How Do You Find The Eye Relief For A Riflescope?
One of the more important features of any riflescope is its eye relief. This is the distance from the scope’s eyepiece to your eye.
At least it is when you have the scope positioned correctly on the air rifle so that you see the whole image through the scope!
Eye relief is a significant feature in scope specifications. And – unlike some other specifications – it’s the same for airgun scopes as for riflescopes intended for firearms use.
But how do you find the actual eye relief for your scope?
The way we do it at Hard Air Magazine is shown here.
We have an optical bench baseplate, to which the scope is attached by a system of posts and rings. The baseplate is very sturdy and precise. It provides a solid way of supporting the scope and – being long – is easy to set horizontally.
Set behind the scope eyepiece is a white card mounted on a rack and pinion stage (as it’s known). This holds the card at right angles to the axis of the scope. It also allows the card to be moved back and forward precisely and under control.
Then a bright light is shone through the objective (front) lens of the scope. The card is moved back-and forward using the rack and pinion stage until the disk of light from the eyepiece is sharp on the card.
When the disk of light is sharpest, the distance between the scope eyepiece and the card is the eye relief for the riflescope. Because everything’s held securely in place by the baseplate, it’s easy to measure the eye relief for that scope precisely.
Oh, and by the way, the diameter of the disk on the card at that point can also be measured. That’s the exit pupil diameter. It’s another typical part of a scope’s specification.
Of course, you don’t need high precision professional equipment like this to get a rough idea of the eye relief and exit pupil diameter of your own scope. You could set your gun on a rest and – with the help of a friend – hold a piece of paper behind the scope to achieve an approximate measurement.
You’ll also notice that both eye relief and exit pupil scope numbers are gives as a range in scope specifications. For example: eye relief 4.3 inches to 3.3 inches. That’s because both eye relief and exit pupil diameter are different at various magnifications of a zoom scope.
Generally, the larger eye relief and exit pupil numbers relate to the lowest magnification for that scope. The smaller numbers are those for maximum magnification.
Understanding scope specifications can help when you make a purchase decision. For example…
A longer eye relief will be useful if you plan to mount the scope on a spring-piston or gas ram-powered air rifle, so that the scope doesn’t hit you when the gun recoils. Most PCPs have next to no recoil and so you often can use a scope with shorter eye relief.
There’s more airgun scope stuff coming in future issues of Hard Air Magazine. We hope you find it interesting!