Part 4 – How to set Ocular Focus
Last but not least in this series of setting up your precision rifle comes ocular focus. A quite a few people I’ve meet over the years of training have been perplexed by the purpose of Diopter located on the back of the optic, otherwise known as the ocular housing or bell. The best way to think of this ocular focus adjustment, is its function is setting the prescription of your optic to match your eye, much the same way the doctor sets the prescription for your eyeglasses. If you don’t take the time to set this adjustment, it can have some pretty painful results depending on the severity of the miss match.
To properly set the ocular focus, first we’ll need to turn the parallax adjustment all the way to infinity (that’s usually denoted by the sideways “8” for you crayon eaters). Next we’re going to back the ocular focus waaaaaaay out. There two types of adjustment that you’ll normally see. Fine and Quick. Quick adjustment oculars will allow you to traverse the range of adjustment in a few turns, but a fine adjustment will have you spinning for a while. Once you are backed out, go ahead and hop down behind the rifle and aim at a nice bright background that allows for high contras of the reticle.
Unless you happen to have some really bad eyes you should almost immediately see that the reticle is fuzz as hell. You’ll probably notice too that if you strain your eye hard enough that it will become a little bit clearer, and that straining of the ocular muscles is exactly what we want to avoid as this fatigue is unproductive for marksmanship and can have negative long-term effects.
The correctly focus reticle is seen above, if the reticle looks like the one below, then you'll need some adjusting.
From here having a buddy on hand will help out a bit and make it go much quicker, although theoretically you could do it by yourself if you have no friends. Basically, while lying behind the rifle, looking at that high contrast background, you’ll want your buddy to cover the optic either with a hand or paper so that it is dark as you look through the optic. Then when you’re ready they will remove the cover and you are looking to see if that reticle is instantly clean and crisply in focus. Key word is instantly, if given enough time (usually just a second or two), your eye will automatically begin to try and focus and that would defeat the whole process.
If that reticle isn’t good instantly, then you’ll need to adjust the ocular focus. If you have the fine adjustment, I’ll normally make 1 or 2 full rotations in, or those with the quick adjusting ocular focus you, can probably get away with a ¼ of a turn. Once adjusted this process of buddy assisted focus checking is then repeated as many times as necessary until you the reticle is presented to you in an instantly clean and crisp image.
It. Is. Finished!
Congratulations, with the ocular focus set you have completed the basic set up for your precision rifle! At this point I normally like to take a paint pen and place a witness mark on the any of the adjustments, like cross bolts, the ring caps and ocular adjustment ring. It is arguable and normally not necessary but it’s cheap insurance if something does come loose and will save you a lot of time and head scratching in the odd chance it does.
Understanding Cold Bore