Some Coaching tips

1. Aperture sight sizes

One of the most problematic issues “new” and some not so new rifle shooters face is choosing aperture sight sizes. Just remember this one rule of thumb to guide you; 

For the front sight bigger is better, and for the rear sight smaller is better. 

· The Rear Aperture.

Most literature that you can find about rear aperture sizes will tell you that the correct range to work in is 1mm to 1.3mm. Not much of a range you say! So why so small? “If the target is dark, why can’t I just open the aperture up until it is bright enough?” Sure, opening up the aperture will allow more light through but it comes at a cost! Here’s why. 

Coaches will tell you to focus on the front sight, that is really important but how do you focus on the target as well? The answer is you don’t have to because the rear sight iris does that for you. A correct sized iris will make sure that the target is in focus at the same time that you are concentrating on keeping the front sight in crisp focus. 

The small size is a way to keep both the front sight and the target in focus at the same time. Quite simply, the smaller the hole the longer the field of view. “Field of view” is the term that photographers use to describe the area that is in focus. For instance, a portrait photograph would be taken with a large aperture so that only the subject will be in full focus and everything beyond will be blurred. The large aperture gave a narrow field of view. Whereas a landscape photograph with some trees in the foreground would be taken with a small aperture size so that everything remains in full focus. The small aperture gave a long field of view. That is all you need to understand because if you try to look deeper at camera aperture sizes and compare them with rifle apertures it gets very confusing. For instance the camera aperture sizes or “f stops” as they are called have weird numbers usually starting at f1.4 (largest) and going through to f22 (smallest). Yes the larger the number the smaller the aperture size on cameras! There is a mathematical formula to work out the actual size but it doesn’t matter, it is the principle that you must understand. The smaller the aperture the longer the field of view. The front sight and the target will be in the best focus. The problem here is the trade off with light. The smaller hole will let less light through! So you make it bigger so that you get enough light. (The new adjustable irises are good like that). Your scores go down and you can’t understand why? Maybe the target has gone fuzzy because you have inadvertently shortened the field of view by too much. You achieved your goal of getting more light but it came at the cost of poor target definition.  

With the help of my stepdaughter who was doing a maths degree at Waikato University we worked out that to get 25% more area (therefore 25% more transmitted light) a 1mm diameter aperture would only need to be enlarged to 1.118mm. To get 50% more light a 1mm diameter aperture would only need to be enlarged to 1.225mm. If like a lot of shooters you use a 1.1mm aperture and open it to 1.230 it will transmit 25% more light. If you open your 1.1mm aperture to just over 1.3mm (1.347mm) you will increase the light transmitted by 50%, that is a big increase in light for just a shade over 0.2mm increase in aperture size. So you don’t have to go silly when the target looks dark and wind your aperture out to 1.5 or more like I have seen. Just keep it within the range of 1mm – 1.3mm and if the light is still not to your liking simply use a coloured filter. 

This is still a “work in progress” as we are constantly looking at aperture sizes and aiming areas with the help of Scatt optical training systems at our Paralympic shooting training sessions. It could be that we might find out that we can get away with a slightly larger than 1.3mm aperture for 10 metre air rifle but for now stick to the tried and true guidelines and understand what changing the rear aperture size does to the depth of field/focus (and your score).

· The Front Aperture.

Another trap for beginners is to think by making the front aperture smaller and closer to the circle of the target, that it will result in accurate shooting. Logic does suggest that this will work well but in practice the opposite is true. The closer the aperture ring gets to the target the harder it becomes to properly define the distance between them. Your eye begins to play tricks on you and you cannot properly judge the distances and movement. The edges can distort, bleed or blend into each other. You spend too much effort on the sight picture and not enough on hold, trigger and follow through. Some people think it is a good idea to hide any movement by reducing the size of the front sight aperture. Movement is caused by poor balance, not the size of the aperture! So, ask yourself just how do you expect changing the front sight size will improve your balance? 

Your scores are dipping and you can’t understand why? Go bigger!! Trust me, your eyes will easily align two circles one inside the other if there is enough space between them to properly define them. As an example, New Zealand’s Mike Johnson is currently ranked in the top five (previously number 1 and Paralympic champion) in the world in two IPC events, the 10 metre air rifle R4 and R5. Mike shoots with his front aperture on his FWB 800 usually set between 4.8mm and 5mm. That is much larger than the 4.5mm that most beginners will start on for air rifle, AND he shoots world records like that!

Dave