f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
have to change either aperture or focal length.

But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Saturday, May 12, 2018 3:21 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
have to change either aperture or focal length.


Bryan