Question about eye relief vs. field of view

[ILoveBigRacks]

Okay, honestly don't know just what it's called but figured I'd toss out a few of the only technical terms I actually knew .

I've got a new Leupold VX-III 4.5-14 mounted on my hunting rifle and am having a few issues I hadn't anticipated. I found a lot of my shots were getting longer (or my eyesight worse more likely ) so wanted to go a bit more magnification over the 2-8 I previously had on it. Anyways, what I'm finding is that when taking shots at 14 power on the scope, to get the full field of view my eye has to be just about an exact distance from the scope - a tiny bit closer or farther than perfect and the black closes into the field. What exactly is this called and is this viewing distance variable from scope to scope? Kind of pointless having a scope with 14 power top end if it's unpractical to use in the field when you need it.

I know, I know, how often do you actually have use for 14 power when hunting? Personally, I like a bit more magnification if I have a good rest - easier on my eyes. Offhand is another matter, but that's why they make variable scopes. Hey, don't judge me .

 

[Panman]

That's called the Depth of Field, and it always shrinks as magnification increases. The Focal Point is the spot at which the image is sharp (the location we call correct eye relief), and the Field of View is the dimension of the viewable image at the given Focal Point.

No real way around it, if you have a zoom lens, your depth of field will shrink as you zoom, and that effect is mirrored on your side of the lens. There are a few ways to increasing the range of the Depth of Field, but all come with a trade-off. In a camera, you would decrease the Aperature (the size of the lens hole) to increase the Depth of Field, but that would mean less light gathering ability, and poor low-light performance. You can compensate for that by increasing the relative size of all components of the system, or with increased quality of glass, but that drives up cost, and may make for bigger, heavier optics. This is why cameras have flashes, to increase the illumination allowing more light to reflect off the subject and compensate for limitations in aperature. All done cheaply. So the simple answer is to carry a big ass flash.

But seriously, each manufacturer will likely have a slightly different depth of field based on the size of the tube, objective, and quality of glass, so shop around. You may be able to pay more and get more, or you may be comfortable to sacrifice something (like extreme low light performance) if you don't need that feature in your style of hunting. Ultimately though, you are going to have to deal with a limited Depth of Field and a narrower focal point in a zoom optic.

 

[Juster]

This is relevant:
http://www.petersenshunting.com/content/scope-bite

Maybe repositioning the scope will make it easier to deal with the problem.

 

[Dogleg]

It sounds like you're talking about "eyebox", the range of distance that your eye can be from the scope and still be useable. There is a point where it's perfect, a certain distance back and forth that is "good enough", then when you exceed that distance you get the doughnut effect.
When you position your scope, do it with the scope set at the highest power in the shooting position that you will be using that power. For me that would be prone with a bipod.
As you noted the "eye-box" is the smallest at the highest powers, so set it up to make the most difficult task as easy as it can be, and let the lower powers take care of themselves.

 

[Mystic Precision]

The ease to see through a high mag scope is directly related to the eyepiece. Some scopes are more sensitive to eye location and position. Others more generous.

That is why I really like the bushnell Elite 4200's. They have an easy to use eyepiece even on their high mag rifles. But some of these have fairly short eye relief. On a lighter kicking target or varmint rifle, excellent. Not so good on a boomer.

I am really enjoying and impressed with the new Gen Sightron SII and SIII scopes. They have a easy eye box and HUGE eye relief. Glass is also right at the top of available scopes at any price point.....yes, they are great that way.

I presently have a SII big sky 6.5X20 and would have no issue using the 20X in the field with good eye relief and a nice field of view.

Jerry