Jump to content


A Quick Question About The F Word

Recommended Posts

The F here means focal ratio, that is the relationship between the diameter of the objective lens or mirror and it's focal length (the point at which light comes to a focus). So the 120mm F/5 has a focal length of 600mm and the F/8 (F/8.3 I expect) is 1000mm.

The F/5 is good for low to medium power viewing, ie: deep sky objects etc while the F/8.3 is better for higher power viewing, ie the planets, the moon etc. Both scopes can do both things but each has their strengths and weaknesses.

Achromat refractors produce something called chromatic abberation (CA) which is where not all wavelengths of light can be bought to focus at exactly the same point. This means you will see some purple fringing (false colour its sometimes called) around the moon, brighter planets and stars. This is more pronounced in the F/5 scope.

Thats some info for starters anyway !.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.