#### lostinpangea

##### n3wb

I purchased one IPC-HDW2231R-ZE to do some testing in multiple locations and found that it worked very well. I'd now like to use the IPC-T5442TM-AS in some of those locations in order to take advantage of the higher resolution.

The challenge is figuring out which focal length to buy. This is because the focal lengths, sensor sizes, resolution, and angle-of-view are different between the cameras. I'm trying to find a way to calculate the correct focal length to choose for the fixed lens camera based on the settings of the varifocus camera.

I know that this has been discussed here: Using vari-focal camera to determine focal length of fixed-focus camera and Two cams with 12mm lens, but VERY different FOV. Why? but I still can't seem to find a solution.

I did some deeper looking and based on the calculator here: Digital Camera Sensor Sizes: How it Influences Your Photography (credit to @wopi82 for finding it), but it doesn't provide information on the 1/2.8" sensor size. I used the calculator plus the wikipedia article (Image sensor format - Wikipedia) to come up with some guesstimate conversion factors.

Based on the above, I've come up with the following guess as to how to convert the different sizes to normalize the focal length to the 35mm standard (focal length multiplier):

Sensor | FLM |

1/3" | 7.2 |

1/2.8" | 6.8* |

1/2.3" | 5.6 |

1/2" | 5.4 |

1/1.8" | 4.8 |

That yields the following zoom positions for the 2231 for the fixed lens sizes of the 5442.

Focal length equivalents:

IPC-T5442TM-AS | IPC-HDW2231R-ZE | 35mm equivalent |

2.8mm | 2mm | 14mm |

3.6mm | 2.6mm | 17mm |

6mm | 4.2mm | 29mm |

Note: the 2231 has a range of 2.7 to 13.5mm, so the 3.6mm 5442 appears to be a close match to the 2231 at its widest.

So, that should answer the question, but here comes the problem. According to Focal Length | Understanding Camera Zoom & Lens Focal Length | Nikon from Nikon, the focal length should not only determine the magnification, but also the angle of view. But the angle of view between the lenses is very different at the same focal length.

IPC-T5442TM-AS | IPC-HDW2231R-ZE | |

Focal length (17mm @ 35mm standard) | 3.6 | 2.7 |

Horizontal AOV | 89 degrees | 106 degrees |

Vertical AOV | 48 degrees | 57 degrees |

This would imply that at the same equivalent focal length, they have very different AOV's - which doesn't seem possible?

Does anyone have any idea how to solve this problem, or where I might have gone wrong in my thought process?

Thanks very much for the help!!!