Whale oil beef hooked! A breakthrough, I can see again ...

[Patrick Chadwick]

What follows is a simplifed explanation that is not scientifically exhaustive, but adequate for present purposes...

No-one of mature years is going to be able to see backsight, foresight and target all clearly at the same time, so you have to make a choice.

The one element you must see clearly is the foresight - whether blade or ring. If you have an aperture backsight, this is usually so close to the eye that a) not even a teenager with 20-20 vision can focus on it and a foresight simultaneously, and b) it is not necessary to focus on the backsight, as the eye naturally tends to center in the fuzzy aperture. Conscious control to make sure that the field of view really is central, and not just accidentally so, is the clue to better aiming with an aperture backsight (and for telescopic sights as well).

So the lens for shooting with an aperture backsight is optimized so the the depth of field (the range over which everything appears adequately focussed) has the foresight as the near point and the field extends out towards the target, without any attempt to get the backsight aperture in focus.

Of course, the depth of field is also dependent on the aperture. The depth of field increases as the aperture is stopped down. So the optician checks the setup with the maximum size of aperture you are likely to use in practice, and if the light enables the use of a smaller stop, the depth of field will be even larger.

With an open backsight the adjustment must be different. There is no automatic centering trend, as with an aperture backsight. So the foresight is taken as the far point, with the depth of field extending back towards the open backsight. Once again, the depth of field is increased by stopping down the optical aperture, which is why those spectacles with a grid array of holes produce a sharper picture with open sights. But a better solution is to have an adjustable iris on the shooting glasses, so that the aperture can be reduced as far as the light permits.

Please note that you cannot shoot well with an aperture backsight combined with those hole-grid spectacles. The aperture backsight is both the optical stop and an aiming reference. An additional aperture behind, whether as an iris or the hole grid can only act as a stop and increase the depth of field if it is optically smaller (i.e. subtends a narrower angle at the eye) than the backsight aperture. Which means that you lose the aiming reference of the backsight aperture. So the iris/hole grid element is only for shooting with open sights!

You will also have noticed that the best solution actually requires two lenses - one for shooting with an aperture backsight, and one for shooting with an open backsight.

The optican who made my shooting glasses has a well-deserved national reputation. He examined my eyes with equipment of a sophistication that I have never seen in an ordinary "downtown" optician's room.

I had wondered for some time why I had a marked tendency to produce double groups, separated vertically by a couple of MOA, even when shooting off sandbags when I could be pretty sure that an unconscious shift of hold was not the explanation.

He (or rather his equipment) was able to detect a flaw in my right eye that causes a double refraction. I literally see double in the vertical direction. The second image is that ominous couple of MOA above the "true" image, and much weaker, but strong enough to allow a false aim if I am not careful. This flawed region is about 1 mm below the center of the iris, so although I wrote above that one should make sure that the eye is looking through the center of any aperture, in my case I can improve the picture by deliberately looking low, so that the defect area is out of the sight line. With this insight, I have been able to vastly improve my aiming capability with open sights by learning to hold the blade very deep in the backsight, almost to the point where it disappears.

Ten to one AM here now!

G'night!