Jump to content


Primer - Focal Lengths and Ratio's

Kaptain Klevtsov

Recommended Posts

Here goes with a quick go at the mystical numbers on your telescopes and how to decode them.

I'll start off with some definitions in case some of these are non-obvious, the whys and wherefores of the f/ number will have to wait until I can condense the f/ concept better as I've just tried and messed up.


This is the diameter of the hole that the light enters the telescope through. A 100mm refractor will have a front lens of roughly 100mm. A reflector will have a tube slightly larger than the aperture as the actual aperture is the primary mirror (the tube is merely a way of holding all the bits together).

Focal length

This is the distance from the primary (front lens in a refractor or the big mirror at the bottom of the tube in a Newtonian) to the focal point. The focal point is where you put the eyepiece or camera.

f/ ratio

This is the relationship between the aperture and the focal length.

So what does all this malarky mean?

The significance of these three numbers is, in simple terms:-


More aperture means that you are collecting more light and more light makes the image appear brighter. Bright things are easier to see and to photograph. More is better.

Focal length

This has a direct bearing on the magnification. No other number is involved as far as the telescope is concerned. The magnification is calculated by dividing this number by the focal length of the eyepiece. Using the above example and a 10mm eyepice gives a magnification of 1000/10 = 100. If you use a 20mm eyepiece the magnification goes down to 1000/20 = 50.

For imagers that doesn't help much so there's another way of looking at it. If you could look through a hole the size of your camera's sensor you would be able, by moving your eye from side to side, to see a section of the sky. Bigger sensor, more sky. To work it out more exactly you need to imagine that all the light comes through a tiny section in the centre of the lens or primary mirror so you can use the focal length of the mirror and the sensor size to work out how much sky you can "see" using that sensor. If you sketch an isosceles triangle with a base dmension equal to your sensor width and a height equal to the focal length of the 'scope, the angle at the pointy end is the angle which also covers the piece of sky that you can image widthwise. Do the same for the height and you get the angle corresponding to the amount of sky you can image in the other direction.

To be continued...

Captain Chaos

  • Like 2
Link to comment
Share on other sites

This topic is now closed to further replies.

  • 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.