Ask Foghorn: Rifle Scope Parallax

A reader asked:

I’m curious about scopes. Top Shot’s George, while describing his setup process for the 1000 yard shot, said (among other things) that he adjusted something because he didn’t like the scope’s “parallax”. I know approximately what parallax means, and can guess how it applies to scopes, but have no idea how to set up a scope for a shot like that.

In short, for those of us not blessed with $1K scopes, what are all the little knobs and dials and fiddly things for?

OK, let me take a shot at this…

Parallax is not some fancy science concept that only physicists think about; we experience it every day. Imagine driving down a divided highway with some telephone poles in the divider, trees on either side, and mountains in the distance. From your point of view, the telephone poles will seem to whip by at lightning speed, the trees will appear to leisurely stroll along, and the mountains won’t appear to move at all. All of these objects are moving in relation to your car, but you perceive them as moving at different speeds depending on their distance from you. This motion is called parallax.

Just stare at the above moving GIF for a bit (shamelessly stolen from Wikipedia). Close objects move faster than far away objects.

I can hear it now. “Hey Foghorn, this is interesting and everything, but what does this have to do with slinging lead?” Think about it a little longer. When we’re shooting, we usually have a close object (the reticle) moving around a distant object (the target). If the parallax isn’t adjusted for, the crosshairs will move around the target depending on where our eye is in relation to the scope. In other words, without adjusting for this parallax the crosshairs will move around from shot to shot.

Let’s take a look at how a scope works to get some more insight on this issue.

In this additional image shamelessly stolen from Wikipedia, the parts of a scope are displayed in a cross section diagram.The red lines indicate the path the light takes through the scope, being focused by the lenses. Remember, the image is flipped twice before it gets to your eye, and the flipping takes place where the red lines cross. Think back to your high school physics class (or just play with a magnifying glass for a bit) and this property of lenses should seem familiar. Those crosses are called “focal points” or “focal planes.”

Just taking a quick detour (since we’re already here, and a nifty attraction is not too far off) when talking about reticles on scopes they’re usually referred to as being in the “first focal plane” (FFP) or the “second focal plane” (SFP).

The image for the reticle needs to be at one of the focal points (in the figure above where the red lines cross) in order for them to be in focus when the crosshairs are on the target. With fixed power scopes it really doesn’t matter which focal plane you pick, but on variable power scopes the difference is gigantic.

• First Focal Plane reticles remain at a constant size compared to the TARGET.
• Second Focal Plane reticles remain at a constant size according to the SHOOTER’s POINT OF VIEW.

In other words, with increased magnification an FFP scope reticle will appear to get larger but a SFP reticle will appear to remain the same size. The reticle (or corsshair or whatever else you want to call it that my spell check doesn’t believe is a word) is printed on a flat piece of glass at the chosen focal point, not a curved piece like the rest of the lenses.

Back to our original question, the reason for this parallax issue is that the focal point for the target isn’t always the same as the focal point for the scope. Telescopic sights are designed to be used at a certain distance (typically 100 yards for hunting, 50 yards for rimfire, 300 yards for military) and will be parallax free in and around those distances.

But when you’re shooting something at distances hundreds of yards off from the designed distance you need to be able to adjust the focus so that the focal planes line up with where they were designed to be. If the focal planes don’t line up, you will only be able to focus on the crosshairs or the target but not both and parallax will be a problem.

TO EXPAND on this concept (as it really is pretty confusing), here are some illustrations. Pretend that the pictures with the dotted lines are the section of the telescopic sight pictured above where the first focal plane and the reticle are (the only flat piece of glass in the tube). My MS paint skills aren’t what they used to be, so bear with me.

The dotted lines in this illustration indicate how the light was DESIGNED to pass through the scope. Just like with the magnifying glass, there’s a distance you can hold it from the ground where the light will create one very intense spot of light (which, I am told, is harmful to ants). That point where the dotted lines cross is exactly like that very bright spot, all of the light is concentrated there.

The red lines in this image indicate the ACTUAL path of the light from the target through the scope. Because they line up with the intended path of the light, the target and the reticle will both be in focus simultaneously. The multi-colored monstrosity on the right is my attempt to draw a target. I know, I failed kindergarten. All of the light is concentrated on the reticle, making both the target and the reticle appear in focus.

But what if the target is at a different distance than the designers of the scope intended for it to be? The focal length will be shorter or longer and change the point at which the light is focused in the scope. The ACTUAL focal point will be different from the DESIGN focal point and the light won’t be properly focused on the reticle.

When the light is improperly focused on the reticle, you will be able to see either the target or the crosshairs in focus at any given time but not both at once.

When either the target or the reticle is out of focus, parallax comes in and starts screwing with your shots. Any slight movement of your head between shots will alter your point of aim, even if your crosshairs still appear to be on the bull’s- eye. I spent a good 3 hours on the range once trying to zero a scope without adjusting for the parallax, and ended up holes AROUND the bull’s-eye but never IN the bull’s-eye.

To fix this issue, scope manufacturers will typically include some way to adjust the scope in order to make the focal planes align with where they are designed to be. The controls for these adjustments are usually either on the front of the scope (moving the “objective” lens, which is the first lens on the scope facing the target) or on the side (moving internal components). Scopes with the adjustments on the front are called “AO” or “A/O” models.

When adjusting for parallax, simply turn the knob until both the reticle and the target are in focus simultaneously (or as close to in focus as possible). This will eliminate the parallax at that distance and make for more accurate shooting.

As to what the other fiddly bits do, the other knobs in the middle of the scope are typically for windage and elevation adjustments. These knobs are secured either by a cap  of some sort, a nut that applies pressure (which you must unscrew and relieve to adjust the scope), or the scope maker may simply make the knobs extremely tough to turn. Scopes may also include an adjustment ring on the back of the scope to adjust the focus, but this is NOT the same as adjusting the parallax. This back ring adjusts the entire image, parallax and all.

TL;DR: Make sure both the target and the crosshairs are in focus simultaneously by turning the knob indicated as adjusting for parallax.

Relevant Wikipedia articles (for further reading or time wasting):

• Parallax
• Telescopic Sights
• Reticles
• Lens

If you have a topic you want to see covered in a future “Ask Foghorn” segment, email [email protected].