If you have an adjustable magnification scope, the adjustment is usually done by a ring just before the ocular housing. For some scopes like Nightforce, the whole ocular housing needs to be rotated to adjust magnification. (This is the reason why I don’t like Nightforcescopes). The target image of all power scopes can be adjusted to increase or decrease in magnification. Some scopes have a reticle that increases in size in proportion to the target image, while others keep the reticle the same throughout the magnification range.
These scopes are known as First Focal plane (FFP), or Second Focal plane (SFP).
First focal plane:First focal plane riflescopes(FFP) adjust reticle with target image. You can use any graduated markings to measure at any magnification setting. This makes it possible to estimate range using Mils. However, this feature comes with some cons. The scope may not be as thick or thin as you need it to be at high magnification ranges. Additionally, it will be more expensive. FFP scopes can be a good choice for long distance target shooting.
Second focal plane:Second focale (SFP) riflescopes only adjust the target image. The reticle is the same size. This can be helpful for always having the right size reticle to aim with and see, but it can also cause problems if the power setting is incorrectly set. You can see that if your reticle has MOA or Mil marks, they will not be as large at full power. You can reduce the power setting to make the marks smaller but the image will shrink. If you want to do the math, you can (e.g. If you use half the power, your marks will be twice as large. This is great for hunting scopes as SFP are stronger (less moving parts) so you won’t lose sight of your reticle even in low light. Nightforce scopes are an exception from the “full power” rule. Nightforce scopes sometimes have a random dot at the power ring, where the marks are precise (reason number two I don’t like them).
STOP USING FULL MANIFICATION ALL OF THE TIME! Lower magnification will allow you to shoot more accurately. There will be less movement, and you will need to focus on your reticle (good), rather than a large target (bad). Lower magnification also makes it easier for you to find your target.