Vignetting in Binoculars (2 Viewers)

[Tringa45]

This is a topic rarely discussed on Birdforum, but one that has an influence on the performance and perhaps subjective appreciation of binoculars.
Vignetting is a loss of brightness towards the field edge and is present in all binoculars. It is caused by obstructions, usually baffles, in the optical path.
The front surface of a prism would usually have a circular baffle and its diameter would, of course, be as large as the prism would allow.
It should also be large enough to accommodate the light cone of a focussed point source on the optical axis. If this were not the case, then the objective diameter would essentially be stopped down. This is sometimes the case for cheap binoculars but two other examples come to mind: early Canon 10x42 IS were effectively 10x38 and a 50 mm Optolyth Porro was effeectively only about 40 mm.

However, binoculars typically have a true field of view of around 8° so the light cone of a focussed object 4° off the optical axis would hit the edge of the baffle and reduce the brightness we see via the eyepiece. Holger Merlitz states that a reduction of 30% (!!) at the field edge would not necessarily be perceived as disturbing. So much for subjective comparisons of minor transmission differences! It does however force the manufacturers to reach some sort of compromise.
Prisms make up a considerable part of a binocular's weight and a reduction of 10% in a linear dimension would result in a reduction of volume and weight of 27%. If a large proportion of potential customers is shouting for reduced weight, who's to blame the manufacturers if they comply with optical compromises? Perhaps that's just another example of negative internet influence. Twenty years ago the manufacturers did what they considered right.

Vignetting can be observed by holding a binocular at arm's length and observing the exit pupils as it is turned off axis. Swarovski allegedly place some emphasis on this (Randpupille) and on three of mine the exit pupils are still gibbous-moon shaped before they occlude. It's not a bad binocular, but on my Kowa Genesis 8x33 they become almond-shaped. An 8x42 NL I looked at was mediocre, but that can be attributed to the extraordinately large AFoV.

Perhaps others, in particular Canip with his large collection, could give some feedback on this. I'm guessing the 2,5 kg Nikon WX would fare very well and the Zeiss SFL less so.

John

 

[Canip]

Interesting topic, John!

Brief remark about terminology: I guess the term "vignetting" applies to all situations in which peripheral rays are obstructed. It is therefore also used for the "square shape" vignetting in porro binoculars which exhibit the use of BaK-7 glass vs. binos with BaK-4 glass that show fully illuminated round exit pupils.

See pic (credit: Yoder/Vukobratovich, Field Guide to Binoculars and Scopes, p.68).

Turning to the "edge pupil" type vignetting which your initial post addresses, I will post some examples of vignetting in a next posts which will show that the phenomenon is not something that is absent in expensive binoculars and only shows in cheap ones. Quite to the contrary. All seems to depend on the size of the prism used vs. the size of the ray bundle (Holger, correct me.)

 

[Canip]

A few samples of vignetting:
SkyRover 8x42 (left) vs NL Pure 8x42 (right)

 

[Canip]

Swaro MY Junior 7x28 (left), AX Visio (middle), APM 6.5x32 (right)

 

[Canip]

Bushnell Rangemaster 7x35 (left), Olympus DPS I 7x35 (right)

 

[Canip]

Zeiss SF 8x32 (left), SFL 8x40 (middle), Nikon WX 10x50 (right)

 

[Canip]

Discussion in a nutshell:

• Is the SkyRover "worse" than the NL? Not noticeably in my view.
• The Junior seems okay, the Visio quite good, the APM appears good at first sight but then exhibits several other anomalies
• The Bushnell with its huge prisms exhibits little vignetting, the Olympus even less (!), but shows the "square shape" vignetting of BaK-7 binos
• The SF and SFL exhibit similar amounts of vignetting, but the SFL shows an odd blueish area at the edge. The WX is no doubt quite good, but not free of vignetting either.

I have never paid much attention to vignetting when reviewing binoculars. Rightfully or wrongfully? Am I wrong in saying that in most modern binoculars, brightness loss at the edge is a actually quite modest?

fwiw Canip

 

[Tringa45]

Brief remark about terminology: I guess the term "vignetting" applies to all situations in which peripheral rays are obstructed. It is therefore also used for the "square shape" vignetting in porro binoculars which exhibit the use of BaK-7 glass vs. binos with BaK-4 glass that show fully illuminated round exit pupils.

According to Holger the limit for BK7 is a focal ratio of f/5. As most binoculars are "faster" than that, some parts of the light cone will be too acute for total internal reflection leading to that squared-off effect.
Of course, the glass path is part of the optical calculation, but for focal ratios slower than f/5 BK7 would be a better choice than BaK4 as it has lower dispersion and somewhat better blue transmission.

Regards,
John

PS: Thanks for the pics. I thought the Nikon WX a little disappointing and was puzzled by the asymmetry of the eyecup. Your vintage 7x42 SLC would be a good benchmark.
The Sky Rover and NL do indeed look similar and the Junior is probably helped by its relatively narrow AFoV.

PPS: I see now that was wide-angle distortion of the camera lens. A longer focal length would give a more "objective" depiction.

