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# Using vari-focal camera to determine focal length of fixed-focus camera (revisited)

#### lostinpangea

##### n3wb
This is my first time installing cameras for my home - thanks to all those who provided advice in my original post: Newbie Request for Camera Recommendations (pretty detailed)

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
*my best guess; other values were pulled from sources

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!!!

#### lostinpangea

##### n3wb
Thanks! I didn't know about that calculator.

But I'm not sure it fully answers the question... I'm trying to use a 2MP 1/2.8" varifocus camera to determine the focal length needed for a 4MP 1/1.8" fixed lens camera. Are the MP and sensor size irrelevant in the calculation? I can imagine how MP wouldn't matter, but it's hard to imagine that sensor size would not?

#### lostinpangea

##### n3wb
Yes... essentially, it's the same formula as the calculator provided by @Mike. But it doesn't seem to take into account when the cameras being compared have different sizes of sensors.

#### lostinpangea

##### n3wb
Or, perhaps a simpler way of describing the problem...

Is the 2.8mm fixed lens on the IPC-T5442TM-AS (1/1.8" sensor, 4MP) extremely close to the same zoom and angle-of-view as the 2.7mm setting on the IPC-HDW2231R-ZE (1/2.8" sensor, 2MP)? or is the 3.6mm fixed lens IPC-T5442TM-AS closer?

#### freddyq

##### Getting the hang of it
I am planning to do exactly the same thing as @lostinpangea except I'll be using a HFW5442E-ZE which is varifocal to determine which fixed lens version of the T5442TM-AS I should order. Difference in my case is that both these cameras have the same MP and sensor size.

I thought I would just setup the HFW5442-ZE, get the focal length set to one I'm happy with and whatever that is, order the nearest fixed focal length of the T5442TM. Is it not that simple?

#### lostinpangea

##### n3wb
If your sensors are the same size and geometry, I think you should be fine doing a direct focal length comparison.

But I don't think that's possible with different sized sensors. I think this explains it better than I can: Understanding Focal Length and Field of View | Edmund Optics

Instead, I think it's possible to apply the same principles and basic algebra used in the calculator, but apply it to angle of view instead of focal length. You should be able to use the proportionate amount of change in the zoom to predict the proportionate amount of change in the angle of view. Then simply choose the fixed lens that most closely matches the angle of view of the varifocal camera at the set zoom position. Angle of view should be comparable across cameras, regardless of focal length and sensor size. And it's the angle of view that should really be more descriptive of what you will see. I hope!

#### freddyq

##### Getting the hang of it
If your sensors are the same size and geometry, I think you should be fine doing a direct focal length comparison.

But I don't think that's possible with different sized sensors. I think this explains it better than I can: Understanding Focal Length and Field of View | Edmund Optics

Instead, I think it's possible to apply the same principles and basic algebra used in the calculator, but apply it to angle of view instead of focal length. You should be able to use the proportionate amount of change in the zoom to predict the proportionate amount of change in the angle of view. Then simply choose the fixed lens that most closely matches the angle of view of the varifocal camera at the set zoom position. Angle of view should be comparable across cameras, regardless of focal length and sensor size. And it's the angle of view that should really be more descriptive of what you will see. I hope!
Hmm thanks. I think I will post here once I have messed with the camera a bit to verify before I buy. With the fixed lens versions you do have to be that extra bit careful. Alternatively, would be amazing if Dahua came out with a varifocal version of the T5442 turret...problem solved!

Sent from my SM-G950F using Tapatalk

#### freddyq

##### Getting the hang of it
I've taken delivery of two IPC-B5442-ZE cams ordered from Andy I'm planning to use one of these to test focal lengths in a couple of locations where I'm planning to put the T5542TM fixed lens camera.

So my question is, assuming I use ipcam viewer lite to access the camera's feed (I won't have the BI setup until a while later) will the app allow me to adjust the B5442's focal length? Or will I need to do that via the camera's web interface? And secondly, just to confirm again given the B5442 has the same sensor size and MP as the T5542TM, can I simply just note the focal length on the B5442 which works best and order the appropriate version of the T5442TM?

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#### Rebelx

##### Getting the hang of it
Great effort on trying compare the field of view on these various cameras.

For Newbs, I think it's best to stick to varifocal as they are super forgiving and you can easily take em with you (and reuse) if you ever move houses.

#### jack7

##### Getting the hang of it
freddyq and lostinpangea, you might like this calculator:
How to:
And scroll up the page a little and see how to use your own view if desired.

#### freddyq

##### Getting the hang of it
freddyq and lostinpangea, you might like this calculator:
How to:
And scroll up the page a little and see how to use your own view if desired.
Thanks yea I've used that quite a lot to come up with my proposed combo of cams for various locations around my house. So on paper the two fixed lens cam should work but I want to confirm that with actual footage to see exactly how it would look. Guess it's just me double checking before buying.

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#### freddyq

##### Getting the hang of it
I've had another play around on the IPVM tool - it looks like two T5442TM-AS cams (3.6mm version) will provide the kind of coverage and FOV I would want. I did a comparison buy putting two 2231s in the same locations and adjusting the coverage to what I would want and it ends up almost the same as the fixed FOV and width that the T5442 would provide. So feels like I can just go for the T5442 - the newbie in me wants a varifocal for it's flexibility but these new 5442 cams provide a real upgraded alternative to the 2231 if you can sacrifice varifocal. And looks like jury is out on whether we might see a varifocal turret in the 5442 range anytime soon so won't wait for that specifically.