Table of Contents
Parallax or parallax error is a phenomenon that occurs when the apparent difference in the position of an object is viewed from different lines of sight.
An everyday example of this is in a car with a speedometer that has a needle to indicate the speed. To the driver the markings on the speedo are directly behind the needle therefore, by design, gives the most accurate reading of the speed indicated. In the passenger seat there may be another set of eyes looking at said instrument. Due to parallax (viewed from a different line of sight) the indication may appear higher or, if you drive on the right side the road where you live, it may appear lower.
This can have a marked difference in accuracy using scopes at longer ranges. We’ll look at the concept of parallax and how it affects rifle shots in this article. We’ll also go over how to compensate for parallax error and achieve the highest level of accuracy possible.
Parallax - Reticles and focal planes
The reticle at the Second Focal Plane
Parallax in Rifle Scopes occurs when the viewed object is on a different focal plane to the reticle. The object is represented as a picture by the objective lens at the First Focal Plane (FFP). If the reticle and object image are not in the same optical plane then this causes the image to be unclear or to move in relation to the reticle.
Note: As the image changes note that the crosshairs move also!
The reticle at the First Focal Plane
Note: As the image changes in this case note that the crosshairs remain in their original position!
Parallax - How to correct it
The key to correcting parallax is to focus both the reticle and the object or target. This would difficult with the naked eye as the reticle might be 15cm from your eye and the object might be 150m. With a scope however, as we have discussed in the previous sections, the image of the object (objective) is represented at the First Focal Plane (FFP) by the objective lens.
Methods of adjusting parallax on scopes with adjustable focus
Optics only have the ability to see clearly (focus) at one distance. The are various methods employed to adjust focus in a scope:
- Adjustable Objective Lens(AO)
- Adjusted by the Adjustable Objective Ring
- Side focus lens
- Located between the objective lens and erecting lenses
- Adjusted by the Parallax or Side Focus Turret
- Rear focus lens
- Located between the erecting lenses and the ocular lens
- Adjusted by the Rear Focus Ring
- Ocular lens
- Eyepiece or Diopter Focus Ring
The first three methods focus the object. Whereas the last method (4) brings the reticle into focus.
Scopes without adjustable parallax or focus
Typically, scopes without adjustable parallax or focus are used at ranges within 200m (a usual hunting scenario) and are of lower magnification (zoom). In these ranges the effect of parallax is much less. Furthermore, scopes with a fixed objective have a very wide field of focus. The objective focus is set by the manufacturer and can be from 50m to 200m.
It really is a matter for horses for courses. If you are operating with low zoom and at small ranges, adjustable focus scopes may be an expense you can do without. On the other hand if you are operating at bigger distances then some form of adjustable focus is probably a very worthy addition.
- Always focus the diopter or eyepiece (reticle) before you touch anything else.
- Scopes with the reticle at the FFP do not need parallax adjustment
- The most common reticle location is at the SFP
- The Adjustable Objective Lens (AO) is the most common focal adjustment
- The further the range the larger the parallax error.