Why do I see the moon as two objects through my DSLR viewfinder if I use eyeglasses?

Question

I use eyeglasses with -1 diopter correction. Few days ago, I photographed the moon during night with my Canon 2000D DSLR. When looking at the moon through the viewfinder, I saw the moon as two distinct bright objects. I thought that there might be something wrong with the diopter adjustment of my 2000D DSLR. The image that I took was just fine, and not out of focus. The viewfinder was practically useless due to the severe dual vision effect. Unfortunately, I didn't test the viewfinder without eyeglasses that day, and today it's rainy and cloudy so I can't repeat the test today.

Today, I tried to replicate the effect indoors with eyeglasses by using the same focal length (250 mm) and focusing at over 5 meter distance. I couldn't replicate the effect. I didn't touch at the diopter adjustment of the DSLR at all, so the same settings were used for astrophotography and this test.

I haven't yet tested looking at the moon through the viewfinder without eyeglasses during the night, so I can't say if the effect appears without eyeglasses. I suspect it might be related to eyeglasses.

My questions:

• Why do I see the moon as two objects through the DSLR viewfinder?
• Is seeing the moon as two objects related to me wearing eyeglasses?
• Can the effect be caused by incorrect diopter adjustment setting on the DSLR?

Answer 1

You may have astigmatism that is insufficiently corrected by your glasses. Astigmatic eyes have multiple focus points that create double images. The high contrast of the moon against a dark sky makes the fainter "ghost" image more visible. If this is the case, you may be able to replicate the effect with a high contrast image, such as black text on a white page under bright light.

Vision also tends to worsen with age (there are some exceptions that temporarily improve). Prescriptions for astigmatism have two diopters (sphere, cylinder) plus an angle (axis). Some contact lenses for astigmatism are weighted to prevent rotation from affecting the axis. (Some contacts use a different correction method that allows them to ignore the axis.) Also, if you tilt your head during the eye exam, the axis will be off and the prescription won't be quite right. Whether you need new glasses depends on your tolerance for slightly imperfect vision.

If you use contacts with the "same" prescription as your glasses, and the problem occurs only with your glasses, the problem is likely something associated with your glasses, such as reflections, as Michael C suggests.

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Some medical conditions and substances (alcohol, THC, antihistamines, medications, etc) can create similar effects by causing the eye ciliary muscles to pull unevenly on the lens. If substance induced, the effect is likely to be temporary.

If you have other medical conditions that can affect vision (including diabetes and hypertension) you may need an eye exam regardless of the need for new glasses. If your vision rapidly worsens, with or without pain, it could be a medical emergency (such as glaucoma). Use your best judgment. Consult a physician.

Answer 2

When you are in a dark setting your pupils dilate to allow more light into your eyes. The moon is much brighter than your surroundings. When an image of the moon is being projected out the rear of your viewfinder, it's almost like a high intensity spotlight.

My guess is that with your eyes so acclimated to your dark surroundings, the light coming out of your viewfinder, reflecting off the front of your eyeglass lens, and then bouncing back from the viewfinder is bright enough that it might be what you are seeing. Does the second image move relative to the first if you change the angle of your glasses relative to the viewfinder?