Image on the left shows a very large depth of field, owing to the fact that the small aperture only allows perfectly directed light rays in.  The photo however doesn't show that to achieve the same brightness as the right photo, the exposure would have to be very long in duration to collect the required number of photons.  (Photo courtesy of exposureguide.com)

Image on the left shows a very large depth of field, owing to the fact that the small aperture only allows perfectly directed light rays in.  The photo however doesn't show that to achieve the same brightness as the right photo, the exposure would have to be very long in duration to collect the required number of photons.  (Photo courtesy of exposureguide.com)

People who are legally blind without their glasses usually realize that they are a broken or lost pair of spectacles away from trouble.  I once had a patient with a -15.00D prescription who lost her glasses when travelling in eastern Europe.  Without being able to speak the local language, she was hopelessly resigned to stumbling around and trying to find her way back to the hotel.  It took her 4 hours to find her way back.  Now we usually recommend that people who are this nearsighted have a back-up pair of glasses (or contact lenses) with them at all times, but sometimes "things" happen, so to speak.  So what could have been done in this scenario?  One solution would be to create a pinhole device.

A Pinhole occluder.

A Pinhole occluder.

A pinhole occluder is simply any opaque object that has a tiny hole or holes poked in it. An actual "manufactured" pinhole occluder that all optometry/ophthalmology offices have for diagnostic testing is made of plastic with numerous 1.12mm diameter holes scattered throughout it.  However, when in a bind, any piece of paper will work (business cards work great for this).  Simply take the card and poke a ~1mm in diameter hole in the centre of the card with a pen, and then hold the card up flush to one of your eyes (while shutting the other eye), and peer through the hole.  Since the pinhole acts like a tiny aperture, and only allows light waves directed right at the centre of your eye to pass through, you should see dramatically better with it.  Infact in a healthy eye and a well-made pinhole, a person should see almost 20/20 with it. 

When an eye needs prescription glasses to see, there is usually an error in the size or shape of the eye, which leads to issues of "where do the light rays entering the eye end up focusing once they are inside".  But even in these eyes, a perfectly directed light ray should pass through the centre of the pupil unimpeded, and focus on the macula in the back of the eye.  It's just the billions of other light rays that enter from different angles that aren't so lucky and end up focusing outside the macula, making these patients see blurry without their glasses.   A pinhole occluder blocks the misdirected photons, and only allows the perfectly aimed ones in, giving the user clear vision.  It is this basic principle of optics that has led to numerous 'companies' selling pinhole based glasses as a replacement to regular glasses, in an effort to make a quick buck.  The problem, however, is that by blocking 99.9% of the misdirected light waves, the majority of light doesn't enter the eye, and the resulting image is pretty dim, (in addition to suffering from aberrations related to diffraction) which is why glasses made of pinholes are not a permanent solution or even an adequate replacement to normal glasses (even if you are told otherwise on late night infomercials).

A good analogy would be if there was a dart throwing competition and the competitors were being judged on accuracy and total score.   Each competitor had 10 darts to throw. If 'Competitor 1' threw and hit the bullseye with all 10 darts, he would be very accurate and have a very high score.  This is analogous to a person who doesn't need glasses and naturally can have every photon (dart) hit the target in the centre (the macula).  'Competitor 2' is less naturally talented but devised a dart that was able to be drawn to the target, no matter how poorly it was aimed.  Once again, all ten darts would land on the target.  'Competitor 2' is like a person who wears glasses, and the lenses refocus and redirect misdirected light waves onto the target. 'Competitor 3' didn't believe in the advances of technology, and instead opted for a piece of plywood with a hole in the middle of it, and placed it directly in front of the dart board.  The hole was the exact same size as the bullseye, and therefore only the perfectly thrown darts were able to pass through the hole and hit the target. All the other poorly thrown darts would hit the plywood and therefore not hit the dartboard and not count towards accuracy (this tournament has weird rules).  'Competitor 3' is obviously the pinhole glasses patient.  All three competitors would achieve the same accuracy score (100% of the darts each competitor threw, that hit the dartboard landed on the bullseye.).  However only Competitors 1 and 2 would have respectable scores, since all 10 darts hit the bullseye.  Competitor 3 would have a very poor score  since only 1 or 2 darts actually made it to the dart board (score in this case is analgous to brightness.)

So, pinhole occluders/pinhole glasses are an acceptable way to help a person see accurately in a pinch, so in theory, they do work.  Its just that they work so much less effectively than normal glasses, that they should never be used as an actual replacement to glasses.

 

Dr. Burke is an optometrist practicing at Calgary Vision Centre.  Opinions above do not constitute medical advice, and readers should consult with their optometrist if they have questions or concerns about their eye health