The first thing to know is that the magnifying power of a telescope is limited by the amount of light that the **telescope aperture** can collects. The two other important values for accurately calculating the magnification are the **focal length of your telescope** and the *focal length of the eyepiece*. These two values are indicated on a label placed on both items. The magnification formula is quite simple:

The telescope FL divided by the eyepiece FL = magnification power

**Example:** Your telescope FL is 1000 mm and your eyepiece FL is 20 mm. 1000/20= 50x! The image seen in your eyepiece is magnified 50 times!

Being able to quickly calculate the magnification is ideal because it gives you a more:

**Flexibility**, because each celestial object has a different brightness (magnitude) and you may want to quickly increase or decrease the magnification when changing targets.**Responsiveness**, because the weather conditions can greatly affect your viewing and you want to be able to adjust your magnification quickly and accordingly.

**Note:** The **focal length of your telescope** is a fixed value that cannot be changed, so the only way to increase or decrease the magnification is to change the eyepiece or Barlow lens.

**Lowest Useful Magnification**: You can calculate your telescope lowest magnification by multiplying your aperture (in inches) by 3 or 4 times. For a 4-inch telescope, the minimum useful magnification is between 12x and 16x. At this power, you will have a wider field of view (FOV) and a brighter image, which is ideal for large celestial objects such as galaxies and nebulae.

**Highest Useful Magnification:** Calculating the maximum useful magnification of your telescope is quite simple. If you work in inches, you multiply this value by 50. So, for an 8-inch telescope, the maximum magnification you can use is x400. If you exceed this value, you will get an overly magnified image.