I know that this topic has been discussed a few times before , but I'm trying to find the flaw in my way of calculating the magnification equivalence for imaging.

Imagine a square postage stamp glued to a wall 10 meters away from a telescope on a tripod. You insert an eyepiece and note that when viewing the postage stamp, the stamp's diagonal exactly fills the field of view.

You then replace the eyepiece with a camera with a square sensor. The image exactly covers the postage stamp; there is no border around the stamp and there is no cropping.

Could you not argue that the camera "magnification" is the same as the telescope-eyepiece magnification? After all, if you look at the image at a later time, you will say, "Yes. That is exactly what I saw in the eyepiece." (Ignoring that the eyepiece view was round and that the image is square.)

**Eyepiece:**

TFoV = (AFoV / TFL) × 60

where

TFov: True Field of View in arc minutes

AFoV: Apparent Field of View

TFL: Telescope Focal Length

Mag = TFL / EFL

where

Mag: Magnification

EFL: Eyepiece Focal Length

**Camera:**

TFoV = (Diag × 3460) / TFL

where

Diag: Diagonal dimension of (square) sensor

So

(AFoV / Mag) × 60 = (Diag × 3460) / TFL

Solving for Mag

Mag = (AFoV × 60 × TFL) / (Diag × 3460)

Example:

Plössl with AFoV of 52°

Telescope with FL of 2000 mm

Sensor with width and height of 11.31mm (Diag = 16mm)

Sensor "Mag" = (52 × 60 × 2000) / (16 × 3460) = 112.7

If I used an 18mm Plössl eyepiece in this telescope, I would see exactly the same field of view as the camera sensor.

What is wrong with my logic?

**Update: **spelling correction

**Edited by grkuntzmd, 17 April 2021 - 08:20 AM.**