Sergio, On Sat, Jul 28, 2012 at 1:41 PM, Sergio Pissanetzky <[email protected]>wrote:
> This is serious business. People don't know these things. I remember how > uncareful they were to measure my IPD at the place where I ordered my > bifocals. > Another pet peeve of mine. Bifocals allow people to stop using their muscles that focus their eyes, so they lose their natural ability to accommodate (known as presbyopia). Sure there is age-related loss, but usually not the total loss that bifocals bring to people who baby their eyes. I just tested them with objects at 4m distance. They make the objects look > a little bigger and much sharper, but do not make them juf to and away from > me. > Sounds like they got your IPD pretty close to correct. One problem is that they round to the nearest millimeter at each stage of the process. For example, suppose your eyes are 61.5 mm apart. The guy reading it might write down 61 mm. Then, computing the distance to each side to make the lenses, the lens maker might divide in half and get 30 mm. However, the lens making machinery is a bit off, so they may only be 29.5 mm on each side. Then, they go into frames that are, say. 0.5 mm too close. Then, you put on glasses with an actual IPD of 58.5 mm onto a head with an actual IPD of 61.5 mm, for a total accumulated error of 3 mm. This example is sort of a worst case, and people usually get "lucky" and have some cancellation of the errors so things are usually within a millimeter or so, but if you have a strong prescription, that is often enough to make you nauseous. Steve =============== > **** > > *From:* Steve Richfield [mailto:[email protected]] > *Sent:* Saturday, July 28, 2012 2:29 PM > > *To:* AGI > *Subject:* Re: [agi] Lessons for AGI from the first glaucoma reversal**** > > ** ** > > Alan,**** > > On Sat, Jul 28, 2012 at 11:44 AM, Alan Grimes <[email protected]> > wrote:**** > > Steve Richfield wrote: > > > Glasses do LOTS of things besides making images sharp. They make images > > larger or smaller. They introduce pincushion or barrel distortion. They > > introduce differences in responses to maintain eye tracking. All of > > these things "warp" your sense of where things are in your world.**** > > =\ > > I had to go without glasses for a week or two. When I got my new pair, > they made me feel like I was 3 feet tall! =\**** > > > That is because they got your interpupillary distance (IPD) wrong. I make > my own measurements using a precision steel machinists ruler and a mirror. > The gizmos that most eyeglass places have are usually badly miscalibrated. > > Note that http://EyeBuyDirect.com has some fancy software where, if you > create an account (no purchase is necessary) and enter in your photo, it > computes your IPD from your photo!!! I tried it. We disagreed by ~1 mm, > which sounds better than your eyeglass place is doing, unless your > prescription is EXTREMELY strong. > > BTW, the quick test for IPD being correct is to look at an object that is > ~4 meters or ~13 feet away, and flip your glasses up and down to see if the > object seems to jump toward and away from you as you alternately look > through your glasses, and look under them. Your present glasses would > obviously fail this simple test. > > I suggest going back and getting them to make their glasses right.**** > > I'm a bit better now that > my brain has adapted, but it was totally freaky.**** > > > Now, you will have a similar hassle sometime in the future when you get > proper glasses. > > Steve **** > > ** ** > > *AGI* | Archives <https://www.listbox.com/member/archive/303/=now> > <https://www.listbox.com/member/archive/rss/303/18883996-f0d58d57>| > Modify<https://www.listbox.com/member/?&;>Your Subscription > **** > > <http://www.listbox.com>**** > > ** ** > *AGI* | Archives <https://www.listbox.com/member/archive/303/=now> > <https://www.listbox.com/member/archive/rss/303/10443978-6f4c28ac> | > Modify<https://www.listbox.com/member/?&;>Your Subscription > <http://www.listbox.com> > -- Full employment can be had with the stoke of a pen. Simply institute a six hour workday. That will easily create enough new jobs to bring back full employment. ------------------------------------------- AGI Archives: https://www.listbox.com/member/archive/303/=now RSS Feed: https://www.listbox.com/member/archive/rss/303/21088071-c97d2393 Modify Your Subscription: https://www.listbox.com/member/?member_id=21088071&id_secret=21088071-2484a968 Powered by Listbox: http://www.listbox.com
