In comparing specs of cameras with different sensor sizes it felt to me like the specs were inconsistent. After a lot of number crunching I'm convinced they are. General conclusions listed first. They're based on only 2 specific camera models so it's premature to apply them widely. Corrections to math and logic errors welcome.
1. The sensor size (1/2.8", 1/1.8", etc.) is approximate and there's a good chance you don't know what it means, other than "bigger is better".
2. The lens focal length spec (3.6mm, 6mm, etc.) can be very inaccurate. Is Dahua fudging (i.e. "normalizing") it to a standard sensor size?
3. The camera FOV spec is fairly accurate.
4. Because of item #2, if you calculate the FOV, the answer could be way off. An online calculator will give garbage out based on the garbage in. Use the Dahua FOV specs.
Details....
What does sensor size mean? My answer is from logical conclusion, not from reading it anywhere. It's the sum of the horizontal and vertical dimensions of the sensor. The 1/2.8" etc. specs. are approximate. Kind of like the "55 inch class" specs used with televisions these days. To get the actual sensor size, you have to find the spec sheet for the sensor which gives the actual diagonal size, then apply the aspect ratio and pixel shape to get the horizontal and vertical dimensions. Fortunately the pixel shape is commonly square and can be ignored in the calculation. The IMX290 sensor in the 2mp starlight cameras is 6.46 mm diagonal from its spec sheet. Applying the 16:9 aspect ratio gives rectangular dimensions of 5.63x3.17mm. Adding these gives 8.80mm. 1/2.8" is 9.07mm, kinda close. The IMX347 sensor in the 4mp starlights is 9.04mm from the spec sheet. Applying 16:9 aspect gives rectangular dimensions of 7.88x4.43mm. Adding these together gives 12.31mm. 1/1.8" is 14.11mm, so unless I made a math error they're cheating a bit on this one. 1/2" instead would be pretty close.
I looked specifically at the horizontal angle of view for 2 cameras, with 3.6 and 6mm lenses, the 2mp HFW4231E-SE and 4mp T5442TM-AS. I'm able to make actual measurements for only the 6mm lenses because that's all I have.
The angle of view is calculated from the sensor width and focal length. I'm comparing the calculated angle, vs. camera spec angle, vs. actual tape measured results with the 6mm lenses.
HFW4231E-SE 3.6mm
Calculated 76 degrees, camera spec 87 degrees
HFW4231E-SE 6mm
Calculated 50.3 degrees, camera spec 52 degrees, actual measurement 51.56 degrees
T5442TM-AS 3.6mm
Calculated 95.2 degrees, camera spec 89 degrees
T5442TM-AS 6mm
Calculated 66.6 degrees, camera spec 56 degrees, actual measurement 54.2 degrees
Same conclusion, for at least the 2 models I was able to measure.: The camera's FOV spec. is pretty close and the lens focal length spec must be incorrect.
Since doing this the hard way I found an online calculator that gives the same answers: Camera Field of View Calculator (FoV)
It looks intimidating until you realize all you have to do is enter the focal length, any old number for the field distance, and the native sensor size. Then hit "recompute" and the answers pop out.
1. The sensor size (1/2.8", 1/1.8", etc.) is approximate and there's a good chance you don't know what it means, other than "bigger is better".
2. The lens focal length spec (3.6mm, 6mm, etc.) can be very inaccurate. Is Dahua fudging (i.e. "normalizing") it to a standard sensor size?
3. The camera FOV spec is fairly accurate.
4. Because of item #2, if you calculate the FOV, the answer could be way off. An online calculator will give garbage out based on the garbage in. Use the Dahua FOV specs.
Details....
What does sensor size mean? My answer is from logical conclusion, not from reading it anywhere. It's the sum of the horizontal and vertical dimensions of the sensor. The 1/2.8" etc. specs. are approximate. Kind of like the "55 inch class" specs used with televisions these days. To get the actual sensor size, you have to find the spec sheet for the sensor which gives the actual diagonal size, then apply the aspect ratio and pixel shape to get the horizontal and vertical dimensions. Fortunately the pixel shape is commonly square and can be ignored in the calculation. The IMX290 sensor in the 2mp starlight cameras is 6.46 mm diagonal from its spec sheet. Applying the 16:9 aspect ratio gives rectangular dimensions of 5.63x3.17mm. Adding these gives 8.80mm. 1/2.8" is 9.07mm, kinda close. The IMX347 sensor in the 4mp starlights is 9.04mm from the spec sheet. Applying 16:9 aspect gives rectangular dimensions of 7.88x4.43mm. Adding these together gives 12.31mm. 1/1.8" is 14.11mm, so unless I made a math error they're cheating a bit on this one. 1/2" instead would be pretty close.
I looked specifically at the horizontal angle of view for 2 cameras, with 3.6 and 6mm lenses, the 2mp HFW4231E-SE and 4mp T5442TM-AS. I'm able to make actual measurements for only the 6mm lenses because that's all I have.
The angle of view is calculated from the sensor width and focal length. I'm comparing the calculated angle, vs. camera spec angle, vs. actual tape measured results with the 6mm lenses.
HFW4231E-SE 3.6mm
Calculated 76 degrees, camera spec 87 degrees
HFW4231E-SE 6mm
Calculated 50.3 degrees, camera spec 52 degrees, actual measurement 51.56 degrees
T5442TM-AS 3.6mm
Calculated 95.2 degrees, camera spec 89 degrees
T5442TM-AS 6mm
Calculated 66.6 degrees, camera spec 56 degrees, actual measurement 54.2 degrees
Same conclusion, for at least the 2 models I was able to measure.: The camera's FOV spec. is pretty close and the lens focal length spec must be incorrect.
Since doing this the hard way I found an online calculator that gives the same answers: Camera Field of View Calculator (FoV)
It looks intimidating until you realize all you have to do is enter the focal length, any old number for the field distance, and the native sensor size. Then hit "recompute" and the answers pop out.
