PING Phil Hobbs... Optics Question

On Wed, 26 Nov 2014 15:05:20 -0500, Phil Hobbs Gave us:

That is the typical real world result I see. Great focus between 5" and about 1.4 feet.

Does not one's current focal capabilities play in as well?

IOW, I used to be able to see tiny particulate between tiny ridges in my fingerprint surfaces. Now, I cannot close focus closer than that 15" mark bare eyed, and the glasses of any number are needed to correct that.

I know. Getting old stinks, but it's better than the alternative (or so we're told by all the materialist folks). ;)

Cheers

Phil Hobbs

How does someone with better eyesight read the inter-pupil distance...

I think it's 66 ??

(Taped a ruler to my forehead and used my webcam ;-) ...Jim Thompson

Looks roughly right, assuming you're converged on infinity. Zennis are cheap--get a few pairs with different IPDs, and see which ones are the most comfortable. I tried 68, 69, and 70, and standardized on 69 mm.

Cheers

Phil Hobbs

I look straight into a mirror, one eye at a time. I mark the mirror with a whiteboard pen or some such, one dot for each eye, then measure.

Mine's 68-69 mm, unusually wide. Subtract 2-3 mm for reading glasses.

Go to Costco -- they'll do it for you.

The other important measurement is the vertical position of the optical axis, which should be directly in front of your pupils when looking straight ahead. A good optician will always measure this for the particular frames you are ordering, but the standard Zenni order form does not spec this. You can however spec it in the notes section of the order if you already have a set of the frames to measure.

That's not nearly as big an issue, because you can adjust it by (a) sliding the glasses up and down your nose, and (b) tipping your head a bit.

Cheers

Phil Hobbs

Beware. My wife got these, and she is *not* happy - too much glare at night, and from glints off the chrome on automobiles.

What she specifically got is a lens with a diffraction grating engraved on each surface. The effect is that there are in fact three foci, and one of the foci is usually close to the retina. The brain is good at seeing the best-focus image and ignoring the rest. The problem is that those other foci are still there, and a bright point of light will cause lacy halos.

Yes, one can specify where the focus will be. Most people specify infinity, and need glasses to read.

Both eyes must be at about the same focus, or the brain will be unable to fuse the left and right images.

I'm nearsighted, which I actually find useful, and so when I had the left eye done, I had the doctor set the focus point to be the same as the right eye. This worked well.

And, I got a standard lens, without diffraction patterns, based on my wife's experience.

Joe Gwinn

Hi Joe,

Happy Holidays!

Hmmm... I'm not at all sure what my neighbor got. He was explaining *his* to me (no idea how the subject even came up) and said his were "experimental" (though no idea if that was 10 years ago or last week!). The take-away (for me) was that he could focus near *and* far whereas most cataract surgery leaves you with one or the other (in most cases, "far"... which seems kinda silly to me! OTOH, it may be that it is easier to get far "almost right" than it is to get *near* equally correct??)

My assumption was that the lens was pliable enough for the eye to still alter its overall shape (?)

Having needed glasses for distance (though "just barely") for decades, I would think it much easier to get used to carrying "driving glasses" and being able to conduct most of your day-to-day "close in" work with unaided vision.

Was your right eye "stable" (ignoring lens clouding)? I.e., do you expect the vision in both (fixed+natural) to remain essentially the same?

I will have to quiz the neighbor when next I see him. ISTR he is "away" for the holiday else I'd holler over the back fence. Maybe just wait for next party...

I believe we went through this in the distant past.

It's difficult to focus on infinity when looking at yourself in a mirror. I just tape a ruler to my forehead, look at something in the distance, and take a photograph (selfie). I print the photo (nowhere near to scale), draw a straight line through the pupils, and draw perpendicular lines through the pupils to the ruler. Then, I just read the scale.

If you want to order reading glasses, look at something at about reading distance and take another photo. The IPD will be less than at infinity.

Mine is narrow, which is probably due to me being narrow minded. Distance: 30.0 + 31.0 = 61.0 mm Computah: 28.0 + 28.0 = 56.0 mm Reading: 27.0 + 28.0 = 55.0 mm Close Inspection: 27.0 + 28.0 = 55.0 mm Note that Zenni wants the IPD specified separately for each eye, in case your nose isn't quite centered.

Mine show a 4-5 mm difference. I prefer to measure the IPD instead of using an approximation.

Vanity:

It doesn't matter if you're focussed at infinity. What matters is that each eye is aimed straight ahead, *as if* it were focussed on infinity.

The mirror thing results in the same measurement, regardless of how far you are from the mirror. But closer is better, more precise.

I just tape a ruler to my forehead, look at something in the

Fine, as long as there is zero parallax error.

On Fri, 28 Nov 2014 15:13:29 -0800, Jeff Liebermann Gave us:

Hey, man... Don't beat yourself up, you might hurt your eyes!

I beg to differ. Distance: 30.0 + 31.0 = 61.0 mm Computah: 28.0 + 28.0 = 56.0 mm Reading: 27.0 + 28.0 = 55.0 mm Close Inspection: 27.0 + 28.0 = 55.0 mm Note the 5-6 mm difference between the various close in measurements (1 to 2 ft) versus the measurement at infinity. I actually tried doing the measurement in a mirror and failed to get an accurate result because I could not find anything to focus on that was sufficiently far away.

I can duplicate the above tests with a camera, tripod, and steel rule. Want me to do it and post photos? (Tomorrow please as I'm recovering from an overdose of turkey).

That's true for the mirror, but not for where my eyes are looking. It might work if I had something sufficiently far behind me, and I were looking at the far objects reflection in the mirror. However, with my bathroom mirror, there's no way to do that because there's a wall behind me.

In my never humble opinion, doing these measurements without a mirror is probably best.

Actually, my eyes have partially fixed themselves. I mentioned this previously, but it's sufficiently interesting to be worth repeating.

I've had a problem with astigmatism for about 30 years. About 9 months ago, I found myself having problems seeing with my prescription driving glasses, especially after dark. Some experimentation showed that I could see better at a distance without my glasses. Once I stopped wearing my driving glasses, my vision improved even more.

According to the best guess of my body mechanic, what happened was that 30 years ago, I started having high blood pressure problems. The blood vessels in the eyes are not uniformly distributed. The increased blood pressure caused the normally spherical shape of the eye to distort asymmetrically, resulting in astigmatism. In 2001, I started on various high blood pressure medications. 13 years later, and lower blood pressure, my eyes seem to have returned to almost their original spherical shape. Unfortunately, I still need reading and computah glasses.

So, if you don't mind, I'll continue beating myself up, as the results seem justified.

A person's IPD is generally a single number, assuming infinite focus. The mirror thing is an accurate way to measure that.

Just do it. When you mark the dot on the mirror, you are necessarily looking straight at the reflection of your own eye. So the line from each eye to the mirror is 90 degrees to the mirror, as if you were focussed at infinity.

Just look at your own pupil in the mirror when you mark the dot. Can't miss.