Hugh explains the difference between a first focal plane reticle and a second focal plane reticle.
There are many factors to consider when shopping for a new variable-power riflescope, from the magnification range to the price range. A potentially important factor that is frequently overlooked is the location of the reticle within the scope. With reticles, as with real estate, location matters.
In a variable-power riflescope with the reticle in the first image plane, changing magnification enlarges or reduces the reticle image along with the target image. The reticle always covers the same percentage of the target area.
The reticle of a variable-power riflescope may be located in the first focal plane, associated with the objective lens; or it may be in the second focal plane, associated with the ocular lens. The practical consequences of the design choice are immediately evident when using the scope. A first-image-plane reticle changes size in lock-step with the target image as you change the magnification setting. Increase the power, and the target and reticle grow simultaneously. Decrease the power, and they both shrink. Either way, the reticle always covers, or subtends, the same amount of the target.
When the reticle is in the second focal plane, changing power setting increases or decreases the size of the target image, but the reticle remains a constant size to your eye. As the target image is enlarged, the reticle covers less of the target. As the target image is reduced, the reticle subtends more of it, possibly obscuring areas you might prefer to keep in view.
For most shooters seeking a general-purpose riflescope, the reticle location isn't really critical. The choice boils down to individual preference. If you would rather look at a reticle that maintains size consistency with the target, go with a first-plane model. If you want to see a constant-size reticle that looks the same regardless of target magnification, choose a riflescope with a second-plane reticle. Reticle location does become critical if you're in the market for a variable-power riflescope with range-estimating capability.
With a variable-power riflescope that has its reticle in the second image plane, changing magnification alters the size of the target image while the reticle image remains constant. At higher magnifications, the reticle covers less of the target area; at lower magnifications, the reticle blocks more of the target area.
Many such systems are based on visual comparison of a target or other object of known size with reticle features such as mil dots, graduated stadia lines, or other designated portions of the reticle. With a first-focal-plane reticle, which expands and contracts in synch with the target image, you will be able to employ the comparator guide marks at any magnification you select as appropriate to making the shot.
If the riflescope has a second-plane reticle, you will be able to use the comparator to estimate range only at a single, specified magnification, which will be indicated in the owner's manual for the scope. That means you will have to discipline yourself to set the power to the proper range-estimating value first, then reset it to the magnification you deem best for executing the shot.
Although this isn't the most onerous procedure, you do have to pay attention to what you're doing. Clearly, any range estimation made at an incorrect power setting will be off the mark, perhaps wildly so. What's the best way to avoid this pitfall? Follow the advice given to the tourist in New York City who asked a New Yorker how to get to Carnegie Hall--practice, practice.