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Large Exit Pupils and an 8x56 SLC (1 Viewer)

Tringa45

Well-known member
Europe
Most would agree that large exit pupils contribute to ease of view but there seem to be two opposing schools of thought; many assert that they make eye placement less critical and others, that one doesn't need exit pupils larger than one's own dilated pupils.

I think that both arguments are somewhat flawed. It's generally advisable to keep one's pupils centred in the exit pupil as one would otherwise experience lateral chromatic aberration. However, small exit pupils can vignette the eye's pupil.
The eye rotates about its centre but the pupil is at a radius of 11-12 mm, so even a diversion of 10° is going to result in a lateral movement of the pupil of about 2 mm. Binoculars for military use have been made in formats such as 8x60 and 10x80 and I don't think think they were intended for use by 10 year-olds with 8 mm pupils!

When comet Neowise was visible last month I went out on successive nights and even naively took a 30x65 birding scope, in which it appeared as a small diffuse blob without tail. The best view of all was with my old (2003 vintage) 7x42 SLC. It would surprise me if my old eyes could dilate to 6 mm, but after becoming adapted to the dark (new moon and little direct artificial light) it was amazing what could be seen under clear suburban skies.

At Birdfairs and at Photokina the 8x56 SLC (and its non-HD predecessor) had often impressed with a very immersive view, a poor man's WX so to say. Mine (early 2020 production date) arrived last week. It's a massive beast at 1225 g and probably will not accompany me on long walks, particularly if I take a scope.
Accessories are a Field-Pro type case, rainguard, objective covers and carrying strap. I prefer the old semi-hard SV-type case which had a bit more room for a notebook, energy bar or cleaning kit and the rainguard is the hard plastic hinged abomination, so a one-piece Zeiss-style oval cover will replace it. The carrying strap (traditional lugs) can be rapidly adjusted in length, which is an advantage if one wants to carry the bin bandolier fashion.

Swarovski quote 24 optical elements, though I could only speculate on their configuration. We know that the big SLCs now have Abbe-König prisms, which no doubt help achieve the claimed 93% transmission, and although AK is not a pre-requisite for an offset of the optical axis, the objective spacing at my 63 mm IPD is 76 mm. IPD range is 57-79 mm.
The long focal length of the large objectives requires eyepieces of correspondingly long focal length, which provide massive eye relief of 23 mm. The eye lenses are 28 mm in diameter and the eyecups can be extended by 4, 12 and 16 mm.

I consistently measured an AFOV of 59,5° (60° spec.) but with corrected vision (glasses) only achieved a close focus of 4,6 m (3,9 m spec.). As I still have a small amount of accommodation, the true figure is probably a little worse than that.
The reason soon became apparent when I simulated short-sightedness with a ca. 15 cm focal length hand magnifier (6,5 dioptre) behind the ocular and was able to focus on the moon with some focus travel left. It would appear then that focus overtravel is at least 7 dioptres (5 dioptre spec.).
Dioptre adjustment left/right is +/- 4 and the focusser requires about a 3/4 turn from 5 m to infinity. It is pleasantly smooth with a damped feel but, as is typical for Swarovski with spring-loaded focussing lenses (eliminates dioptre shift), the torque differs clockwise/anticlockwise.
Strangely, it gets lighter towards infinity, just like my old 7x42 although the 8x56 has -ve. focussing lenses (move towards the objectives to infinity) and the 7x42 has +ve focussing lenses. My 10x42 SV also has -ve. focussing lenses and gets lighter towards near-focus!

Another typical Swarovski attribute is the so-called "Randpupille". If one holds the binocular at arm's length and rotates it, the exit pupils are still gibbous moon shaped before they occlude. On many binoculars with undersized prisms they become almond shaped.
False pupils are quite apparent, but so far outside the large exit pupils that they could not cause problems and I could detect neither flare nor ghosting.
Pincussion distortion is moderate, so rolling ball would not be a problem for those sensitive to it.
With the binocular mounted on a tripod and viewing a squared pattern, I saw some astigmatism at the field edges. This was moderate compared to a 10x42 FL I once owned but not anywhere near as well corrected as on my 10x42 SV. There was no benefit to be obtained by refocussing. Despite requiring a cylindrical correction of 0,75 dioptres on one eye I seemed to achieve better edge sharpness without my glasses and question whether glasses impair the view when the eyes move outside the optical axes.

The white paper test did not reveal any colour bias, not surprising with such high transmission, and it was here that the improved coatings and Abbe-König prisms showed a small improvement over the old SLC. CA is also better corrected.
Depth of field of an 8x bin is so much closer to 7x than it is to 10x. The reduced steadiness of the latter combined with reduced FOV and exit pupil size (for the same objective diameter) causes me to question the sense of 10x for hand-held use. 7x50 is the standard choice for marine use and the legendary Zeiss Jena 7x40 DF was ideal for its dubious purpose.
In retrospect, my uninformed choice of a 7x42 for my first good binocular 16 years ago was a very lucky one.
The 8x56 is much too heavy for general birding use and is a superfluous luxury for which I make no apologies.

John
 
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Omid

Well-known member
United States
Great post! Thank for sharing your thouthts John. Here are some notes and thoughts to add to your ascellent comments:


Most would agree that large exit pupils contribute to ease of view but there seem to be two opposing schools of thought; many assert that they make eye placement less critical and others, that one doesn't need exit pupils larger than one's own dilated pupils.

Large exit pupils when accompanied with a generous eye relief (18mm or more) have the key benefit of allowing for proper eye rotaion (bonocular convergance). The proper way of holding binoculars is to position your eyes' rotation center at the exit pupil. This is not possible as you noted if the binoculars have short eye relief: any eye rotation will lead to pupil shearing which then leads to light loss and abbarations.


The eye rotates about its centre but the pupil is at a radius of 11-12 mm, so even a diversion of 10° is going to result in a lateral movement of the pupil of about 2 mm. Binoculars for military use have been made in formats such as 8x60 and 10x80 and I don't think think they were intended for use by 10 year-olds with 8 mm pupils!

Exactly as you said. When I buy/use a riflescope, I make sure that it has at least 10mm exit pupil diameter. It has nothing to do with my eye pupil dialation; it is about comfortable eye placemant behind the eyepiece.

Perceived luminance does not grow proportionally to pupil dialation (i.e., it is not proportional to pupil area) due to a phenomenon known as Stiles–Crawford Effect. The "percieved luminance" decreases quickly as a function of distance from the center of the pupil. Your 7X42 provide a beautiful view due to other ergonomic and physiological factors one of which is the effect I explained above. Other reasons remain to be explored..

There is much more to binoculars/human-vision interaction than the classical theories of magnification, brightness and exit pupil. I have explored a few such topics in the thread called New Horizons II.

Finally, I agree with you that the new Swarovski 8X56 SLC provides outstanding views. This view is not duplicated in the 10X56 and 15X56 models.

-Omid
 
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