# How to calculate lenses parameters to record a pc monitor

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

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

• Sorry the screen size is 40in. What kind of lens should I have if I position camera 100cm away (disregard distortion for now)? Aug 14, 2022 at 17:04
• Just get a zoom. You still didn't tell us the camera frame size. This really isn't anything I've ever bothered to try calculate. You just hold up the camera & pick a lens. Previous experience will tell you a ballpark figure, the rest is simply empiricism. Aug 14, 2022 at 17:14
• I thought to get camera for the lense. I don't even know what frame size is. And I am sure to select a zoom lenses I will have about the same amount of questions or even more. My previous experience is custom cctv system for my house 10 years ago. Also I did photography when I was a child. On film, developing and printing it myself. That was 45 yrs ago. Aug 14, 2022 at 17:25
• I edited question - took first good looking item from aliexpress. They aren't difficult to find Aug 14, 2022 at 17:32
• Honestly, if you're about to throw \$1000 at a camera system, you really ought to do some research first. At least visit a camera shop & talk to the staff. You're starting with a difficult goal, one which will probably not give good results & no clue of what equipment you need. This is not a good place to start. The lens you just added seemingly at random would not be even vaguely suitable, even if you can figure out what camera it would fit. Aug 14, 2022 at 17:37