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ZEISS. Discover the fascinating world of birds, and win a birding trip to Columbia

Myopia and close focusing distance (1 Viewer)

Arthur Red Rod

Well-known member
Hello everyone,

Today I was feeling bored in front of the television and decided to use my 7x50 Fuji's to look at Tolkien's orcs in greater detail. To my surprise, I was able to achieve razor-sharp focus and even had some remaining adjustment range in the individual diopter knobs at a mere ~3m away from the television. I proceeded to test the focus range for several minutes and realized that I could read the small lettering on an Amazon box label (approx size 10 font) less than 2m away.

The min. focus distance of this model is 5m per current Fuji specs, so I know this likely isn't an issue of manufacturing tolerances. With my limited understanding of optical physics, I concluded that it must have something to do with my myopia. As of my last exam in 2018, I have a -3.75 diopter in my left eye and a -4.00 diopter in my right eye.

I believe this is severe enough of a defect to give me a permanent "boost" in close focus distance compared to people with good vision. Of course, the trade-off is I cannot use binoculars with a narrow dioptre range without glasses, as the "negative" correction will often top out long before I get a good view. Have other members with corrected vision noticed something similar with their eyes? Perhaps I should look into insect study. 3:)
 

mpeace

Well-known member
Hi, I've been very interested in the effect of myopia and also prescription glasses on the view through binoculars. Certainly myopia results in achieving a closer focus. The effect is noticeable, but going from 5m to 2m is a bit extreme and I think a lot of that may be down to the manufacturer under-stating the close focus that often seems to happen.

With my Swarovski 8.5x42 ELs with my -2.75 eyes close focus shifts from the stated 1.5m to about 1.35m, which is still noticeable and useful. Other effects are a slightly increased apparent field of view and magnification. I've increased this effect on my binoculars by fitting the lenses of reading glasses to the eye cups and the focus tolerance still allows focus at infinity. If interested see https://www.birdforum.net/showthread.php?t=392597
 

Arthur Red Rod

Well-known member
Hi, I've been very interested in the effect of myopia and also prescription glasses on the view through binoculars. Certainly myopia results in achieving a closer focus. The effect is noticeable, but going from 5m to 2m is a bit extreme and I think a lot of that may be down to the manufacturer under-stating the close focus that often seems to happen.

With my Swarovski 8.5x42 ELs with my -2.75 eyes close focus shifts from the stated 1.5m to about 1.35m, which is still noticeable and useful. Other effects are a slightly increased apparent field of view and magnification. I've increased this effect on my binoculars by fitting the lenses of reading glasses to the eye cups and the focus tolerance still allows focus at infinity. If interested see https://www.birdforum.net/showthread.php?t=392597

That is a brilliant set-up and worth making into a mass-produced device. I should note that all of my close observations have been done without glasses. With glasses, I get focusing distances close to manufacturer spec, presumably because my eyes are temporarily "fixed" to the correct diopter of 0.

-2.75 is a pretty significant difference from -3.75 and -4.0 so perhaps that is why you are not seeing as amplified an effect? I have a friend with a -5.0 going in for Lasik in a few weeks, so I must remember to test with his eyes before the opportunity is lost!
 

PeterPS

MEMBER
Hi, I've been very interested in the effect of myopia and also prescription glasses on the view through binoculars. Certainly myopia results in achieving a closer focus. The effect is noticeable, but going from 5m to 2m is a bit extreme and I think a lot of that may be down to the manufacturer under-stating the close focus that often seems to happen.

With my Swarovski 8.5x42 ELs with my -2.75 eyes close focus shifts from the stated 1.5m to about 1.35m, which is still noticeable and useful. Other effects are a slightly increased apparent field of view and magnification. I've increased this effect on my binoculars by fitting the lenses of reading glasses to the eye cups and the focus tolerance still allows focus at infinity. If interested see https://www.birdforum.net/showthread.php?t=392597
I agree that an effect of myopia is to decrease the close focus. Other effects are a decreased fov and magnification (above you appear to say the opposite). I have noticed the decreased apparent magnification, but I am not sure about the decreased fov---any intuitive explanation for that?
Also let's say that your myopia is -1d in OS and -5d in OD---which eye will determine the intensity of the above effects?
 

mpeace

Well-known member
I agree that an effect of myopia is to decrease the close focus. Other effects are a decreased fov and magnification (above you appear to say the opposite). I have noticed the decreased apparent magnification, but I am not sure about the decreased fov---any intuitive explanation for that?
Also let's say that your myopia is -1d in OS and -5d in OD---which eye will determine the intensity of the above effects?

I think I'm getting my +/- confused and thinking about my prescription rather than my eyes. So my prescription is -2.75, so I guess my eyes are +2.75. Anyway I'm short sighted (myopia) and that has the effect of decreasing the close focus. The whole fov with my glasses on is visible, the same as with my glasses off. The difference though is that I see a larger apparent field of view with my glasses off than with them on. The larger AFOV also means a slight increase in magnification. I did some measurements of the effect here: https://www.birdforum.net/showthread.php?t=385385
 
ZEISS. Discover the fascinating world of birds, and win a birding trip to Columbia
ZEISS. Discover the fascinating world of birds, and win a birding trip to Columbia
ZEISS. Discover the fascinating world of birds, and win a birding trip to Columbia
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