Andy McLachlan explains why sometimes smaller really is better when it comes to focusing
Many shooters new to the sport of airgunning, let alone outdoor target shooting, will be faced with the dilemma of purchasing a scope that is best suited to their particular needs.
Very often, people believe bigger is better, and a scope with a large objective lens will be considered preferable to one with a lens considerably smaller. For example, looking at the available options, a potential purchaser might consider that a scope with an objective lens (the big end) is bound to be better for shooting, as its 50mm lens must be able to let more light into the scope’s internal lens assemblies than one with a 40mm lens. This is true; shooters wanting to maximise all available light at both early morning and late evening are best served by a good-quality scope with both a large and high-quality lens – and,
just as importantly, lens coatings.
However, the larger the front objective lens becomes, the smaller the oft-used term ‘depth of field’ becomes. What do we mean by depth of field? Well, imagine that you are lying down facing a knockdown target that is positioned within the confines of a wooded area with little light. You are shooting with a friend whose outfit is armed with a scope of 4-12 x 56mm (four to twelve times magnification, with a large 56mm objective lens). Your friend takes the shot first. He is very unimpressed as his shot fails to even hit the faceplate, and he turns to you with a look of astonishment upon his face. How could he possibly fail to miss the target completely, he wonders?
Not being sure why your friend has drawn a blank score, you settle down with your own outfit, which is armed with a much smaller objective lens of ‘only’ 36mm. Due to your friend’s shot completely missing the target, you suspect that his pellet has been deflected by a wayward blade of grass or leaf that he failed to notice. Carefully following the trajectory curve of the pellet, and remembering of course that it will be higher above the zero point upon the zenith of its journey mid-flight from its 40-yard zero, you spot a tiny branch that is right in the path of the travelling pellet.
The only way to deal with this hazard is to reposition yourself on the peg and hope that a different angle can negate the hazard. Sure enough, you manage to thread your pellet through and the target falls with a satisfying ‘clank’.
Not being sure why he missed the target, your friend asks if you spotted anything that might have caused the miss. It is at this point that you use the classic “You didn’t see the twig because your big-lensed scope does not have sufficient depth of field” comment. “Eh?” your friend asks.
Depth of field is a term to describe how much of an image seen through a lens is sharply in focus around the actual focal point. For example, for a given focus everything between, say, 25 to 35 yards away might be sharp, with objects nearer and farther looking progressively blurrier. The actual range is determined by the amount of light entering the lens and its focal length. A larger objective lens allows more light into the scope, but this in turn leads to a shallower depth of field – so a narrower range of objects before and after the focus point are sharply focused.
Large objective lenses, such as those used by Field Target shooters who tend to use monsters with objective lenses of 50 or even 60mm, are able to parallax each individual shot. This means that they focus their optic for every shot, and in their case read the range prior to winding in an individual firing solution. They’re not really bothered about depth of field because they are adjusting their scopes all the time to calculate range.
It’s very different for the shooter choosing to compete in Hunter Field Target, however. As many of you will be aware, the competition rules forbid us to make any adjustments at all to our scopes once the competition has begun. So you’re better served by an optic that enables you to see everything between the minimum of eight and maximum of 45 yards as clearly as possible.
An ideal scope for HFT will have a smaller objective lens than that used by a regular hunting scope. Remember, the smaller the objective lens becomes, the more the depth of field increases (generally speaking). In other words, if your friend had been using your scope, or one with an objective even smaller than yours, he would most likely have spotted the branch hazard and secured the shot and the points that go with it. The fact that your scope with the 36mm lens was able to clearly identify the hazard and your friend’s 56mm lens was not perfectly illustrates why, in this case, smaller really is better!
So what, I hear you ask, is the ideal scope size for HFT shooting? One of the other major advantages of scopes with smaller objective lenses is that they tend to suffer much less from the major enemy that is parallax error. Basically, this means that your gun is pointing in a slightly different direction from your crosshairs, and is an obvious cause of many misses on the target course, particularly for the unwary.
An ideal size will be something along the lines of 2-10x magnification, a 36mm objective lens, parallax adjustment (for the one-off adjustment to 25 yards mid-HFT course distance) illuminated reticle for those murky woods, and a good standard of build that means you don’t have to re-zero the scope due to internal movement of the components on a regular basis. The specifications of such a scope would be described as ‘2-10×36 AO IR’. Like many other shooters, I also consider that a good reticle helps for deciding upon individual firing solutions during a round of HFT, with my own and some of my friends’ preference being a half-milliradian scope which tends to bring in our aim points perfectly when zeroed at 40 yards.
I have written many reviews of some excellent scopes that perfectly fit the requirements of the ideal HFT scope, and
I have no intention of naming them all here. Some of the scopes cost four-figure sums, and you do, with optics, tend to get what you pay for. I will, though, mention one relatively low-cost, but truly superb scope that will hopefully turn you onto the many advantages of the ‘small is best’ optic. This one does not cost the earth and represents the optical advantages of a scope which you might normally overlook.
The scope is the Hawke Airmax 2-7×32 AO with the AMX reticle (model number 13100.) It’s available for a very reasonable £140, but even cheaper than this if you shop around. It is certainly not perfect for an HFT optic, but it does represent a specification that comes very close and one that will literally open your eyes to the not inconsiderable advantages of what is possible with a small and well-priced optic.