26
$\begingroup$

My book defines:

The closest distance for which the lens can focus light on the retina is called the least distance of distinct vision or the near point. The standard value (for normal vision) taken here is $25 \text{cm}$ (the near point is given the symbol $D$.)

However, in normal everyday life, I've always observed that I can still see objects clearly and distinctly for distances even at around $10 \text{cm}$, which is much less than the value $D=25 \text{cm}$. Yes, it does strain my eye to be looking at objects so close at $10 \text{cm}$ but I still can see them anyway, distinctly and clearly.

The Wikipedia article on LDDV is a stub. I couldn't any other useful information elsewhere.

Can anyone please resolve this dispute I've arrived at. Thanks!

$\endgroup$
  • 3
    $\begingroup$ Do you wear spectacles which correct for short-sight? $\endgroup$ – Farcher Dec 5 '17 at 8:19
  • 2
    $\begingroup$ I suspect that the LDDV in cm has a strong correlation with the age of the person in years. Fits with me, at least (both around 53). $\endgroup$ – Peter A. Schneider Dec 5 '17 at 16:44
  • 3
    $\begingroup$ "The standard value (for normal vision)" - you wear corrective lenses because your eyes don't conform to the standard value because you don't have normal vision. $\endgroup$ – Todd Wilcox Dec 5 '17 at 19:00
  • 5
    $\begingroup$ This is literally the very definition of "near sightedness". $\endgroup$ – Beanluc Dec 5 '17 at 23:30
  • 1
    $\begingroup$ Half the stuff you think you see is added by the brain in post-processing anyway :-) $\endgroup$ – user89712 Dec 7 '17 at 1:36
21
$\begingroup$

The focusing of the eye is the result of two things: the curvature of the cornea (which is responsible for the majority of the refraction of light into the eye), and the state of the lens.

When you are young, the lens is very pliable and it allows you to change the focus over a wide range of distances. To go from a focus of "infinity" to 25 cm, you need to be able to change the refractive power of the eye by 4 diopters. That's typical for a healthy young eye - but there are two things that can change where you comfortably focus. The first of these is the shape of the cornea - if your cornea has "greater than average" curvature, this means that light from closer up will be in focus while the lens is relaxed: now add 4 diopters, and the final focal distance you can achieve may well be less than 25 cm. You would be considered "near sighted", and might need glasses (with negative power) to see properly in the distance.

If your eye is insufficiently curved, a healthy lens may not be able to get you to focus close up; in that case you might need a corrective lens with a positive power.

Finally, as you get older, the lens's ability to adjust diminishes; and sooner than you would like, you will need corrective lenses (maybe even bifocals or even more complex lenses) to cover the full range of distances. Perhaps it's just computer glasses so your eye is more relaxed while looking at a screen (our eyes did not evolve to be focused at a short range for long periods of time), but eventually it's one pair for driving, one for reading, one for...

UPDATE

From the comments, it is clear that you have "defective eyesight" which is corrected with a diverging lens (negative power). When you take the glasses off, your eye is naturally focused closer than "average". This is the explanation why in your case you can see objects in focus at a distance of 10 cm.

$\endgroup$
  • $\begingroup$ So, I understand that the result of my being able to see objects so up close is because I'm young, my cornea/lens is flexible and thus my "focussable range" is very broad (infinity to nearly 10cm). I also understand that my "focussable range" will decrease with age. Am I correct? $\endgroup$ – Gaurang Tandon Dec 5 '17 at 4:41
  • 1
    $\begingroup$ Probably. I don't know if you are young but you do... And yes this ability will diminish with age. Are you able to see clearly in the distance? Some young people can focus really close - but they can't see the blackboard in the classroom. So their whole range of focus is shifter to shorter distances. $\endgroup$ – Floris Dec 5 '17 at 4:55
  • $\begingroup$ Well, I have defective vision since childhood. I got spectacles in grade 2. I still wear spectacles, though my spectacle power is steady since the past 4 years. $\endgroup$ – Gaurang Tandon Dec 5 '17 at 4:56
  • 1
    $\begingroup$ In that case I am betting your prescription is negative - in other words, without glasses you see better close up than far away. Right? $\endgroup$ – Floris Dec 5 '17 at 5:06
  • 1
    $\begingroup$ @Michael not necessarily. The range over which you can adjust gets smaller - but depending on the shape of your eye, you may be "permanently staring in the distance", or "permanently focused up close", or "somewhere in the middle", or "nowhere at all". $\endgroup$ – Floris Dec 5 '17 at 17:52
45
$\begingroup$

The least distance of distinct vision is the minimum distance your eye lens can focus on an object without any strain. This means the eye is in a relaxed state. But eye is a self adjusting lens. When you try to see an object closer than 25 cm(for a normal eye), your eye automatically adjusts the focal length thus decreasing it. This is why your eye gets strained.

