Help choosing Lens for fixed lens camera

[Fozzie Bear]

We are looking to live stream video to YouTube from a CCTV camera in our Village Hall using RTMP protocol. this is so village residents can view Parish Council and other meetings held in the hall during Covid-19 and possibly later to reach a wider audience.
The main hall is approximately 15m long and 6m wide. Meetings will be held near the stage at one end of the hall approximately 13-14m from the camera position at the other end of the hall above the entrance doors.
The camera being considered is a Exvist ZSD8PSH16F fixed lens 8Mp 4K IP camera. This comes with either a 2.8mm, 3.6mm, 6mm or 8mm lens. I suspect the 2.8 and 3.6mm are two wide angle so probably the 6mm or 8mm lens. Is there an online calculator that can calculate focal length and lens required for a fixed object? Alternatively can someone give me some advice on which ens to choose for my particular scenario.
Many thanks
Fozzie

 

[Dan]

Hi @Fozzie Bear

Hikvision's Lens selection tool will give you a rough idea of the area covered by different focal lengths, obviously, you won't get exact measurements because this tool won't feature your specific model but as long as you select a similar 8MP/4K bullet camera you will get a rough idea of each focal length.

 

[Fozzie Bear]

Thanks Dan, I have used a Hikvision 4K camera but not really sure what I am looking at and how to analyse the results ie what does Confirmed, Recognised and Detected relate to?
Fozzie

 

[Dan]

The Confirmed, Recognised, and Detected relate to the clarity of faces at those distance. So a face in the confirmed area will be very clear and easily identifiable as a specific person, whereas detected faces will likely have little detail and you will likely only be able to identify that it is a human being, and the area between will vary in detail depending on how close you are to the other 2 sections.

In your scenario, our recommendation would be that if you want to cover everything including the area directly in front of the camera then we would recommend a 2.8mm model, but if you want more detail in the faces of people sitting in the centre of the room then we would recommend either a 4mm or 6mm.

 

[Fozzie Bear]

Many thanks Dan thats really helpful,
One final point. The height of the camera seems to have quite a large bearing on the three zones Confirmed, Recognised and Detected. e.g if you have the camera quite low down at say 2.5m with an angle of less than 10 degrees the zones are very narrow. However if you lift the camera up to say 4 m then the zones band are a lot wider. Is this a normal feature of CCTV cameras?
I am now wondering if the camera should be mounted a lot higher than I had previously thought. It might distort the image slightly by being at an angle but might mean more of the area could be within the confirmed zone.
Fozzie

 

[Dan]

Hi @Fozzie Bear

I am not sure which camera model you are using to get these measurements because when I use a 2085G1 8MP Hikvision Mini Bullet I get the below results with a 4mm model:

The left image is at 2.5m and the right image is 4m and as you can see the areas don't really get any smaller (maybe half a meter reduction at most)

The key thing for you to take in is that the area you want to focus on (13-15m) is in the blue 'detected' area which means people will be visible but the detail will not be great, if we switch to a 6mm model you will see from the below images that there is a big difference:

Now the 13-15m area is in the 'recognised' area and so you will have much greater detail and the FoV is around 14m wide which means much less of the FoV is being wasted on the walls and area that surrounds where the meeting is taking place (with 4mm you will see it is more like 24-25m at that distance)

So we would recommend if the meeting area is your main focus that to achieve the highest level of detail you should use an 8MP camera with a 6mm focal length. (I see the model you are looking at does also come in an 8mm which if available maybe even better as it will narrow the FoV further (to around 8-10m) and give you even greater detail)