discussion of that issue last year, but I guess the threads were lost.

As stated above, I believe one reason we perceive the 10x25 to be brighter is because it can be pointed selectively at a smaller real field that probably is brighter than the more uniformly dark field seen by an 8x20 in dwindling light. Add the advantage of more magnification to aid acuity, and VOILA!

Whatever the explanations, I continue to agree with you that the 10x25 is a more effective configuration than the 8x20. In the case of Swaro pocket binoculars the AFOVs are very close. The larger 10x25 allows me to wrap the last two fingers of each hand around the objective tubes and focus with the middle finger while the thumbs fit underneath. I can hold it rock steady (even though I've been diagnosed with essential tremor) for long periods. By comparison, the 8x20s jiggle around like a bowl of jello.

Regards,
-elk

 

[elkcub]

... In this particular case the larger size of an object in the image of the 10X25 precisely matches the increase in objective size so the surface brightness of the object and the "intensity per square millimeter of light energy" on the retina is identical in the 10X25 and the 8X20, no matter what the lighting conditions. Its very likely that you will see the object more clearly through the 10X25 in low light, but only because it's larger, not because it's brighter. ...

Henry,

This is the part I'm not (quite) following. Let's say we are dealing with a unit square reflective target on a uniform black background. If the AFOV is the same in each binocular, then the larger projected image of the target takes up a larger portion of the apparent field. Although the amount of light per unit area on the target's retinal projection remains constant, as you say, the total light energy stimulating the retina increases, i.e., the integral over the target area. I don't know if this translates to an increase in apparent or perceived overall brightness, but I wouldn't be too surprised if it did.

-elk

 

[kabsetz]

Gentlemen,

Let me suggest you take an 8x20 and a 10x25 of the same model, set them on a tripod side-by-side pointed at uniformly lit sky or such, and take digital photos through the eyepieces of both using aperture priority mode, maximum aperture. Was there a difference in exposure time, and how much? (be sure to block stray light from entering between the eyepiece and the camera).

Kimmo

 

[Atomic Chicken]

Greetings!

Kimmo - I just did exactly what you recommended... using a diffused 75W lightbulb as the target. I set my camera (Sony DSC-T1) to manual mode and placed the lens so the lightbulb filled the full field of view after passing through the tripod mounted binoculars. In both cases, the camera defaulted to F/5.6 - the widest aperature it is capable of. The resulting shutter speeds are as follows:

Zeiss Victory 8x20 - 1/125 Second
Nikon HG 10x25 - 1/200 Second

This would seem to support my contention all along - that the 10x25 configuration is delivering more light to the same size exit pupil (and thus the same size retinal image) under the same lighting conditions. I was surprised that the result so closely matched my calculation, but I know the camera has a fixed "set" of shutter speeds so I'm not sure how precise the results are... but the results are definitely suggestive nonetheless.

Best wishes,
Bawko

 

[Swedpat]

Hi Bawko!

I'm sharing your result with big interest but have some questions. If your experiment proof your argument, then this fact would be clearly visible by the naked eye tests. A 10x50 would be considerable brighter than a 5x25 undependent of different light transmissions between them, but my experience is that my 5x25 and 10x50 appears as equivalent.

I would like to make this test: Take a 8x20 and place it at 10m distance from an object. Place then a 80x200 telescope and place it at 100m distance. Place an ocular to the telecope with the same appearant FOV as the 8x20. Both of them has the same light transmission and exit pupil. In this case you will see exactly the same visible area and size of the object. But according to your calculation it would be almost impossible to use a 200mm telescope at daytime because with 80x power it would be even brighter at dawn than a 7x50 bino despite only 2,5mm exit pupil. Even a 20x80 binocular would be a perfect night glass, but it isn't. The 4mm exit pupil of a 20x80 will not bring out details in an almost completely dark forest at night. But a 7x50 does.

If a larger aperture gives better brightness despite the same exit pupil according to your calculation, that means a large instrument like an astronomical telescope, if it's large enough, would be usable as a night glass despite only 1mm exit pupil. I dare to claim that 1mm exit pupil will never be usable for other purposes than daytime studies or astronomy, not even with 1000x power. Your test has not convinced me...

