Field of View question

[Donald Samuels]

I use a RedCat 51 on a Canon 90D.  I am trying to figure out what its
focal length is with this combination.

The 90D has the APS-C sensor so normal calculation of a full frame lens
on this camera is to multiply the focal length by 1.6 giving a 400mm
focal length.  When I look at one of my submissions to nova.astrometry.net,

https://nova.astrometry.net/user_images/12272827#annotated

It says the field of view was 5.13 by 3.42 degrees. When I use a
calculation to convert this to focal length (d/(2*tan(theta/2))), it is
giving me approximately 230 by 250 Can someone explain what I am doing
wrong?  I think FOV is the correct way to do the calculation.

[Dustin Lang]

For 5.13 deg, I get

35. / (2*np.tan(np.deg2rad(5.13)/2))

= 390.6

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[Donald Samuels]

Sensor size on a 90d is 22.3mm by 14.8. I am getting 223mm focal length. Using your 35mm size I don't match you, I get 350.9 (both by excel and an old hp11c). Perhaps some one has put up on astometry an image with a full size sensor and a 250mm lens??

[Dustin Lang]

Hi,

If you plug in 22.3 mm and 5.13 deg, that's 249 mm:

https://www.wolframalpha.com/input?i=22.3+%2F+%282*tan%28radians%285.13%29%2F2%29%29

or 14.8 mm and 3.42 degrees,  that's 244 mm:

https://www.wolframalpha.com/input?i=14.8+%2F+%282*tan%28radians%283.48%29%2F2%29%29

cheers,

dustin