Field of view in different scopes (1 Viewer)

[dantheman]

Had a quick scan through threads on here but couldn't see the answer to the following simple question (could be answered through knowledge or a very basic test of a few examples, neither of which I have) -

Telescopes - does field of view change with objective size or magnification?

Essentially will Person A) with a big scope with 90mm objective and a 30x eyepiece be able to see a lot more, or the same amount as Person B) with a small scope with 50mm objective and a 30x eyepiece? (Allowing for different scope manufacturers/designs)

(Eg a Kowa 90mm/Nikon 50mm - modern scope design)

Or does telescope design not work that way? My intuition is getting confused.

Cheers! Dan

 

[Pluraldave]

It depends on the eyepieces. When you look into an eyepiece you see a circle of light with black outside of it. The width of this circle is the apparent field of view (measured in degrees). This will vary from eyepiece to eyepiece depending on the design. You can approximate the true field of view (in degrees) using the equation

True field of view = apparent field of view / magnification

So if you happen to have two different scopes with 30x eyepieces and both eyepieces are designed to have the same apparent field of view then both scopes will show the same true field of view. However, the larger scope will have a brighter image.

If the apparent fields of view of the two 30x eyepieces are different then whichever eyepiece has the larger apparent field of view will show a wider true field of view, not which telescope has the larger objective lens.

If you instead had a 60x eyepiece and a 30x eyepiece, both with the same apparent field of view, then because the magnification is doubled the 60x eyepiece will have a true field half the width of that seen through the 30x.

 

[dantheman]

Thankyou. Clears that one up nicely. :t: