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Blackouts and objective lens size - 30/32mm vs. 40/42mm (1 Viewer)


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United States
Hi - this may be a somewhat elementary question but I wasn't able to find a consistent answer on other sites and other threads.

Thanks in advance.

Is there a correlation between blackouts or an 'unforgiving' eye box and the size of the objective lens? While this may be different for everyone since all faces and hand holding positions are different, assume that the person is constant and they are looking through various models in both the 32mm and 42mm, would you bet that they have more blackouts on the smaller or larger objective?

I seem to have issues with the smaller objective bins. I initially believed it was from the small ocular lens and small/thin eye cup which barely allowed the length of my eye and eyelashes to fit within the rubber eye cup.

Any insight would be appreciated as I don't want to take unnecessary risk when buying another pair of bins online that I have not tried...

Thank you
Blackouts are usually from not getting your eye where the binocular exit pupil is. For binoculars with a long eye, if you don’t wear glasses you may need to have your eyes someway back from the binocular to see the full field with blackouts if you’re too close or far away. This is what many modern binos have adjustable length eyecups so everyone can use then, unlike the older binos with either very short eye relief or fixed rubber eyecups. Others can comment on possible
Correlations with specific specifications.

I find that eyecups that don’t come out very far cause blackouts for me. Yesterday I did a modification with o rings to lengthen the eyecups on my Maven B3s just 2mm. It made all the difference in the world. You need a bino with eyecups that screw out to do this though. That feature makes it easy to give the lens a good cleaning. I’m amazed how many expensive binos don’t have this feature. My Mavens retail for $550 so it can’t be that expensive to add these.
Nothing to do with objective size per se. Exit pupils need to be smaller than even the 3.2mm of a 10x32 (which I've used for 20+ years) to be likely to induce blackouts from misalignment. "Eye box" issues that make positioning more finicky in some models are due to details of eyepiece design, not objectives. For users without eyeglasses, eyecups of insufficient depth are the usual cause of kidney-bean blackouts (right image below). This has become more common in recent years due to failure to compensate for eyepieces with higher eye relief.


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Yes, because for a given magnification, a larger objective will have a larger exit pupil.
I've noticed the same thing - the binos with smaller exit pupil tend to be the ones with more difficult eye placement. This is why my 10x binoculars are 10x56, I love the easy eye placement. Special mention to the Nikon 10x35 E2 which seems to defy the rule, it has comfortable eye placement even w/ the small e.p.

If the eyecups aren't high enough that makes everything worse.
Exit pupil size does influence ease of eye placement (the larger the exit pupil the more forgiving). Blackouts, however, are/should be an eye relief issue - if your eye is too close, blackouts happen. Binoculars with longer eye relief, designed for glasses/spectacles wearers, can be prone to blackouts for those fortunate enough not to need glasses; in some cases (eg Zeiss Conquest) longer eyecups can be obtained to help resolve this.

If blackouts are the only problem the OP is experiencing, binoculars with shorter eye relief may be the solution. Birdforum member Canip has an excellent site where an extensive range of binoculars have been measured for eye relief using the same criteria - it should be a useful reference aid. If the binocular seems somewhat fiddly or finicky with regard to eye placement, a larger exit pupil should make for greater ease of use.
It seems to me, the more complex designs that try to achieve large FoVs with long eye relief are to blame in some cases. All my old wide angle porros in 8x30 with very short eye relief, don't suffer all that much from blackouts, while more modern designs do. That's one reason why I am such a fan of these old porros. Very comfy view, if you don't need glasses to observe.

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