Then you will need to consider your price point, the power setting on the scope, its durability, the reticle, adjustments, size of objective, low light capabilities and of course any accessories.
The price of a scope can run you from $100 up but on average most people will spend between $400 and $1200 to get a good rifle scope. Your scope must be durable to undergo the challenges of the environment and the scenarios you will be using it in. So after choosing a price point you want to look at the adjustments and do some tests. Can you shoot a square even if it isn’t perfect. If not this may not be the scope for you. When turning the adjustments you should feel and hear a clicking sound.
The power setting is also imperative and to determine this you have to take into account what you will be using your gun for. Are you working in law enforcement or are you shooting at deer? How far are your targets? 100? 1000 yards away? Getting a scope with a variable up to 20 is best if you will be shooting far distances and in less than optimal conditions. But then again the power is also dependent on what and where you are shooting from. For a low caliber a power of 4 to 6 is best and if shooting in a lot of wind and mirage a lower power is also ideal.
Check out how you will have to make the adjustments on the scope. Will you need special tools or will you be able to use your hand. If you are looking for a tactical scope it must be one that will turn with your hand. There are scopes with MOA adjustments and those with bullet drop compensators.
As far as the reticle goes you can choose from cross hair, mildot and duplex to name a few. The mildot reticle has been in use for many years by the military. They are great for range estimation as well as holds in elevation and wind and are great for use as leads on mobile targets. But this may not be necessary to all and if not will mean a waste of money. If you won’t be going beyond 100 yards then you can leave these alone.
There are illuminated reticles which are great for low light situations where you couldn’t otherwise see the crosshairs. You will find some that illuminate the entire reticle while there are others that only light up the center of the reticles.
The next thing that is better than illuminated reticles is a night vision scope. These can also have illuminated reticles especially in day/night models. If your scope needs batteries ensure you have extras so you will never be out.
If your weapon has an ocular lens you may not need an expensive scope, all you may need is a one inch chemlight which you will tape to the inside of the ocular lens. This will illuminate the crosshairs but you will want to tape the majority of it up so that the light is not as bright.
A larger objective lens will let in more light so the crosshairs in your scope will remain visible longer even while it is getting darker. The scope is also raised higher above the barrel if it is bigger so this will mean you will need higher cheek pieces and higher rings. This will give a higher silhouette though it is just a small difference. A larger lens will provide more field of view and also lower power.