Regards, Patric

 

[henry link]

Bawko,

Experiments are good, but I think something has gone awry here, either in the controls or the instrumentation of this one. I can suggest at least two possibilities: 1) the surface of the light bulb is not actually uniform in brightnes so that a brighter area was sampled by the 10X25. 2) the amount of vignetting of the exit pupil by the prism apertures of the two binoculars is different. The full exit pupil of every binocular suffers from some vignette (one side of the exit pupil is not fully illuminated by the same side of the objective), but the amount of vignette varies from model to model. Or perhaps, as you suggest, the camera would have picked closer intermediate shutter speeds if they were available.

Let me try another "thought experiment" example, in which the light source is truly uniform and the imaginary binoculars are completely vignette free. Imagine that you are looking at a simple target, the full moon, with 8X20 and 10X25 binoculars. Field width (real or apparent) will be irrelevant since the moon's image occupies only a small part of either field and is surrounded by black. I think you will agree that the moonlight that falls on the the objectives of the two binoculars is equally bright per square mm of glass surface. Of course the 25mm objective collects more moonlight because of its larger total area. The light collected by the objectives travels through the binoculars and the lens of the eye and is brought to focus on the retina. Each binocular will produce a tiny focused image of the moon on the retina, but the sizes of those images will differ according to the magnification. I don't know exactly how large these images are but for the sake of simplicity I will say the 8X image of the moon on the retina is 2mm in diameter and the 10X image is 2.5mm. I think you can see where this is going. The ratio of the size difference between the two images projected on the retina is exactly the same as the ratio of the size difference between the two objective lenses. Therefore, the brightness of the moonlight falling on the retina, per square mm of area, is identical in the two binoculars, just as it was when the very same moonlight fell on the different sized objectives.

I actually don't disagree with you and elkcub (in his last post) that, in low light, larger images of objects in binoculars and telescopes can appear brighter even when the actual surface brightness is equal. This is well known to observers of dim fuzzy objects in astronomy. One would hope that the eye/brain would do the right thing and register that the object is only bigger, not brighter. I don't have an explanation for why this seems to happen in our vision system, but to understand it will certainly first require an accurate model of how much light from different types of binoculars is actually falling on the individual receptors on the retina.

 

[Atomic Chicken]

Henry,

There is another possibility...
When I did the experiment, I placed the lightbulb close enough to the binoculars that it's light completely filled the field of view of the 8x20, and also the field of view of the 10x25. Perhaps I would get more accurate results if I placed the lightbulb several feet away so it appears as a small part of the image?

However, if we are talking about binocular performance in low light conditions, wouldn't the available light from the entire scene be focused through both sets of binoculars, not just a bright "angular source" like the moon you used in your thought experiment? In that case, having the lightbulb up close so it fills the field of view would be a more accurate approximation of the real-world viewing conditions we are attempting to duplicate.

While I follow your thought experiment completely and agree 100%, I'm not sure that it is accurate in terms of comparing 2 binoculars under low-light conditions - mainly because the object being viewed in the thought experiment is a small part of the overall field, thus skewing the results in favor of the smaller aperature. When the total available light fills the entire field of view, I still maintain that the total light falling on the retina will be greater when using the larger aperature binoculars, assuming the AFOV and exit pupil size remain identical.

Best wishes,
Bawko

 

[elkcub]

... but to understand it will certainly first require an accurate model of how much light from different types of binoculars is actually falling on the individual receptors on the retina.

Be careful, Henry, modeling is where I fell in a hole. :h?:

I think your general observations are correct for this reason. The light entering the objective comes from the real field. As the objective size is increased it does catch more light, as Bawko says, but the power must also be increased to maintain a constant EP. As the power is increased the real field diminishes and the light throuput would tend to remain constant per unit area. The formulas would determine if it's an exact tradeoff, but the optics must generally work in that way. (Unless I've fallen in another hole.)

This reasoning essentially applies to a uniformly illuminated field, where it would not be evident that the real field is diminishing if the apparent (i.e., projected retinal) field remains more or less constant. Under more usual birding circumstances, where there are defined objects in a non-uniformly illuminated field, the user must select something to look at. In diminishing light, he probably searches out the brightest most contrasty objects, which are more easily isolated within a narrow real field that also maximizes source brightness. So, the binoculars are credited with being brighter, which in this limited sense they are.

