“I acquired a 12×50 mm binoculars a few months ago and I was totally impressed using its clarity and brightness. The Field of View was also cool. The only problem I had with this binocular was the blackout problems. These were too conspicuous, especially during the day. It had been difficult to start to see the whole image especially without taking time to position your eye correctly.”
This is what my sister came complaining about last weekend. While I helped her out, it occurred to me that lots of guys out there were lost in this same problem. I’ve created this post to greatly help anyone who can’t reach me.
Having said that, the “blackout” problem that my sister was seeing is called the kidney-bean effect. You can even call it the flying shadow effect.
What is the Kidney Bean Effect?
The kidney bean effect can be referred to as a flying shadow effect. This is a mishap connected with wide-field eyepieces. However, it majorly results from the spherical abnormality of the bins exit pupil design.
The binocular’s exit pupil is curved. For this reason, it isn’t possible, in most cases, to perfectly focus the complete field of view as well. So, you should release the bin and hold it slightly further from your own eye. This will enable to easily focus on one zone better.
Basically, there is the position that your iris cuts of light from a zone, usually between your center and the zone’s periphery. This happens when your eyes are not aligned correctly with the eyepiece optical axis.
When this happens, what you would get are “flying shadows” that look similar to the kidney bean. This effect is often pronounced throughout the day. Additionally it is very visible if you are watching a bright moon at night.
What Causes Kidney Bean Effect?
The basic reason behind the kidney bean effect in binoculars is when the Eye Relief and IPD of the binocular usually do not match together with your eyes.
A practical explanation is this when you consider the figure above, it shows how light converges behind the lens to create an image.
The primary light ray that reflects differently behind the lens is the principal ray. It really is the key ray that results in the field. When the field is illuminated fully (whenever your binocular is subjected to bright light) and your eye’s pupil is bigger than the littlest cross-section of the rays (in the above image 3.5mm.), you will get a full image without distortions.
However, if the pupil is smaller than this tiniest cross-section of the rays (3.5mm) you’ll get the kidney-bean effect. This happens since the rays that are on the outer part of the key field of view are blocked from coming into the eye. In a nutshell, the image isn’t fully reflected.
Addressing Kidney Bean Effect in Your Binoculars
To manage the kidney-bean effect and focus on the entire field of view, you must contain the bin slightly away from your eye to focus one zone much better than the other.
You must also align your eyes correctly with your optical axis. This will prevent your iris from cutting off some light in one zone between the center of the FoV and the periphery.
Because the effect is pronounced throughout the day so when watching bright moon, you can choose to use your binoculars in a dull area or when the moon isn’t so bright.
The attention relief shouldn’t also too much time as well as too short. Instead, it must be an actual match with the attention to enable you to correctly position your eye behind your eyepieces.
NOTE: Kidney bean effect is majorly linked to improper eye relief. It is also triggered when your eyes aren’t centered correctly on the eyepieces. So no matter the sort of bin you buy, it is necessary to adapt your binoculars properly until you get the perfect eye relief and the proper interpupillary distance that suits your preferences.
How exactly to Adjust your Binoculars Settings for a Clear All-Round View
There are two things that you can do to your binocular and wrap up with the proper adjustment that clears the blackout spots.
Adjust the Bino’s Eye Cups
The exit pupil will remain in the heart of the field when you observe an image under your binoculars. However, this is only possible if you position your eyes properly behind the eyepieces.
Luckily for you, most eyepieces could have an eyecup. The eyecup allows you to properly position your eye in the light’s axis.
You can move them in or out and properly align your eyes to the right distance from your eyepiece lenses and within the binocular’s eye relief.
All eyecups are adjustable. They’ll comfortably accommodate an assortment of people and varying shapes of face. They even work for every kind of eye socket depths and for guys who wear eyeglasses or not.
Adjust Your Bino’s Hinge
When you move the guts hinge of your binoculars inwards or outwards your eyepieces may also re-align. In so doing, they will provide you with the right interpupillary distance that comfortably aligns your eyes to the eyepieces. Subsequently, you will get the very best coverage of the exit pupil no loss of light.
NOTE: in the event that you do both of these things correctly, you shouldn’t have any issues with binoculars blackouts.
What You GOT TO KNOW about the Kidney Bean Effect
From what you have observed, the kidney-bean effect is often associated with a certain fraction of wide-field eyepieces which may have large eye relief.
It results from the exit pupil’s spherical aberration. Because the exit pupil is generally curved it is not capable of keeping the complete field of view under focus.
With an eyepiece that’s curved outwards, it is ideal to really get your eye nearer to the binocular’s eye lens so that you can center your eye’s pupil towards the shadow. However, with an eyepiece whose edges are curved inwards, you should center the attention from the blackened shadow.
If the exit pupil of your optics is larger than your eye pupil, some light area of the light cone will be cut out. This blocks the light from coming through fully and so leaves you with dimmer or blackout around your image.