Vignetting in Binoculars (1 Viewer)

[LUCCHINI1968]

Questo è un argomento raramente discusso su Birdforum, ma che ha un'influenza sulle prestazioni e forse sull'apprezzamento soggettivo del binocolo.
La vignettatura è una perdita di luminosità verso il bordo del campo ed è presente in tutti i binocoli. È causata da ostruzioni, solitamente deflettori, nel percorso ottico.
La superficie frontale di un prisma avrebbe solitamente un deflettore circolare e il suo diametro sarebbe, ovviamente, grande quanto consentito dal prisma.
Dovrebbe anche essere sufficientemente grande da accogliere il cono di luce di una sorgente puntiforme focalizzata sull'asse ottico. Se così non fosse, il diametro dell'obiettivo sarebbe sostanzialmente ridotto. Questo a volte è il caso dei binocoli economici, ma mi vengono in mente altri due esempi: i primi Canon 10x42 IS erano effettivamente 10x38 e un Optolyth Porro da 50 mm era effettivamente solo circa 40 mm.

Tuttavia, i binocoli hanno tipicamente un campo visivo reale di circa 8°, quindi il cono di luce di un oggetto messo a fuoco a 4° dall'asse ottico colpirebbe il bordo del deflettore e ridurrebbe la luminosità che vediamo attraverso l'oculare. Holger Merlitz afferma che una riduzione del 30% (!!) ai margini del campo non sarebbe necessariamente percepita come disturbante. Questo per quanto riguarda i confronti soggettivi di piccole differenze di trasmissione! Ciò costringe tuttavia i produttori a raggiungere una sorta di compromesso.
I prismi costituiscono una parte considerevole del peso di un binocolo e una riduzione del 10% della dimensione lineare comporterebbe una riduzione del volume e del peso del 27%. Se gran parte dei potenziali clienti chiede una riduzione del peso, chi è la colpa dei produttori se accettano compromessi ottici? Forse questo è solo un altro esempio di influenza negativa di Internet. Vent'anni fa i produttori facevano ciò che ritenevano giusto.

La vignettatura può essere osservata tenendo un binocolo a distanza di un braccio e osservando le pupille di uscita mentre è ruotato fuori asse. Swarovski presumibilmente pone una certa enfasi su questo (Randpupille) e su tre dei miei le pupille di uscita sono ancora a forma di luna gibbosa prima di occludersi. Non è un brutto binocolo, ma sul mio Kowa Genesis 8x33 diventano a mandorla. Un 8x42 NL che ho visto era mediocre, ma ciò può essere attribuito all'AFoV straordinariamente grande.

Forse altri, in particolare Canip con la sua vasta collezione, potrebbero dare qualche feedback in merito. Immagino che la Nikon WX da 2,5 kg se la caverebbe molto bene e la Zeiss SFL meno.

John

Nei vostri confronti date sempre per scontato che i binocoli che provate siano ben collimati, vi assicuro che non è così. Soprattutto nelle Marche secondarie non c'è uniformità di qualità e allineamento nei parametri. Tolleranze accettabili, 0,30/0,40 verticale, 0,60/0,80 convergenza orizzontale, 0,40/0,60 divergenza orizzontale.

 

[Jos Stratford]

Nei vostri confronti date sempre per scontato che i binocoli che provate siano ben collimati, vi assicuro che non è così. Soprattutto nelle Marche secondarie non c'è uniformità di qualità e allineamento nei parametri. Tolleranze accettabili, 0,30/0,40 verticale, 0,60/0,80 convergenza orizzontale, 0,40/0,60 divergenza orizzontale.

Hi,
As an international forum, please post in English - Google translate etc may help you if needed.
Thanks

 

[LUCCHINI1968]

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

In your comparisons, always assume that the binoculars you try are well collimated, I assure you that this is not the case. Especially in the secondary Marche there is no uniformity of quality and alignment in the parameters. Acceptable tolerances, 0.30/0.40 vertical, 0.60/0.80 horizontal toe-in, 0.40/0.60 horizontal toe-out.

 

[Swedpat]

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

Let me be questioning: is it even relevant to talk about vignetting when you look from aside?
When you use the optics you look from ahead and then these vignettings are not visible.

 

[Maljunulo]

In your comparisons, always assume that the binoculars you try are well collimated, I assure you that this is not the case. Especially in the secondary Marche there is no uniformity of quality and alignment in the parameters. Acceptable tolerances, 0.30/0.40 vertical, 0.60/0.80 horizontal toe-in, 0.40/0.60 horizontal toe-out.

0.30-0.40 what?

What are the units?

Did I miss it?

 

[Ignatius]

0.30-0.40 what?

What are the units?

Did I miss it?

Looking at his yellow attachment, he probably means degrees.

 

[Tringa45]

Let me be questioning: is it even relevant to talk about vignetting when you look from aside?
When you use the optics you look from ahead and then these vignettings are not visible.

Vignetting is an obstruction leading to a loss of illumination towards the field edge (hopefully not on axis).
By tilting the binocular and observing the reduced exit pupil you can evaluate it.

John

 

[normjackson]

Not sure what explanation Holger has given but reckon I'd need a Ladybird book one (dunce emoji here).

Guess the suggestion is that with the binocular to your face, as you swivel your eyes to the outer parts of the field of view you will be presented with a smaller exit pupil (in accordance with Canip's photos) than the round one you enjoyed looking straight ahead. Depending on your pupil size at the time, that might mean the whole views dims a bit (unless you're using an Olympus7x35 DPS I). Is that right and do folks see it?

 

[Swedpat]

Vignetting is an obstruction leading to a loss of illumination towards the field edge (hopefully not on axis).
By tilting the binocular and observing the reduced exit pupil you can evaluate it.

John

Ok! Is this possible to avoid at all? Actually I have never thought about it. Sharpness loss towards the edge is usual, and even distorsion but I don't think I ever reacted about brightness loss. Only when eye relief is too short and I don't come close enough, but that is another thing.

 

[normjackson]

Looking on the dull side, looks like maybe most of those vignetted exit pupils contain a higher proportion of the (higher quality) centre of the entrance pupil? Guess the Nikon WX 7x50 would show less vignetting because of its lower AFOV. Small AFOV of the Takahashi also a factor.

 

[Tringa45]

Not sure what explanation Holger has given but reckon I'd need a Ladybird book one (dunce emoji here).

Holger wrote that tests had shown that a darkening at the field edge of around 30% would not be perceived as disturbing.
Canip's photos indicate that in some cases it is even more than that and confirm my suspicion that Zeiss have scrimped on the prisms of the SFL to achieve such low weight.
I mentioned earlier that about the best I have seen is the 7x42 Swarovski SLC but I think attempts to market a binocular in that format with a weight of 950 g would today be doomed to failure.

Guess the Nikon WX 7x50 would show less vignetting because of its lower AFOV. Small AFOV of the Takahashi also a factor.

I think true FoV is the determining factor. For the same AFoV binoculars with a higher magnification have shorter focal length eyepieces with a correspondingly smaller field stop and thus make use of a smaller part of the objective's image.
The 22x60 Takahashi Astronomer only had an AFoV of around 45° IIRC so would have had a very narrow TFoV.

It occurs to me that binoculars with Abbe-König prisms are at a disadvantage with regards to vignetting. Their length would place the prism stop closer to the objective, where the diameter of the light cone is larger. Perhaps Porro II is the ideal configuration!

John