How does focal length change Exposure? [Astrophotography]

[koulea]

Hi, this is my first post to this forum...so bare with me. (I'm super excited!!!)
So, I read in an article about night photography how the focal length can affect star trails.
I understand exposure and the way focal length changes depth of field but in no way do I understand how focal length can have an effect on star trails.
Does focal length really affect the trails or is it just the focal length that limits the aperture that in turn changes the exposure?
I hope someone can clear away my confusion

Thank you in advance

-Koulea

 

[Overread]

I would assume that a long focal length records less of the sky and thus you'd get straighter star trails because you've "cut" a more selective part of the sky out. Whilst a shorter focal length lens is going to have a much wider angle of view and thus show a larger section of the sky, enough that the trails can now show their curvature more clearly.

 

[WayneF]

Speaking of camera lenses, focal length will change the width of the view you see, but if at the same f/stop number, the exposure should remain the same. That is what f/stop is... the same exposure at the same f/stop number, regardless of focal length.

 

[KmH]

If you are not using a dark site for your astrophotography, a longer focal length will have a narrower/smaller field-of-view (FoV) and less light pollution in the FoV will yield a somewhat darker sky when you do a long exposure.

A longer focal length will also magnify the length and width of any star trails, just as it magnifies terrestrial objects like tree limbs.
By necessity star trails in photographs are stars that have been over exposed. That over exposure makes the star look a lot bigger in a photo than it really is and makes the star trails bigger too. Note that dimmer stars have noticeably narrower star trails than do the brightest stars in the photo.