How focal length compares between an EP and a camera lens

[JudeHS]

So I was wondering why a telescope eyepiece and camera lens behave differently when it comes to focal length and magnification. I don't have any experience with camera lenses but from what I understand a longer focal length will result in a smaller field of view. However, with a telescope eyepiece, a shorter focal length will result in a bigger f.o.v. Any ideas as to why this is - I'm guessing it's to do with the telescope and camera being different? 

[Ruud]

The eyepiece is a magnifying glass with which you study the image from the telescope's objective. A magnifying glass magnifies more when it allows you to study an object from a shorter distance. Thus the shorter the focal length of an eyepiece, the larger the magnification it provides.

 

 

 

Edited September 26, 2019 by Ruud

[Anthonyexmouth]

you have to compare focal length of telescopes and camera lenses.. then they work the same way

eye pieces are an additional step 

[vlaiv]

Different things.

Lens and telescope when used at prime focus (in this case also acting as a lens) is a projection device. It transforms angle into linear distance. Star at a certain angle to optical axis will be focused at a certain distance in focal plane.

Telescope + eyepiece is more like angle amplifying device. Light coming out of the eyepiece is not converging, it is still parallel rays but at a larger angle. So star at a certain angle to the optical axis will end up as being parallel rays exiting eyepiece at larger angle (ratio of angles being equal to magnification you are using).

Human eye is also projection device much like lens or telescope at prime focus - it converges parallel beams (for object at infinity) to a spot on retina and photo sensitive cells pick this up and form a signal. Actual image is a projection on your retina in the same way lens projects small image onto camera sensor surface.

If we extend this analogy - telescope + eyepiece is more like teleconverter then camera lens - it changes magnification/focal length of human eyeball.

Because of this, different maths applies to calculating things. With lens it is focal length that is proportional to size of projection, while with telescope+eyepiece (being compound lens akin to teleconverter), magnification is determined by ratio of focal lengths of telescope and eyepiece.

[JudeHS]

I understand now thanks for the replies!