The requirements for a varmint or benchrest rifle scope are far different (though still sharing the basic concerns) from those for medium to big game.
The varminter or benchrest shooter is usually most concerned with high magnification, ease of adjustment, repeatability of adjustment and parallax adjustment. The hunter is more concerned with generous eye-relief, light gathering characteristics, weight and durability. Both rate clarity high on their lists of "wants" when shopping for scopes.
Let's take a look at the above requirements and see how they translate to real-world scope offerings.
The varmint/benchrest crowd, as a rule, like magnification levels much higher than those reasonable or desirable in a hunting scope. It is not unusual to see scopes of this type in the 32 to 40 power magnification range, with many variable power scopes spanning several levels within this range. These scopes generally are long and bulky, made even more so with the addition of various lengths of sunshades to keep the glare off the objective lens, and have protruding "target turret" adjustments. The goal with these scopes is to give as clear a picture of tiny distant targets as possible while adhering to the adage "aim small; miss small".
The seasoned hunter, on the other hand, wants either a sturdy fixed power or fairly compact variable power scope with a minimum of bulk and protuberances. Magnification usually runs from a fixed 4 power or less to variables in the 4 to 12 power range with a lot of interest still generated by the old stand-bye 3-9 or 2-7 power variables. The intention of these scopes is a clear picture of much larger targets while providing a large field of view and ease of acquiring the game animal in the scope picture.
Eye Relief is the distance between the ocular, or eye-piece, of the scope and the shooter's eye required to achieve proper alignment and focus. This distance is not an absolute. A scope listed as having an eye relief of 4" may actually range from 3 to 5" depending on the shooter's vision and head position. For the varmint/benchrest crowd, a generous eye-relief is generally not a big issue. Most rifles used in these pursuits are of low to medium recoil levels and offer limited danger of "scope eyebrow" as long as the shooter doesn't "crawl" the stock.
As the rifle calibers increase to the game field levels, eye relief becomes critical. The 3 to3 1/2" eye relief offered in most scopes becomes the minimum safe distance. Many are the shooters who have come back from the range with crescent-shaped cuts over the shooting eye as a result of a combination of too-short eye relief and the shooter positioning his head too far forward while firing, a common occurrence when shooting from a bench. I've been known to garner the occasional one myself-once from a hot loaded 45-70 that literally brought me to my knees.
3" of eye relief is the absolute minimum for your own safety. Some modern "high-end" scopes now offer eye relief distances as great as 5" and are a great idea. Scopes intended for mid-barrel mounting or handguns can have eye reliefs of around 7 to 24".
Complete articles can be (and have been) written concerning the mechanics and theories of parallax. To put it in its most simple terms; parallax is the apparent movement of the crosshairs resulting from changing the position of your eye behind the scope. This movement is caused by the refractive qualities of the lens system housed within the scope body and varies from scope to scope as well as by distance.
Most varmint/benchrest scopes, and many of the newer hunting scopes, have an adjustable objective (AO) to eliminate parallax at any given distance. By focusing on the target and turning the adjustment ring, apparent parallax can be minimized for that distance. Those scopes not having this feature are usually adjusted by the factory to be parallax free at 100 to 150 yards. This, of course, depends a great deal on the shooter's vision and focal plane.
Note: The yardage markings on AO scopes is intended for reference only. They seldom are actual indications that parallax will be eliminated at the specific distance indicated. Only adjustments made while looking through the scope are accurate.
Scopes can be tested for parallax by sighting on a distant object with a supported rifle. Without touching the rifle, slowly move your eye from side-to-side and up and down while watching the crosshairs. Parallax will make itself known by the crosshairs appearing to move around the target. If the crosshairs remain centered on the target, the scope is parallax-free at that distance. Occasionally minor parallax can be adjusted out by rotating the focus ring on the ocular. Scopes mounted properly to allow the crosshairs to be in the exact center of the lens system will show far less parallax than those that are not centered.
Clarity is the ability of a scope to translate a clean, crisp focused picture to the shooter's eye. Clarity is dependent on precision lens polishing techniques and superior materials for lens construction. The view through a quality scope should be crisp and sharp from the center of the lens all the way out to the edges.