 

[tenex]

I was looking forward to your starting this thread. First a basic observation that may not be immediately obvious: the asymmetry of the "almond" pupil in photographs is an artifact of the direction the bin is tilted, and must be mentally rotated through 360° to give an idea of overall vignetting. (Also, it's specifically 8x bins that typically have ~8° field.)

Holger Merlitz states that a reduction of 30% (!!) at the field edge would not necessarily be perceived as disturbing. So much for subjective comparisons of minor transmission differences!

I've been pondering this since the first time you said it, and the logic is still not clear to me. Vignetting toward the edge is not perceptually the same as dimming of the center field, especially if unaided vision exhibits vignetting itself -- is this why vignetting seems unnoticable in bins, as it just replicates the natural vignetting that would occur further out in the full visual field?

Twenty years ago the manufacturers did what they considered right.

Has this changed recently? It seems to me that compactness has ruled roof-prism design ever since the 1950s. Can one really see a secular trend in vignetting from older models to newer?

Am I wrong in saying that in most modern binoculars, brightness loss at the edge is a actually quite modest?

The test results or photos I've seen illustrating color differences etc don't seem to show much vignetting.

 

[Binastro]

In the daytime the pupils of the eye, especially with an 8x binocular, are smaller than the exit pupils of the binocular.

Some makes, such as Nikon, generally are fairly honest in the statement of objective diameter.

But many of the cheaper models are smaller than stated.
The 15x70 cheap Chinese binoculars are about 15x62 or 15x63.
But a Revelation 15x70 fully multicoated example in good alignment and fairly light weight shows a great deal at night.
For me the better 15x70 Quantam needs a tripod, so was little used.

10x50s are often 10x47 or so.

The Celestron 8x30 6.7x27.

The Optolyth 12x50 is interesting as it is actually 12x42 with a very strange exit pupils, yet the owners love it and it is light weight.

In general, I don't think vignetting is necessarily even noticed, even with squared off exit pupils of older binoculars.

The 77mm Apo Televid spotting scope is at most 75.5mm.

The real nasties are single glass objective cheap scopes, which claim to be 25mm but are actually 10mm with a stop just behind the objective.

And 160mm aperture binoculars that may be 70mm or 50mm. Who knows?

Regards,
B.

 

[Tringa45]

The Optolyth 12x50 is interesting as it is actually 12x42 with a very strange exit pupils, yet the owners love it and it is light weight.

That's the one I was thinking of.

 

[Tringa45]

First a basic observation that may not be immediately obvious: the asymmetry of the "almond" pupil in photographs is an artifact of the direction the bin is tilted, and must be mentally rotated through 360° to give an idea of overall vignetting. (Also, it's specifically 8x bins that typically have ~8° field.)

Irrespective of the tilt direction, the orientation of the vignetted exit pupil to the field edge will always be the same. The tangential diameter will remain largely unchanged, whereas the radial diameter diminishes the closer it gets to the field edge. Btw, I should have written "up to 8°".

I've been pondering this since the first time you said it, and the logic is still not clear to me. Vignetting toward the edge is not perceptually the same as dimming of the center field, especially if unaided vision exhibits vignetting itself -- is this why vignetting seems unnoticable in bins, as it just replicates the natural vignetting that would occur further out in the full visual field?

I think subjective claims to be able to differentiate between axial transmissions of 3% or 4% are not very plausible when edge losses of 30%, or more as Canip's photos would suggest, go unnoticed.

Has this changed recently? It seems to me that compactness has ruled roof-prism design ever since the 1950s. Can one really see a secular trend in vignetting from older models to newer?

Yes. The manufacturers are bowing to demands, probably to a large extent from this forum, for 32 mm versions, higher magnifications, lighter weight, wide AFoVs, shorter close focus etc., etc. Most of these involve additional complexity and/or optical compromises. But that's another topic.

John

 

[Ignatius]

...

Vignetting can be observed by holding a binocular at arm's length and observing the exit pupils as it is turned off axis. Swarovski allegedly place some emphasis on this (Randpupille) and on three of mine the exit pupils are still gibbous-moon shaped before they occlude. It's not a bad binocular, but on my Kowa Genesis 8x33 they become almond-shaped. An 8x42 NL I looked at was mediocre, but that can be attributed to the extraordinately large AFoV.

...

John

Pardon my ignorance, but why would you hold the binoculars at arms' length and skew them to observe vignetting, when most people hold them to their eyes to observe the world?
Is vignetting then not a problem that affects normal viewing? And if it does not affect normal viewing, why is it a problem?
I only know vignetting from photography, where wide open apertures may produce a darkish ring in the image. Also happens with wide-angle lenses and filters.

 

[Tringa45]

Pardon my ignorance, but why would you hold the binoculars at arms' length and skew them to observe vignetting, when most people hold them to their eyes to observe the world?

It's only a test to enable comparisons and was suggested by Holger Merlitz in his book.

Is vignetting then not a problem that affects normal viewing? And if it does not affect normal viewing, why is it a problem?

