Determine Focal Length

Chura

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Hi Guys,

I have few Dahua 2231s today, and I'm about to order one more cam.
I'm thinking this time to go with non-verifocal camera such as (Dahua 5442 or Hikvision IPC-T2385G-I)
How can I determine what is the focal length i'm currently using in my 2231 ?
There's only slider without any numbers
1573765793854.png
 

Arjun

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To be frank, if you're asking the value of that slider in the web UI, its just a rough guess. If your varifocal goes only up to 13mm, half of that is is 7.5mm. I know its not precise, but it will do ;)
 
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gwminor48

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Check out this area of the thread on the 5231 and see if it helps:
 

Chura

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Thanks everyone.
I can't use IPVM, the resolution for Google Maps Israel is unusable :(

Since my camera (like most people) are used for peripheral security, why would I ever choose zoomed versions ?
Here is a picture from local GIS (Better resolution then Gmaps)
1573859657662.png

The green represent current 2231 Turret that cover that area (Focal at Min), But since its too far, I can't recognize faces in the parking (where all the building material now)
So I want to add another one where the purple dot.
Since I've never change focal for the green one, any reason I would like narrower focal in any other cam, or specifically the purple one ?

Thanks Guys, learned a lot here!
 

Walrus

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You have it zoomed out almost all the way, so it's set to around 2.7 or 2.8mm. Any low MP camera (2MP) at that zoom/lens is useless except for an overall view, or very close ID. You either need to zoom in, or go with higher MP.

You need to test the location with a temporary setup. If you buy a 4mm lens camera and put it where the purple dot is, you might find it too close and it can't see what's directly in front of it. You might be better off putting a 2nd camera where your current one is, and zooming them both in so one is looking at the parking, and the 2nd at the road. Kind of like as if the green line with the 20 meter writing is the edge of one view and start of the other.
 
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quest100

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Use some simple geometry - similar triangles and ratios. Look at the specs for one of the cameras you are looking at. The ratio of the sensor width to the camera focal length will be the same as the ratio of the FOV to the distance to the object. Thus

FOV = (distance) * (sensor size) / (focal length)

Sensor size can be horizontal, vertical, or diagonal and the FOV will be the corresponding h, v, or d. This ratio breaks down when the camera has lots of distortion. If you use the size of a sensor pixel the FOV will be what an individual pixel would see. Even for a fisheye lens this will be correct in the center of the FOV.
 
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