Zoom and lense size math

bug99

bug99

Pulling my weight
Dec 27, 2016
398
154
I hope someone can help shed some light on this for me. I do not understand why the lens range and the zoom range are so far off, according to Dahua's data sheets.

For example, for the DH-IPC-HFW5231E-Z5, i see that the zoom range is from 7mm to 35mm (5 to 1), but the actual zoo is from 35 degrees to 12 degrees (~3:1). The Z version is from 2.7mm to 12mm (4.4:1) but the real zoom multiplier is 97 deg/34 deg = 2.8, or ~3:1 again. What caught my eye, and is messing me up is the Z5 starts at 7mm, but has the same FOV as the Z at 12mm, with the same sensor. What is up with that? To further poor salt into the confusion wounds here, the Z12 version actually seems to be 12:1 zoom, both in mm and in FOV (59 deg to 5 deg, 5.3mm to 64mm).

Is this some sort of math thing that is not obvious to me? Is this the same for other manufactures? Does the IPMV calculator take beam angle as an entry or lens size?

Beyond clearing this technological oddity up, i hope someone does a good comparison between the Z, Z5 and Z12 with regradrs to its actual zoom. Perhaps @nayr will take it on. If not i think i might do it soon.
 
Does anyone know of another good avenue that i might explore to better understand this zoom discrepancy issue (mm ratio vs FOV ratio)?
 
Mike Mo said:
Zoom is generally a ratio of focal lengths.l, and is related to the apparent size of the image. FOV is dependent on sensor size.

Understanding Focal Length and Field of View
Click to expand...
@Mike Mo , Thank you for that link. It took me a bit to think through the information and review the math, but i think i have it now, at least to a fair degree.

You are correct in that the manufacturer reported zoom is a rough ratio of focal lengths, rather than the actual apparent zoom (size big/size small). This differs greatly at the small end of lens sizes, but is almost identical for large lens sizes. This is due to the "sine of small angles" issue (sine of small angle is equal to the angle, but sine of a not small angle is not equal to the angle). It crops up here in the inverse tangent represented in your linked document.

I can now use this understanding to make some decent informed decisions, and i hope this short summary helps others in a similar decision boat.

I have yet to get a Z12 version of any of the Dahua Starlight cameras to review, but it seems to me that the value is substantial compared to the Z5 for the 12:1 zoom vs 3:1 or less.
 
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