user39062: I want to make a video audit of my pc screen (4k). The distance would be about 40cm to the centre. Can it be done with much of distortion of the geometry (I cannot place the camera at the centre obviously - this is where my head is) ? How to calculate focus lenght of the lense and other relevant parameters?
Screen size is 40in and it is curved.
P.S. This has to be a separate camera. Software solutions are unacceptable

user12233: There is too much information missing to even start to calculate this. We'd need the screen dimensions [4k doesn't tell us anything we need to know] and camera frame size too [crop factor determines focal length compensation].
Anyway… what if your calculation showed you would need a 30mm lens? You can't buy one of those [easily], so you'd have to use a zoom & set it manually… which would be the same as if you hadn't done all the calculations in the first place.
Anyway, a 27" screen from about 40cm on a full-frame camera would need about a 30mm lens [I did this by eye using as zoom lens as above]. You will still get keystoning because you're not square on, possibly some barrel distortion because of the short lens & softness to the edges unless you close the aperture down a fair way.
The 'trick' to flat field capture is actually to use a long lens from much further away, but this wouldn't fix your keystoning.
All in all, it would be far easier to just record with something like OBS

39062: <p>I want to make a video audit of my pc screen (4k). The distance would be about 40cm to the centre. Can it be done with much of distortion of the geometry (I cannot place the camera at the centre obviously - this is where my head is) ? How to calculate focus lenght of the lense and other relevant parameters?</p>
<p>Screen size is 40in and it is curved.</p>
<p>P.S. This has to be a separate camera. Software solutions are unacceptable</p>
 12233: <p>There is too much information missing to even start to calculate this. We'd need the screen dimensions [4k doesn't tell us anything we need to know] and camera frame size too [crop factor determines focal length compensation].<br />
Anyway… what if your calculation showed you would need a 30mm lens? You can't buy one of those [easily], so you'd have to use a zoom &amp; set it manually… which would be the same as if you hadn't done all the calculations in the first place.</p>
<p>Anyway, a 27&quot; screen from about 40cm on a full-frame camera would need <em>about</em> a 30mm lens [I did this by eye using as zoom lens as above]. You will still get <a href="https://en.wikipedia.org/wiki/Keystone_effect" rel="nofollow noreferrer">keystoning</a> because you're not square on, possibly some <a href="https://en.wikipedia.org/wiki/Distortion_(optics)" rel="nofollow noreferrer">barrel distortion</a> because of the short lens &amp; softness to the edges unless you close the aperture down a fair way.<br />
The 'trick' to flat field capture is actually to use a long lens from much further away, but this wouldn't fix your keystoning.</p>
<p>All in all, it would be far easier to just record with something like <a href="https://obsproject.com" rel="nofollow noreferrer">OBS</a></p>