$\endgroup$
  • 1
    $\begingroup$ Is it? Oh, I believe my book's definition is then incorrect :/ $\endgroup$ – Gaurang Tandon Dec 5 '17 at 4:32
  • 3
    $\begingroup$ It is not exactly incorrect. At LDDV the eye is just not stressed. You can see distances less than it because your eye lens bulges making focal length less $\endgroup$ – V .Kiran Bharadwaj Dec 5 '17 at 4:42
  • $\begingroup$ When I was younger I thought I could see closer by simply crossing my eyes. But now I'm not so sure - with a close object even crossing my eyes it doesn't look sharp. $\endgroup$ – Michael Dec 5 '17 at 17:47
  • 1
    $\begingroup$ @Michael crossing your eyes allows for stereoscopic vision... at very close distances though, the two eyes see the object from very different angles and that's not very helpful for depth perception. Seeing "in focus" is a property of the individual eye. $\endgroup$ – Floris Dec 5 '17 at 18:00
8
$\begingroup$

Since the OP wears spectacles to correct for shortsightedness the lens system cornea and flexible lens) of his eye is too powerful to produce a sharp image of a distant object on the retina.

enter image description here

Note that because the optical system of the eye is too strong a real image of a distant object is formed before the retina. A diverging lens can be used to correct for this defect. However this being so means that the lens system is able to bend light sufficiently to allow for a sharp image which is less than 25 cm from the eye to be formed on the retina. So the least distance of distinct vision is less than that for the "average" eye which is taken to be 10 inches or 25 cm.

If the OP takes the spectacles off and positions them a little way away from some print and looks at the print through them the print should look smaller as the correction lens for shortsightedness is overall concave/divergent.
If the spectacle lens system is not very powerful then the different in size may not be immediate apparent.

enter image description here

It practical terms it means that the radius of curvature of the spectacle lens nearest the eye (the divergent bit) is smaller than the radius of curvature of the surface remote from the eye which is convergent. The divergent surface is more powerful than the convergent surface is more powerful then the convergent surface so over the lens is divergent. The lens is constructed this way so that it does not foul the movement of the eye lashes and the eye lid.

A magnified image of the print would indicate that the spectacle lens was overall convergent and such a lens would be used to correct for farsightedness as is shown in the diagrams below. A person with such a defect would have the near point further than 25 cm from the eye. You may have observed older people reading something from quite a distance because their flexible eye lens is no longer flexible and the optical system of the eye is too weak to focus objects which are close to the eye.

enter image description here

Note that the 25 cm is just a rule of thumb and differs from person to person.

The eye is only truly relaxed when the ciliary muscles used for controlling the shape of the flexible lens in the eye are not in contraction and that is why one might forego the maximum magnification of a microscope (when the final image is at the near point) and observe a specimen with the final image is at infinity thus induce less fatigue.

$\endgroup$
  • $\begingroup$ I'm sorry but you have too many points under one answer. Can you just organize them under two-three paragraphs, instead of one sentence per line? I'm having a hard time understanding how you're moving from conclusion to conclusion... $\endgroup$ – Gaurang Tandon Dec 5 '17 at 15:42
  • $\begingroup$ The pictures help. $\endgroup$ – Floris Dec 5 '17 at 17:56
  • $\begingroup$ @Floris yes, now I can comprehend it well (they weren't there when I originally commented) $\endgroup$ – Gaurang Tandon Dec 6 '17 at 2:24
  • $\begingroup$ I believe the shortsightedness is a red herring here. I don't wear any glasses yet I'm able to focus on objects much closer than 25cm away. $\endgroup$ – Dmitry Grigoryev Dec 7 '17 at 8:26
  • $\begingroup$ @DmitryGrigoryev My answer was in response to the OP being short-sighted. The 25 cm is a rule of thumb and as with most biological systems there can be a large variation. You are obviously able to change the focal length of your flexible lens more than the average and so can adjust it to see both distant objects and onbjects close to you clearly. $\endgroup$ – Farcher Dec 7 '17 at 16:12

protected by Qmechanic Dec 5 '17 at 15:19

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.