Enfield Optics > Explanation & Lesson > Spectacles, Diffraction and Aiming Off
The Use of a Head-Mounted Aperture
In optics there is a study known as ‘Diffraction of Light at an edge’. Light waves get a bit bent when they pass very near to the edge of an opaque object. A small aperture is a special case of an angle bent round in the form of a circle. If it is small enough, the edge diffraction effect all adds up in the central area of the hole. This small hole acts similarly to a lens and can focus images of distant objects on to a screen. This is a pin-hole camera. No glass lens is needed and the depth of focus is quite great. Placed in the lens of a camera, this hole (often adjustable in diameter) has the effect of increasing the range of distances of objects that will be in sharp focus on the film surface. We have a variable aperture in the eye itself but because it does not ever get quite small enough and is not under voluntary control, it is of limited use in shooting optics. Its presence is optically immaterial when a smaller artificial hole is placed near the eye. So placed, the eye is then a severely stopped-down camera and the depth of focus is much increased; so, a foresight that cannot be sharply focused on the retina without its use can now be focused or, at least, is less out of focus. Its advantage over a corrective lens is that it can clear the foresight (and the open rear sight to a degree) without fuzzing up the aiming mark. For me it has a considerable disadvantage in prone shooting. There are now five things to line up into a straight line; aiming mark, foresight, rear open-sight, aperture and the centre of the eye-lens. With the chin in its suitably accustomed position on the stock, I find that I cannot easily always line them up. The position of the aperture relative to the pupil then has to be altered or an inconvenient head position has to be taken up ie I have to ‘crane’. From shot to shot one tends to take up a slightly different geometry and the aperture is frequently not in the absolutely correct, comfortable position. I then tend to lose concentration due to a higher ‘modulus of sodularity’ in the environment. If it does not bother you then seriously consider its use. In offhand or pistol shooting, the mobility of the head over a wider range allows one to line-up the five points more easily.
Incidentally, whether used on the spectacle frame for open sights or mounted on the rifle as an actual rear-sight, the diameter of the aperture is fairly critical. An optical pundit once proved theoretically to my satisfaction that for the vast majority of eyes, under a very wide range of light intensities and at whatever target distance, a diameter of 1.2 mm is about optimum. From my experience I suggest you get an aperture of this size and stick with it. If you keep worrying that the diameter is not optimum at any given time, you have an extra thing to worry about, unnecessarily, and you have quite enough other things to think about!
Using Diffraction in Another Way
With a ‘regulation’ aim, the top of the foresight is on the bisector of the V-notch angle and at the same time in the horizontal line joining the shoulders of the V. With a fuzzy couple of V shoulders and perhaps a fuzzy foresight, this sight picture can be horrendous and capable of giving large errors. I used it for years, being a well-trained traditionalist, and with good young eye-sight it is not too bad. I now find it virtually unusable.
Because diffraction is an edge effect, in a very small area near the V bottom, there is some helpful clearing effect. Think of the V bottom as a sort of deformed and broken aperture. Nowadays I put my foresight top into this area and this drops the aiming mark well down into the V. Without glasses, the foresight tip is noticeably clearer and I can use a weaker lens which in turn reduces fuzz on the aiming mark. The helpful area is very small in compass and my rule is ‘See only enough foresight to be sure that it is actually there and that you are not just kidding yourself that it is.’
It is a bit hairy at first but you soon get the idea. You are looking for about 0.5 mm of foresight tip or less. Note that because the helpful area is so small, provided the foresight tip is in it, windage and elevation misalignment errors will necessarily be small also. Note also that if you change from ‘regulation’ to ‘tiny pip’ you will need to increase the elevation setting on the rear-sight for any given range.
Aiming Off
This may be due to wind effects or to the misalignment of the sight line to the bore line. The traditional way to aim off is to take a regulation aim and then move the foresight (‘cemented’ to the rear sight) a judged distance left or right from the vertical bisector of the aiming mark. Using my sight picture, the ‘black’ is well down in the V. For aiming off I find it best to use the following drill for the last two or more seconds of the aim.
- Get the elevation fixed by taking a dead central aim. The ‘concentration’ at this time is on the size of the gap between the foresight tip and the aiming mark centre bottom.
- Switch concentration to the gaps between the sides of the V and the sides of the aiming mark. If (A) was central, these two gaps are equal. Now make the gaps unequal in the correct direction and by the time they be judged sufficiently unequal, the sear release pressure which has been gradually mounting is brought neatly and smoothly to ‘Pop’.
The temptation is to swop concentration several times between (A) and (B) and this must be resisted because there is no logical end to it and the eye just gets strained. With the traditional method one is merely looking at an inequality and I think the eye is better at judging this. After all, when one gap is made bigger the other gets smaller by an equal amount so the judgment is two-edged or if you like has a magnification of times two compared with the judgment of a singular linear displacement, or so I kid myself and that is all that matters!
Sighting and sear co-ordination is a most personal, withdrawn and introspective matter and what suits one temperament may not suit another. Anyway, it was useful to me to write down how I ‘see’ things and it might help someone to experiment fruitfully as his eye lens gets progressively less flexible which it most certainly will.