“I got an 8×40 mm binoculars a few months ago and I was totally impressed with its clarity and brightness. The Field of View was also cool. The only problem I had with this binocular was the blackout problems. They were too conspicuous, especially during the day. It was difficult 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 many guys out there were lost in this same problem. I have created this post to help anyone who needs help on how to focus binoculars correctly to solve this problem.
With that said, the “blackout” problem that my sister was seeing is called the kidney-bean effect. You can also call it the flying shadow effect.
What is the Kidney Bean Effect?
The kidney bean effect is also referred to as a flying shadow effect while looking through binoculars. It is a mishap associated 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. Due to this, it is not possible, in most cases, to perfectly focus the entire field of view at the same time. So, you have to release the bin and hold it slightly further from your 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 the center and the zone’s periphery. This happens when your eyes are not aligned perfectly with the eyepiece optical axis.
When this happens, what you will get are “flying shadows” that look more like the kidney bean. This effect is often produced during the day. It is also very visible if you are watching a bright moon at night with stargazing binoculars.
What Causes Kidney Bean Effect?
The basic cause of the kidney bean effect in binoculars is when the Eye Relief and IPD of the binocular do not match with your eyes while seeing through binoculars.
A practical explanation is this when you look at the figure above, it shows how light converges behind the lens to produce an image.
The main light ray that reflects differently behind the lens is the principal ray. It is the main ray that comes across the field. When the field is illuminated fully (when your binocular is exposed to bright light) and your eye’s pupil is larger than the tiniest cross-section of the rays (in the above image 3.5mm.), you will get a full image with no distortions.
However, if the pupil is smaller than this tiniest cross-section of the rays (3.5mm) you will get the kidney-bean effect. This happens because the rays that are on the outer part of the main field of view are blocked from coming into the eye. In short, the image is not 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 know how to use binoculars.
Usually, you will need to hold the binos slightly away from your eye to focus one zone better than the other.
You must also align your eyes perfectly with your optical axis. This will prevent your iris from cutting off some light from one zone between the center of the FoV and the periphery.
Since the effect is pronounced during the day and when watching bright moon, you can opt to use your binoculars in a dull area or when the moon isn’t so bright.
The eye relief shouldn’t also too long or even too short. Instead, it should be an exact match with the eye to enable you to perfectly position your eye behind your eyepieces.
NOTE: Kidney bean effect is majorly related to improper eye relief. It is also triggered when your eyes are not centered perfectly on the eyepieces. So regardless of the type of bin you buy, it is important to adjust your binoculars properly until you get the perfect eye relief and the right interpupillary distance that suits your needs.
How to Adjust your Binoculars Settings for a Clear All-Round View
There are two things you can do to clear the blackout spots while viewing through binoculars.
Adjust the Bino’s Eye Cups
The exit pupil will stay in the center 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 binos will have a pair of eyecups. The eyecups help 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 correct distance from your eyepiece lenses and within the binocular’s eye relief.
All eyecups are adjustable. They will comfortably accommodate a variety of people and varying shapes of face.
They even work for every type of eye socket depths and for guys who wear eyeglasses or not.
Using binoculars with glasses: If you wear glasses, you may not need to extend the eye-cups. In that case, just keep them in place. If you remove the eyeglasses, you can move out the eyecups to adjust as needed.
Adjust Your Bino’s Hinge to Set Your Interpupillary Distance (IPD)
When you move the barrels inward and outward around the central hinge your eyepieces will also re-align with the binoculars barrels. By doing so, they will give you the right interpupillary distance that comfortably aligns your eyes to the eyepieces. In turn, you will get the best coverage of the exit pupil and no loss of light.
NOTE: if you do these two things correctly, you shouldn’t have any problems with binoculars blackouts.
What You Must Know about the Kidney Bean Effect
From what you have seen, the kidney-bean effect is commonly associated with a certain fraction of wide-field eyepieces that have large eye relief.
It results from the exit pupil’s spherical aberration. Since the exit pupil is normally curved it is incapable of keeping the entire field of view under focus.
With an eyepiece that’s curved outwards, it is ideal to get your eye closer 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 eye away from the blackened shadow.
If the exit pupil of your optics is larger than your eye pupil, some light part of the light cone will be cut out. This blocks the light from coming through fully and thus leaves you with dimmer or blackout around your image.