SCOPE QUESTION

[robkarrob]

I see the specs on gun scopes, but do not understand the end spec, such as 3-9x 40mm. I know the 3-9 is a variable power magnification, but what is the 40mm. I've seen 40mm, 32mm, 50mm, etc. Could you please explain what this means and what it affects. Thanks

Bob

 

[adluginb]

40mm is the size of the end of the scope. It mainly determines how much light can get into the scope when using it at dusk or dawn. Typically the higher the number the brighter the scope. Of course there are exceptions when you get into higher priced scopes that have better glass in them. I have looked through 32mm scopes that rival 50mm scopes in terms of brightness just because the 32mm scope used better glass.

 

[Charlie Foxtrott]

Like the above poster pointed out.

The third number is the size of the objective lens. As stated this determines how much light the scope will capture. This makes a diference in low light situations.

However, the larger your objective lens is. the higher that you have to mount it above the bore axis of the barrel of your rifle. This in turn increases your parallax. This being more or less the diference between where your barrel is pointing and where your scope is looking. Ideally you want to have the least amount of parallax possible.

Now you can not get away from having some parallax. But the general rule of thumb when mounting a scope is to have it as low down and far forward as possible.

 

[robkarrob]

Thanks for the replies. I also got another question answered without asking, parallax. I did not know what this meant and now I do. I have laser sights on my handguns and would guess this parallax also affects the laser and POI, to some degree, although not as much because of much shorter shooting distances.

Bob