-elk

 

[kabsetz]

I'm risking being sloppy here, since I intend to go out into the sunshine in a few minutes, to watch birds of all things, and haven't spent enough time thinking about it. However, this very moment I completely agree with Henry's assesment of his experiment with a light source which does not fill the entire field of either instrument. I also find myself agreeing with Bawko's and Elk's interpretation that when viewing a uniformly-lit surface (and, in practice, probably any view where brightness variations are much less extreme than with celestial objects agains a dark sky), the extra light gathered by the 25mm objective must make the equal-size TOTAL retinal projection circle to have, on the average, higher surface brighness. It is not a large leap of faith to think that in terrestrial viewing, this would also make it easier to pick what you wish to focus on in the overall image. All of this, of course, only applies until we reach an exit pupil size which exceeds that of the eye under the prevailing light levels, but since we are here talking about a 2.5mm exit pupil, this could only happen in bright daylight.

Slightly off the subject, I have been using an 8x25 for a week or so now, and find that the increase in exit pupil size over an 8x20 makes for rather noticeably more comfortable viewing experience. I'm yet to make twilight comparisons between it and an 8x20.


Kimmo

 

[Atomic Chicken]

Kimmo,

I'm very curious... what 8x25 have you been using? I've been looking for a high quality 8x25 for several months now, the best thing I've found so far is the Brunton Echo 8x25. It's far from perfect, however... I'm still looking for something better.

Best wishes,
Bawko

 

[hinnark]

Kimmo,

I'm very curious... what 8x25 have you been using? I've been looking for a high quality 8x25 for several months now, the best thing I've found so far is the Brunton Echo 8x25. It's far from perfect, however... I'm still looking for something better.

Best wishes,
Bawko

Hi Bawko,

the Canon 8x25 IS is of good optical quality and - believe it or not - the IS is still helpfull. Unfortunately they need a type of battery of which recharchables aren´t available AFAIK. BTW I agree with Henry´s explanations but think also that we discuss some academic questions here. In practice the differences between 8x20 and 10x25 I mentioned above are most important for a decision between both types IMHO.

Steve

 

[elkcub]

Kimmo, et al,

I'm going to hit the hay in a second, but I did some quick calculations on specs for my Swaro pocket binocs. The 8x20 has a Real FOV = 356, and the 10x25 has a Real FOV = 285. They each have an AFOV of 54.3 deg.

Going from the 8x to 10x, the objective area ratio is 1.56. Going from the 10x to 8x the Real FOV area ratio is 1.56. In other words there is an exact tradeoff. As the objective area of the binocs increases the real FOV decreases to compensate. Since the EP is 2.5 in each case, the total amount of light from a uniform field of light will be the same, and of course the light per unit area will also be the same. QED

It would be interesting to verify that this relationship holds for other compact pairs, and also across manufacturers.
-elk

 

[Atomic Chicken]

Steve,

Thanks for the Canon 8x25 IS recommendation, but my main reason for using an 8x25 is small size and weight. The Canon 8x25IS weighs approx. 1lb, far too heavy. If I am going to carry something weighing a pound, I'll grab an 8x32... and get superior optical quality.

My dream compact is a 7x25 or 8x25 that weighs under 10oz... as small as possible with phase coated optics and rugged, waterproof construction.

Best wishes,
Bawko

 

[hinnark]

Steve,

Thanks for the Canon 8x25 IS recommendation, but my main reason for using an 8x25 is small size and weight. The Canon 8x25IS weighs approx. 1lb, far too heavy. If I am going to carry something weighing a pound, I'll grab an 8x32... and get superior optical quality.

My dream compact is a 7x25 or 8x25 that weighs under 10oz... as small as possible with phase coated optics and rugged, waterproof construction.

Best wishes,
Bawko

There is an 8x25 available called 8x25 Celestron Regal LS built by the Japanese manufactorer Vixen. They are waterproof, nitrogen filled and phase coated AFAIK. In Europe these are called Vixen Apex Pro. Unfortunately I only know the 10x25 of these which disappointed me in respect of optical quality. But as you surely know it´s not fair to conclude from one type of bino to another one. So maybe you´ll get a chance somewhere to try them.
BTW these 10x25 had a type of eye cups I found interesting. They work as push- in-pull-out ones but with continous click-stops.

Steve

 

[Swedpat]

I have a Minolta 8x25 D WP XL, which I find to be a very good value for the money. I chose this model instead of a 8x20 or 10x25 because of the significantly better brightness. Waterproof and fogproof. Sharp and contrasty image almost until the edges, even if not in par to a 3,5 times more expensive Swarovski. The only disadvantage I can see is the AFOV of 44 degrees, but the 18mm ER allows to see the absolutely entire FOV even with glasses on. In that respect it's superior to all the Swarovski, Zeiss and Leica compacts.

