Originally Posted by lightwind
Bruce: that is an interesting observation. You mention dialing, but is the aberration due to looking at the edge of the view or only when dialing? The reason I ask is that with a BDR you are not dialing, but rather looking at a position of dead center for the scope.
Not an easy question to answer, but I'll try. Optical aberrations that cause image blur can occur in either the objective lens, the erector tube optics, the eyepiece, or all three. The optical aberrations that result from the objective occur when the target is more than about 25 MOA away from the scope tube axis. Such large elevation adjustments can easily be needed for long range targets (>800 yds), especially if there is a 5-15 MOA bore sight misalignment between the scope rings and the rifle bore in the wrong direction. Image blur resulting from this aberration is more apparent when the magnification is high, which is usually also the case for long range targets. It’s a limitation with doublet objective lenses that triplet lenses are able to overcome.
The field of view is what your eye sees when you look into the scope and scan from top to bottom. This field of view is determined by the erector optics. There are other aberrations that occur in the erector tube optics at the edge of the erector field of view, but they are usually more apparent when the magnification is low, which is usually the case for short range targets. These aberrations add to the aberrations from the objective, especially for intermediate magnifications that are sometimes used with FFP scopes to allow the entire reticle to be viewed for long range shots.
BDC reticles are not typically used for really long range targets. They are useful for up to about 10-15 MOA of bullet drop, which corresponds to about 600-750 yds in target range. Above that range, BDC reticles are difficult to make accurate at all ranges simultaneously. At these low elevation adjustments or hold-offs, optical aberrations in either the objective lens or the erector tube optics are typically too small to notice.