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Subject: Need a little help

Posted By: turtle - Registered User

Posts: 175

Posted At: (5/30/02 8:49 am)

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I'm in the process of developing loads for a Rem. 25/06. I've found a good shooting load but have a question regarding parralax in scopes. My bench is not T shaped but more like a picnic table. When I am in position to shoot it is very hard to get a full field of view without pressing my chest into the table. Without doing this the field of view isn't full but black around the edges. This appears to contribute to larger group sizes. The scope is a 4X14 Leopold Vari X III set on 14X when shooting. Should the field of view be full when precision shooting? and if it is not will this effect group size? Thanks


Subject: The Factotum Speaks

Posted By: ML - Registered User

Posts: 286

Posted At: (5/30/02 11:43 am)

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Rifle-Scope Arcania

Once again, I see that my second-intellect (or what remains of it) is called upon to shed the most feeble light in the darkness of ignorance. Or something like that.

I imagine many Forum members are less than satisfied with their telescopic sights, and now we have an entire generation which has grown up shooting with nothing but telescopic sights.

The problem you describe ("a question regarding parralax in scopes . . . . When I am in position to shoot it is very hard to get a full field of view without pressing my chest into the table. Without doing this the field of view isn't full but black around the edges") isn’t parallax at all, nor is it really Field of View; rather, it’s eye-relief and exit-pupil related. So let’s define a couple to terms.

Eye relief is the distance your eye must be from the ocular lens (the lens closest to your face). In the case of the Leupold 4.5-14 Vari X III, the eye relief is between five inches (at the lower magnification range) and 3.7 inches (at the higher magnification range). Depending on the design of the scope itself, the "sweet spot" of the eye-relief’s range can be critical--especially at the higher magnifications. With a 14-power glass, I imagine you’ve got to position your eye within five millimeters or so, exactly, in order to get an accurate, consistant sight picture.

Why is this? Partly because of the scope’s exit pupil diameter--again exacerbated and increasingly critical at high magnifications. Exit pupil diameter is the amount of light (or in this instance the diameter of the image) transmitted to your eye through the ocular lens of the sight. It’s easy to compute for telescopic sights, binoculars, and so on: simply take the diameter of the objective lens in millimeters (the lens far away from your eye) and divide that number by the magnification. For example, a pair of seven-power binoculars with 35mm objective lenses has a 5mm exit pupil diameter (35 divided by 7 = 5); a pair of seven-power binoculars with 50mm objective lenses has a 7.1mm exit pupil diameter (50 divided by 7 = 7.14).

Your Leupold Vari X III 4.5-14 may have either a 40mm or a 50mm objective lens (Leupold produces two versions). At 14 power, you can see that the exit pupil is only 3.6mm (for the 50mm objective) or a even smaller 2.8mm (for the 40mm objective). While both of these figures are quite good for such a high magnification, they’re still modest nonetheless.

(An aside here--as the iris in the human eye opens and closes, it reveals a pupil pf about 2.5mm under very bright light, of about 5mm in the light of early morning or evening, and of a maximum of about 7mm at night--although as one’s eyes age, the range constricts, too. Therefore, theoretically we never need a pair of binoculars or a scope with an exit pupil diameter in excess of 7mm for light transmission--our eyes can’t use more--although larger exit pupils are easier to pick up ard are less critical in their viewing.)

Parallax, now, is something else entirely, and while of interest it’s not your problem. Parallax is the apparent movement of the target in the scope as you move your eye around the ocular lens.

Still with us? Here’s what you need to do to resolve your particular problem.

One, you MUST line the center of your eye’s pupil with the optical center of the telescopic sight. You mention your shooting bench’s construction as an issue. Fix it. The bench itself is not the problem--it’s the way you, your rifle, the bench and your seat all line up. Simply, sandbag the rifle higher (use anything solid--40-pound sack of cat litter or dog food will work just fine). Just as good, shoot from prone over a solid rest. And remember, that rest (whether on the bench, prone, or in the field) must never contact the barrel directly, but only the rifle’s stock).

Big glass makes precise aiming easier, but the tradeoff is often ergonomic unfriendliness--exactly what you’re experiencing. At the whole other end of the scale, the Leupold M8 2.5-power Intermediate Eye Relief (IER) "Scout Scope" on my Steyr offers a whopping 11mm exit pupil and more than nine inches of non-critical eye relief. Yes, it’s an apples-and-oranges comparison to your 14-power dilemma, but it serves to illustrate the point. I’ve shot with scopes as high as 32 power, and they’re often not very user friendly.

You might try dialing down your Leupold’s magnification for a bit--it is a variable, after all. And take a look at the entire package--how far you can slide the scope in the rings, how high the ring/base combination puts the scope above your rifle’s bore, how high and straight your stock’s comb is.