Clarity does not mean that the larger the objective the better the clarity. Objective size rapidly reaches the point of diminishing returns. Objectives ranging from 32mm to 44mm are about the largest useable sizes. Today's trend towards 50mm and larger objectives in the interest of "light-gathering ability" make proper mounting a nightmare and increase sun-glare on the lenses dramatically.
The adjustments for positioning the crosshairs within a scope vary from scope to scope. Precision scopes often offer one "click" for 1/8" point-off-impact movement at 100 yards. Hunting grade scopes range from 1/4" to 1/2" for the same distance. Logic tells us that the scopes that offer the most precise adjustments are the most desirable for accuracy. These adjustments should be crisp and accurate. Scopes using "target" turrets or range-finding adjustments should be repeatable and dependable.
Again, requirements differ between varmint/benchrest shooters and hunters. Most game hunters sight their rifles for a specific distance and then utilize "hold-over" for extended distances since game seldom stands still for you to adjust a scope for a perfect aim. Varmint hunters or benchrest shooters, on the other hand, often have the luxury of time to adjust and focus for a particularly long or difficult shot. For them, the "target" turret scopes make sense.
Price vs. Quality
Logic tells us that as the price increases, so does the quality when buying a scope. This is not necessarily true. There are many medium priced scopes that are quite accurate and tough without breaking your budget or requiring a second mortgage. There are also many premium priced scopes that are little better than junk. Many of today's manufactured scopes are better than the same brands of the past........but many are also worse.
I currently have a fair number of the older, original Tasco Target/Silhouette scopes ranging from a couple fixed 24 powers through 6 to 24 power variables and a couple 8 to 32 power variables. These scopes have proved to be tough and precise and have served me very well on a number of varmint caliber rifles. Unfortunately, I can not recommend the current crop of Tascos. Quality seems to have slipped drastically and they do not seem to be of any real value.
I recently tested a "varmint" scope that refused to hold adjustment from shot to shot. Each shot of my .223 (hardly a high recoil cartridge) moved the point-of-impact 2 to 3 "clicks" left. Re-adjust, shoot and again the impact moved. Bad manufacturer or just a bad scope? Since the maker is considered to be one of the better ones, reason says that it must be the individual scope. The manufacturer quickly replaced the scope and the new one shows no sign of the above aberration, but I will not really trust it for quite awhile.
There are several other new makers out there offering a lot of features for modest prices. Some are good....others not. Concentrate on the basics of good scope performance and evaluate your situation before spending your hard earned cash.
Now that you have determined the magnification you want, whether you want fixed or variable and your price range, let's take a look at the rest of the picture.
Make sure that the scope you want is compatible with the firearm for which it is intended:
Recoil is a factor to be considered seriously. Rifles with heavy to excessive recoil will eventually destroy any scope.....period. It is just a matter of time. That time range can be extended by making sure that you have chosen a scope rated for these types of weapons.
Ease of mounting the scope with proper alignment is essential. Many of today's scopes have massive ocular and objective lens housings. These make it nearly impossible to mount a scope low enough to allow proper eye position without having to have your head out of proper position. Avoid these like the plague. They will do nothing to assist your shooting or accuracy.
Matte or satin finishes are preferable to gloss finishes for mounting on rifles or handguns with any degree of recoil. The matte finish allows the rings better grip on the tube body without over tightening.
The scope chosen should have enough body length to properly fit in the mounting system on the intended rifle. Don't laugh. As much as I love Ruger #1 rifles, their fixed distance between rings make scope selection difficult with some of the modern lines of scopes. Many of today's scopes (particularly the "compact models) have such little actual body located between the "bell flares" of the ocular and objective housings that there is not enough to fit into the ring spacing on many mounts and still provide proper eye-relief........if they even fit between the rings at all! (Rings designed to extend or otherwise alter positioning to scopes should be avoided. These provide excellent leverage points to bend or flex a scope tube.)
Recommending a particular brand or model scope is about as risky as criticizing a guy's wife. Tastes vary and what is required varies. My personal choices for hunting medium to big game run to the old steel-tube Weaver K4 models that can still occasionally be found. Otherwise, a good quality 2-7 variable will handle all of my needs. For varmint/target in light to moderate recoiling calibers, I generally toss on one of the above mentioned early production Tascos and feel well served.
Read and understand the terms and qualities listed above, do your research and choose wisely and you will be rewarded with a scope that performs to your expectations.
Good shooting to all.