A Lesson in Low Powers

I had a chance to do an informal experiment today. It was time for my annual eye exam, and my pupil was still dilated to about 8mm when I came home. It was a rather cloudy day, but I still needed my sunglasses. It was also quite obvious that my visual acuity suffered greatly with the large pupil opening.

I have a pair of 7x42 binoculars that "snap" nicely into focus, and it is obvious when they are correctly focused. During the day, of course, the pupil of my eye stops their 6mm exit pupil down to probably 3mm or less. Out of curiosity, I tried them with my dilated pupil.

No longer did they seem to "snap" into focus. It seemed like I would approach focus, but not quite get there, and then a little more motion of the focus knob in the same direction would make it worse. I also tried my 12x36 binoculars. Their 3mm exit pupil provided a more obviously focussed view. They did not seem quite as sharp as in normal use, and I suspect I did not have their exit pupil quite centered in my eys's pupil.

At any rate, it was a nice demonstation that with large exit pupils and a dark adapted eye you are limited by the eye, and not the aberrations of the eyepiece/telescope.

Clear skies, Alan

Hi Alan,

We just had a similar discussion in the Binocular group. I tired to explain that a 15x50 bino would seem sharp on stars simply because the exit pupil would be small compared with normal binoculars. I don't think anyone really believed me .

Roland Christen

Hi again Alan,

My 20X65mm binos are my favorite binos for the same reason. I have just enough astigmatism to prevent good views through larger exit pupils.

I do the same exam annually. Impossible to get nice resolution until the dilation wears off.

Thanks,
John

Hi Ed,

Our eyes must be nearly the same. I don't have trouble with obstructed scopes with medium to small exit pupils. I can't help there...

A few years ago I asked an optometrist to order slightly overcorrected glasses with good astigmatism correction as well. I asked for actual glass instead of the normal polymer material. No bifocals, only infinity.

I normally wear contacts but vision can vary with them for a number of reasons.

Wow, what a difference with an eyepiece or just lookin' up!! I haven't seen this well from birth. 20/15 and no astigmatism. Everything is crisp!

Thanks,
John