Lens Field of View

[groberts]

I want to input the field-of-view of a photography lens and DSLR camera into CdC to asssist with framing.  This is quite simple with a telescope etc but when considering a camera lens set-up seems to become more complicated.

For this pupose - is the correct aperture to use e.g  for a 100mm lens with at f4 the the aperture is 25mm or 50mm at f2?

Is therefore the correct number to use i.e. in this case either 25mm or 50mm depending on what f-stop you use, or is the size of the front lens element relevent?

However, I presume that with say a DSLR the crop factor also needs to be included e.g. the CF for the Canon  700D is (I think) 1.61 so the previous apetures now become 54.33mm or 108.60mm (135 x CF / fstop).

Is this correct or am I missing something?

Graham

  

[Kyle Allen]

I don’t see how the crop factor would affect aperture of the lens. Surely, you just tell it your sensor size and focal length to work out the field of view. 🤔

[groberts]

1 hour ago, Kyle Allen said:

I don’t see how the crop factor would affect aperture of the lens. Surely, you just tell it your sensor size and focal length to work out the field of view. 🤔

OK maybe I'm getting things muddled - but am I therefore correct that putting that aside the aperture for say 100mm @ f2.0 is 50mm? 

[Kyle Allen]

Yes that’s correct. 🙂

[michael8554]

On 25/06/2020 at 15:58, groberts said:

a 100mm lens with at f4 the the aperture is 25mm or 50mm at f2?

For a photographic lens, the aperture - the iris opening - is specified with f numbers, so f4 and f2 are the aperture in your example, not 25mm or 50mm

Astronomical telescopes don't have an iris so the aperture equivalent is the size of the opening, the diameter of the objective.

So for your FOV frame you enter the focal length and camera sensor dimension, and if necessary add the front element diameter as the Aperture.

Michael

[johneta]

Try stellarium too. Its quite easy to setup a FOV for telescopes or lenses. Ive got my scope saved and about 8 different lens focal lengths.

You input you sensor dimensions and it calculates the crop factor for you.

[endless-sky]

Hi, in order to calculate the FOV of your imaging lens / camera combination, you do not need to enter the aperture. All the program (Cartes du Ciel, Stellarium, etc.) needs is focal length and sensor dimensions. Since in most cases the "real" focal length is not always exactly equal to the "nominal" focal length, to get the exact field of view you could also plate-solve an image of the night sky you took with that camera / lens combination (it doesn't need to be an extremely long exposure, a few seconds at high ISO is good enoug: the program just needs a few stars to be visible in the picture).

This is an online solver that you could use: https://nova.astrometry.net/upload

Choose the image file from the location of the PC where you saved it and click "Upload". Wait a few minutes and you will get the exact results of your camera / lens FOV. Then you can input that in the planetarium software you use and have a perfectly matched FOV to your camera / lens.