On "Cloudy Nights" Ed Zarenski referred to typical edge illumination on many binoculars being as little as 40%. Canip's photos would seem to support that.
There is a potential problem with overall brightness and ease of view, but it would require a direct comparison within a specific format to confirm this.
The binocular with the lowest vignetting Zarenski tested, btw, was the 22x60 Takahashi Astronomer. Its outstanding qualities may have had additional reasons.

 

[Binastro]

The 22x60 Takahashi had a long focal length and fluorite crystal elements.

It was heavy and hardly a typical binocular.

As to wide angle photo lenses some had radially darkened filters to even up illumination.

Some even had special enlarging lenses to counter the distortion.

I also have a large red filter with radially different densities for aerial lenses.
It is actually crudely made with steps rather than an even grading.

Regards,
B.

 

[Patudo]

Vignetting toward the edge is not perceptually the same as dimming of the center field, especially if unaided vision exhibits vignetting itself -- is this why vignetting seems unnoticable in bins, as it just replicates the natural vignetting that would occur further out in the full visual field?

Interesting - could you provide any reference links that go into the vignetting of unaided vision in more detail, please?

For what it's worth, when using the one binocular I have that has oversized prisms (CZJ Nobilem Spezial) I cannot really discern that the image is brighter towards the edge. It may well be - I just can't perceive it.

Funnily enough, I do think I can perceive a difference in brightness between single and multi-coated binoculars, and I'm convinced that dielectric prism coatings make Schmit-Pechan binoculars slightly, but visibly, brighter than silver-coated (maybe due to the truer colour rendition).

The manufacturers are bowing to demands, probably to a large extent from this forum, for 32 mm versions,

Funny thing is - you could argue that some binoculars have gotten bigger. See the SF and NL 8x32s compared to things like the 8x32 FL; the entire SF series are bigger (though well balanced) than the equivalent FLs. It's a trend that has been going on for some time, too; Leica's 10x42 Trinovid "brick" was a lot bulkier and heavier than the predecessor 10x40 (there is a good photo on Ken Rockwell's site demonstrating this).

Wide field ditto. It was only with the NL that FOV of mainstream birding binoculars exceeded the old Zeiss standard of 150m for 8x30 and 130m for 10x50.

 

[tenex]

Yes.

Let me try again: if even significant vignetting is so hard to notice, how are you detecting a secular worsening in the last two decades? I was hoping you could specify what models you've tested and how, etc. Perhaps by using Holger's method described here?

Interesting - could you provide any reference links that go into the vignetting of unaided vision in more detail, please?

No, I was asking to what extent unaided vision vignettes, since that could explain why the effect is hard to notice in bins, and hoping someone like Ed might know or provide references.

 

[Binastro]

The eye covers a much wider angle than most binocular's AFOV.

The retina is curved and varies in sensitivity at different positions.
It is the brain that enables us to detect straight lines and horizontal to high degrees of accuracy.

There is also the blind spot, usually unnoticed.

There are also often local defects that go unnoticed.

The actual central vision is only about 1.5 degrees across and is used for detail.

Far off axis there is indeed a loss of sensitivity but normally very good detection of movement or a flash of not very bright light.

So a full treatise on human vision is a daunting task, and probably not understood even now.

Land of Polaroid did a lot of research on vision.

It is clear that moving away sideways from the computer screen, the screen is less bright, but I think this is to do with less sensitivity rather than vignetting.

Regards,
B.

 

[agus_m]

Interesting discussion. Are we talking about a vignetting of the edges when looking at the center of the FOV, that we cannot detect? Or when we roll our eyeballs and look at the edges, or both? I assume this vignetting would only happen in dim lighting, otherwise in bright light our pupils would be contracted and can easily fit in the deformed exit pupil.

 

[Tringa45]

Funny thing is - you could argue that some binoculars have gotten bigger. See the SF and NL 8x32s compared to things like the 8x32 FL; the entire SF series are bigger (though well balanced) than the equivalent FLs. It's a trend that has been going on for some time, too; Leica's 10x42 Trinovid "brick" was a lot bulkier and heavier than the predecessor 10x40 (there is a good photo on Ken Rockwell's site demonstrating this).

The introduction of 42 mm ELs, SFs and NLs all led to demands for 32 mm versions. So far Leica have deferred on a 32 mm Noctivid.

Wide field ditto. It was only with the NL that FOV of mainstream birding binoculars exceeded the old Zeiss standard of 150m for 8x30 and 130m for 10x50.

Those values were only achieved before the introduction of long eye relief eyepieces for glasses wearers, for me a welcome development!

Let me try again: if even significant vignetting is so hard to notice, how are you detecting a secular worsening in the last two decades? I was hoping you could specify what models you've tested and how, etc. Perhaps by using Holger's method described here?

I was not referring to vignetting specificaly but to "improvements" in specifications as a result of (internet) customer demand. Lighter weight as a result of reductions in prism size could, of course, lead to increased vignetting.
I have few opportunities for direct comparisons; Chuck and Canip are possibilities if they are interested,
I would suggest the Swarovski 7x42 as a benchmark (which they both own). My 10x42 EL SV and 8x56 SLC were also good. Your 10x56 SLC could be marginally better.

John