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Optics with reticles in the first focal plane will most of-ten be found in scopes designed for general purpose or hunting applications. In a second focal plane optic, the reticle is installed towards the rear of the erector, past the lenses that help control the magnification of the image.ow, there are one or two manufacturers that have scopes with reticles in both focal planes but this is more the exception than the rule. The focus of this article though will be the differences between FFP and SFP reticles and what that means to the shooter.
Eyepieces have a focal length too. To use an eyepiece with a telescope, the eyepiece is mounted so that the focal planes of the telescope and the eyepiece are in the same location. Focal ratio, or f/ratio, is the focal length of a lens or optical system divided by its diameter. A telescope with a lensor mirror `that's 90mm in diamter and has a 900mm focal length has an f/ratio of f/10. But what does this mean? F/ratio isn't a measure of quality. There are good f/15 scopes and good f/3.3 scopes. Varying the f/ratio allows designers to trade off different scope characteristics. For example, for a given design, the lower the f/ratio, the smaller the scope. But at the same time, the longer the scope, the easier it is to achieve good optical performance.