Your rifle is a machine which needs to fit you as well as a comfortable pair of shore. It sounds as though yours doesn’t just yet--at least not in combination with your shooting bench. Change things until it does--and start with the way the rifle sits on the bench.

Best of luck with that .25-’06--you’ve got a good piece of glass there, albeit a bit on the powerful side. Make it work for you.

Regards, and hope this has been of some help,



Subject: ML...

Posted By: turtle - Registered User

Posts: 176

Posted At: (5/30/02 1:20 pm)

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Many thanks for the information. As usual, you came through. So a little clarification. Eye relief is important and the "sweet spot" should be found. Your eye must be exactly centered with the rear lens or the bullet could print differently? I understand I have work to do in positioning myself at the bench I believe I'll start from scratch. By the way, once I obtained a "full field" of view my second 3 shot group was .636 (Extreme to Extreme and backing out .257). This was with IMR 4831 87 gr. Sierra bullets seated .001 off the lands. Again I Thank You for your help.


Subject: Additional Clarification

Posted By: ML - Registered User

Posts: 287

Posted At: (5/30/02 2:50 pm)

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Your question: "Eye relief is important and the ‘sweet spot’ should be found. Your eye must be exactly centered with the rear lens or the bullet could print differently?"

The two (centering your eye with the optical center of the telescopic sight and proper eye relief) are related.

All telescopic sights or binoculars have a range of eye relief (the "sweet spot") where the image is sharp from edge to edge with none of the black fuzziness around the edges you’ve previously described. Most of the time, as magnification increases, that range decreases, and consequently becomes more critical. In practical terms most low-magnification rifle scopes offer as much as two inches of eye-relief range, and some high-magnification models as little as a quarter-inch. (The eye-relief number you see quoted by most manufacturers is not the range per se, but the distance from the ocular lens to the middle of the range.) So long as your eye is within that range, exact placement of your pupil within the scope’s centerline, while desirable, is not absolutely essential, and the target should not appear to shift its location on the aiming reticle--if it does, then the issue is parallax. When you are outside of the range (the situation you originally described), accurate shooting, while still possible, is exponentially more difficult, and requires that the optical center of the scope and the center of the eye be precisely in line from shot to shot--a difficult task.

So first, you need to do all your shooting within the (short) range of proper eye relief when using your scope set at 14 power. As you’ve noted, when you did, you achieved superior results.

* * * * *

A couple of thoughts on magnification

"If a little is good, and more is better, than too much must be just right." Whether it’s telescopic sight magnification, horsepower, or alcohol consumption, that philosophy--while probably no stranger to most on this Forum--leads unswervingly to one point, and it’s not pretty.

High-magnification riflescopes seem like such a fine idea--the magnification makes truly hair-splitting points of aim possible. And when shooting truly small targets at great distance with a precision rifle, high magnification makes sense--to a point.

High-magnification scopes do have some significant drawbacks, though. First, they tend to be expensive. Second, they’re almost always heavy. Third, in order to produce a usable exit pupil, they must use large objective lenses, often in the 50mm range, and this means they must be mounted high above the bore, a condition which precludes proper head position on many rifle stocks. Above 10-12X, the exit-pupil size diminishes enough to warrant note in anything but bright light. Eye relief becomes critical. They offer more limited fields of view, making it more difficult to locate a target you’ve spotted with the naked eye or with a more modest pair of binoculars (a condition known as "getting lost in the scope). Finally, because they exaggerate errors in hold, in anything but a rock-solid shooting position they result in "chasing the scope" (trying to compensate for the crosshairs moving on the target).

A 14X sight on a .25-’06, by today’s standards, is probably not viewed as excessive by most, providing the shooter is gunning for squirrel-sized targets in the 300-yard range. When shooting something as large as a deer at 100 yards, 14X is a huge liability.

Most shooters who purchase scopes up in this magnification range choose variables, believing them to allow more versatility. They do, but only if used correctly. Always move from shot to shot with your scope set on its lowest power, and only dial up the magnification once you have located and decided upon your target.

Some shooters choose heavy, expensive high-magnification variables because they think, in the long run, they’re actually saving the expense and weight of having to purchase and carry a spotting scope or a pair of binoculars. Wrong! It is an exceedingly poor practice to intentionally point a firearm at anything you do not tend to destroy (Rule Number Two). NEVER use your rifle’s scope to "glass" a target, and don’t shoot with anyone who does.

High-magnification scopes have their place, but they are specialized tools. Before you opt for one, make sure you need its features and are willing to live with its drawbacks. And while those drawbacks are apparent on the bench, often in the field, as the scope’s magnification goes up, often so does the magnification of the drawbacks.


The Four Rules:


All firearms are loaded.


Never let the muzzle of a firearm point at anything you are not willing to destroy.


Keep your finger off the trigger until your sights are on the target.


Be sure of your target and what is behind it.