QUESTION: 'Normal' minimum focal length

Discussion in 'Optometry Archives' started by The Central Scrutinizer, Jan 31, 2006.

  1. Hey, all.

    I am nearsighted, to the tune of 3.5, corrected by contacts. I was just
    having a close look at a polished stone I bought, and observed that I
    lose the ability to focus at about the 15cm mark; anything closer and I
    can't focus on it. I just wondered if that was a 'normal' distance for
    such a threshold? Is this distance something that one would expect to
    increase with age?

    Just semi-idle curiosity... ;)

    The Central Scrutinizer, Jan 31, 2006
  2. The Central Scrutinizer

    acemanvx Guest

    If you are young then its good. If you are over 25 then its great! I am
    23 and theres NO way I can focus to 15cm with full power contacts.
    Maybe 25cm if that. My sister is only 18 so her accomodation is good,
    she can focus down to 15cm. Without correction, I can focus as close as
    10cm with my -4 pescription. Thats with both eyes open and crossing
    them and straining. Using both eyes increases the depth of focus and
    you can cross them for fusion of accomodation. However accomodative
    amplitude is measured with one eye open at a time. Uncorrected I see
    fine down to 15cm one eye at a time Thats 6.5 diopters of accomodation
    minus the 4 diopters of myopia I have. With full power contacts id be
    looking at 40cm minimum! I can not see fine print with contacts and
    newspaper print is somewhat blurry unless I hold it arms length. A
    slight undercorrection makes a huge difference.
    acemanvx, Jan 31, 2006
  3. If you are over 25 then its great!

    I'm 37. Guess that's alright, then. I never made a point of seeing how
    close I could get when I was younger, but I'd be surprised if I was
    ever able to get much closer than that.

    If only I could get rid of the damned floaters. ;)
    The Central Scrutinizer, Jan 31, 2006
  4. The Central Scrutinizer

    acemanvx Guest

    If you can still accomodate down to about 8 inches clearly, thats
    better than average accomodation for a 37 year old. Maybe your contacts
    slightly undercorrect? I find that as little as half diopter
    undercorrection makes a significent difference in how well I see from
    near. How well do you see uncorrected? corrected?
    acemanvx, Jan 31, 2006
  5. How well do you see uncorrected? corrected?

    Corrected is _pretty_ good for me. Better with glasses. I have slight
    astigmatism in my left eye, and it was decided that it was mild enough
    not to sweat it in terms of the contacts. So there's a very subtle
    difference between right and left with my contacts. Almost
    unnoticeable. Certainly a 'hit' I'm willing to take to get off the

    I figure that if the contacts undercorrected I would experience eye
    strain, which I don't.

    Uncorrected is just silly. I'm useless. And I thought _my_ prescription
    was strong. I have a good friend who's 9.5. Yuck.
    The Central Scrutinizer, Jan 31, 2006
  6. The Central Scrutinizer

    acemanvx Guest

    9.5 is insane, much more so if contacts due to vertex distance. Your
    glasses pescription should be -4.5 give or take a half diopter.
    Different people appear to behaive differently in how much contact
    power they need vs. glasses. I know someone that sees 20/20 with -3.5
    contacts but needs -5 glasses to see perfect. I know another that needs
    -4 contacts but only -4.5 in glasses. The average is 1.25 meaning you
    multiply your contacts to arrive at glasses. My -3.25 contacts
    multiplied by 1.25 come at about -4 in glasses(I rounded) I see MUCH
    better with -3.25 contacts than -3.25 glasses and in fact theres next
    to no difference between -3.25 contacts and -4.25 glasses. I know some
    optometrists disagre but like I said, it depends on the person and
    their eyes. You can compare glasses vs. glasses but you cant compare
    contacts vs. anything as its variable on the person and their eyes.

    Undercorrection isnt silly, you will understand in a few years once you
    get presbyopia. Imagine not seeing clearly as much as 2 meters in
    advanced presbyopia. Youll need reading glasses for everything 2 meters
    and closer. Talk to someone across the table? His face will be blurry!
    Try to use the computer? You wont be able to read a thing on the
    screen! Newspaper? Forget it your arms arent long enough! I dont like
    contacts for several reasons, one is they arent comfortable, they can
    be felt. Two is I cant see clearly less than about a foot with contacts
    and my eyes feel strained and ache if I do any near work with them. I
    do not wish to incude more myopia that way. Three is I can just take my
    glasses off and see great from near. I am happier with glasses than
    contacts. Ill only wear contacts for seeing in the distance such as to
    play sports.
    acemanvx, Jan 31, 2006
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