Patric

 

[henry link]

I'm risking being sloppy here, since I intend to go out into the sunshine in a few minutes, to watch birds of all things, and haven't spent enough time thinking about it. However, this very moment I completely agree with Henry's assesment of his experiment with a light source which does not fill the entire field of either instrument. I also find myself agreeing with Bawko's and Elk's interpretation that when viewing a uniformly-lit surface (and, in practice, probably any view where brightness variations are much less extreme than with celestial objects agains a dark sky), the extra light gathered by the 25mm objective must make the equal-size TOTAL retinal projection circle to have, on the average, higher surface brighness. It is not a large leap of faith to think that in terrestrial viewing, this would also make it easier to pick what you wish to focus on in the overall image. All of this, of course, only applies until we reach an exit pupil size which exceeds that of the eye under the prevailing light levels, but since we are here talking about a 2.5mm exit pupil, this could only happen in bright daylight.

Slightly off the subject, I have been using an 8x25 for a week or so now, and find that the increase in exit pupil size over an 8x20 makes for rather noticeably more comfortable viewing experience. I'm yet to make twilight comparisons between it and an 8x20.


Kimmo

Kimmo,

You're the last person I would accuse of sloppyness in these matters, but I wonder if reading elkcub's #33 post above might change your mind about surface brightness within the retinal projection circle from a uniformly lit surface seen through binoculars of different magnifications but the same exit pupil.

The one type of visual target that should be brighter in a 10X25 vs an 8X20 would be a point source, like a star. In that case (at least theoretically) the area of retinal projection of the star point remains the same whether the magnification is 8X or 10X, so only aperture determines brightness.

Henry

 

[kabsetz]

Henry,

Reading Elk's post #33 actually did the opposite.

We here have the situation where the 10x binocular uses an objective lens 1,56x larger than that of the 8x to gather light from a 1.56x SMALLER circle (since the TFOV is different by the 8/10 ratio) of somewhat uniformly lit scenery, and passes this through an equal-sized exit pupil onto an equal-sized retinal projection circle (since the AFOV is identical).

Also, theoretically, can we not view a uniformly-lit scenery as an infinite group of adjacent and overlapping point sources? If so, your star-example would show that the 10x25 produces a brighter retinal projection also of a uniformly-lit area, not only of individual point sources.

Kimmo

 

[Tero]

Don, try to test them out, no tripods. Or test some, ANY, 10x pair to see if you are comfortable. I like 10x most times, but use a 7x pair near dusk. Most people seem to like 8x overall. I see much more detail at 10x.

 

[henry link]

Henry,

Reading Elk's post #33 actually did the opposite.

We here have the situation where the 10x binocular uses an objective lens 1,56x larger than that of the 8x to gather light from a 1.56x SMALLER circle (since the TFOV is different by the 8/10 ratio) of somewhat uniformly lit scenery, and passes this through an equal-sized exit pupil onto an equal-sized retinal projection circle (since the AFOV is identical).

Also, theoretically, can we not view a uniformly-lit scenery as an infinite group of adjacent and overlapping point sources? If so, your star-example would show that the 10x25 produces a brighter retinal projection also of a uniformly-lit area, not only of individual point sources.

Kimmo

Kimmo,

Uh...oh. Looks like I'm in up to my neck now. FWIW here's my take on it.

I think elkcub's post speaks for itself, but consider the opposite situation. What if the TFOV of the two binoculars were the same? The eyepiece fieldstop would have to be larger in the 10X25 and as a result more total light would exit the eyepiece. However, its AFOV would be larger in the now familar 10 to 8 ratio, so the size of its retinal projection circle would be larger by the same amount, exactly compensating for the larger amount of light that exited the eyepiece and keeping the surface brightness on the retina equal to the 8X20. Once again I agree with everyone that higher magnification makes things look brighter, but I don't think that phenomenon can be attributed to higher surface brightness on the retina if exit pupils are equal.

Remember that a true point source, like a star, can't be magnified. Its "size" is simply the size of the Airy disc determined by the objective aperture. A 25mm objective's Airy disc at 10X is the same size as a 20mm objective's at 8X so the size of the retinal projection of star points (as opposed to extended objects) is equal in the two binoculars, but the amount of light within the Airy disc is larger in the 10X25. Now my head is starting to hurt.

Henry