Comparison of a Fixed Lens and a Varifocal Lens Camera's FOV

samplenhold

samplenhold

IPCT+ Member
Aug 8, 2018
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Spring, Texas
I was testing out a new Dahua T5442T-ZE 2.7-12mm Varifocal camera (from @EMPIRETECANDY ) and was going to compare it's performance to the Dahua HDW5231R-ZE and the HDW5431R-ZE cameras, which are 2.7-13.5mm varifocals, along with the T5442TM-AS fixed lens at 6mm camera. The HDW5431R-ZE has been covering my driveway from the left side since 2018. The HDW5231R-ZE was covering my driveway from the right side since 2018 until I replaced it with a T5442TM-AS in 6mm this past June.

So I placed the HDW5231R-ZE and the T5442T-ZE on my test rig and set that rig next to the T5442TM-AS in 6mm. See shot below.
test cam positions.JPG

When the HDW5231R-ZE was installed, it was set at about 6mm. So when I wanted a replacement the 6mm T5442 was used. When I set up this test rig, I set the HDW5321 to a zoom of 730 to approximate 6mm. I set the T5442T-ZE to 800 to approximate 6mm. See calculations below.
HDW5231R-ZE zoom calcs .JPG T5442T-ZE zoom calcs .JPG

Then I placed the test rig next to the T5442TM-AS and attempted to get the same views on the rig cams as the installed T5442 6mm cam. To my surprise, the T5442 varifocal was not even close to the same FOV as the T5442 in 6mm. It took some additional zoom to get it about right, 1540, which according to the calculations is about 9mm.
two 5442 at 6mm.jpg two 5442 at 6 and 9mm.jpg

Both of the 5442's are on the same 1/1.8" sensor and are both 4MP. To me this makes no sense. When I replaced the 5231 (2MP on a 1/2.7" sensor) with the 6mm 5442, the FOV was a little different, but close. I assumed this difference was due to the difference in MP and sensor size. Below is the FOV from the 5231 in this test. you can see it is not much different.
C-TEST.20210104_160020524.97.jpg Here are the other two cams at full size shots. DRRT.20210104_160020598.96.jpg T5442T-ZE set at 6mm.jpg

I am at a loss to explain this. Anyone have any ideas? I wish I had thought to compare the 5231 and 5442 varifocals at zero zoom since they are supposed to be both 2.7mm. Also should have set the 5231 to 12mm and compared it to the 5442 at full zoom. Maybe I will if I get some time.

One should keep this issue in mind if you are using a T5442 varifocal to determine what fixed lens cam you need.
 
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OK IPCAMERers, what's up? Over a 100 views of this thread and not a single comment or 'like'? Did I piss all of you off?
 
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I went through a similar exercise a while back comparing fixed focal length 5231s to 5442s, calculating FOVs based on sensor size and focal length. I set up a carefully constructed FOV jig in my back yard to get the actual measurements. My final conclusion was that calculating FOV was useless and the actual observations were very close to the camera specs for FOV. My suspicion was that Dahua might be fudging or "normalizing" the focal length specs so that cameras with different sensor sizes have a similar FOV for a given focal length spec. No proof, just speculation to try to resolve the actual FOVs from the calculated FOVs.

For the record I'm not pissed off at you :):)
 
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I think FOV is a rough guide, not to be taken as totally accurate, especially between camera models or even between a zoom and fixed in the same model. They're more of an approximation from what I've seen and @tigerwillow went and confirmed that.

I'm not pissed either. Just waited for the true experts since my opinion is based on anecdotal observations.
 
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What seems amazing to me is that the 5231 varifocal at 6mm basically matches the 5442 fixed 6mm. Two different models with different sensors and pixel numbers.

But the varifocal 5442 set at 6mm is wider than the fixed 6mm 5442. If it was the other way around, I could just chalk it up to the difference in sensor size.
 
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the difference is due to the varying SENSOR sizes. the larger sensor in the 5442 results in a wider FOV for any specific lens that is not accounted for in your mm-to-zoom conversion...
IE< a 5442 (1/1.8" class sensor) with 6mm lens will be wider than a 5231 (1/2.8" class sensor) with a '6mm' lens.
also, there's more to the lens than its focal point (in mm or 'zoom number'.) The mount type (m12 or d14 or m16 or CS) affects the maximum iris (or minimum f/stop) which in turns affect the DoF (depth of field, or range of distance that things can be in focus)
 
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The 2 cameras do have a different lens spec. 2231 has a greater zoom range.

5442: 2.7 - 12mm Horizontal FOV 114 to 47 degrees
2231: 2.7 to 13.5mm Horizontal FOV 109 to 28 degrees

FOV is 5 deg different when at the zoomed out 27.mm focal length, 19 deg different when zoomed to the different maximum focal lengths.
 
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yes, different lens (tho both d14 mount v/f's) . same sensor.
i wasn't saying 5231 to 2231 is apple-to-apples, just that 2231 to 5542 is not, and that explains the differing FoV at the same 'zoom' level...

aside: one of my 2231's has a 6-22mm zoom range, thanx to a lens replacement... :)
 
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@pozzello any idea on the original post discrepancies? Why does the 5442 varifocal need to be at 9mm to match the 5442 6mm fixed?
 
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once again, different lenses.


While they use the same 1/1.8" sensor, fixed has an m16 while the Z uses a d14 VF lens,
Specs for the fixie says 'board-in', whatever that means, but I've opened one of mine and swapped lenses and I know it uses an m16.
Don't have the 5442 VF. Anyone wanna send me one? :-)
 
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I think the lesson to be learned here is to not judge a camera from it's focal length spec, but from the actual field of view spec.

The 5442TM with a 6mm lens has a horizontal 56 degree FOV. The 5442T with a 2.7-12mm lens has a horizontal FOV from 114-47 degrees. The 56 degree FOV of the TM is obviously close to the end of the range of the T variant (which is 47 degrees) so it makes sense that it is about 9mm focal length on that camera and not 6mm which one might assume because it would match the TM focal length.

It doesn't really matter why (obviously related to the sensor and lens combination), just that you understand which spec is most important when planning camera placement. This is also why I really like using the IPVM calculator when trying to plan for camera placement because it handles this information for you and you can see the FOV overlaid on an aerial view of the proposed location.
 
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Fascinating stuff. But I'm wondering, should I get another 5442TM-AS, or experiment with something different? Something with 1/1.8 sensor or larger... Any recommendations?
 
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