Subscriber Discussion

Attention IPVMU Grads! FOV Calculation Verification Requested

Chris Dearing

•Dec 18, 2013

Slogging my way thru the IPVM back-catalog I came across this post about viewing a face at 3000 meters.

I might have clicked out had John's following statement not caught my eye (emphasis added):

Wow, 3000m is really really really hard.

Bombast and hyperbole you say? Though a frequent user of the singleton 'really', he has (to my knowledge) only ever employed the doubled 'really' once, and never til now has been a triple-threat.

However, before I risk the ridicule of the lurking quants in the original post, I would like to run this proposed solutiion by anyone who feels they can validate it, IPVMU alumnus or not...

Here's the proposed camera: (Cohu HD 8800)

Cohu HD 8800

Required Distance: 3000 m

Focal length: 1680 mm

H Sensor Size: 9.398 mm (1/2.7")

HFOV = Distance / (Focal Length / H Sesnsor Size)

16.75m (55 ft) = 3000m / (1680mm / 9.398mm)

PPF = H Resolution / HFOV

35 PPF = 1920px / 55ft

35 Pixels per foot is at the very bottom range for face recognition, but can anyone say that 35 PPF is even correct?

Thanks!

P.S. For extra credit take a look at this graphic included by John in his reply to the original poster:

tamron lens calculator

Since 1/3=.333 and 1/2 = .5 and 2/3 = .666 and 1/1.8 = .555 wouldn't you expect to see the focal length of the 1/1.8 one between the 1/2 and 2/3 ones, yet it is smaller the the 1/2" one....

What am I missing?

Horace Lasell

•Dec 18, 2013

Your numbers look good to me. The Cohu 8800 HD Googled as 1080p with 0.2 degrees HFOV. At long ranges, the hypoteneuse and adjacent sides of a right triangle are near enough to the same length so that we can just use the distance without any inverse cosine calculation. We didn't need the quants on this one, just high school trig: 3000 m x sin(0.2 degrees) ~ 10.5 m ~ 34.5 ft HFOV. 1080p/34.5 ft ~ 31.5 ppf.

Most IPVM members would probably agree that "how much ppf is enough," "depends." Recalling the many excellent IPVM evaluations, many cameras simply don't function at some lighting level. Still, for the general mostly-well-lighted case, the power of PPF is in its precision and its repeatability. I look at Brian's examples (great post, Brian!) and say to myself, "20 ppf might be enough -- I could go with that." This may fail to take customer viewpoint into account. I think about my experiences as a customer who has procured the services of a professional, and how many times have I used a professional "solution" and thought, "Did those guys really know their field?" Not that anything was terribly wrong, just that in some conditions their solution was marginally inadequate. Still, doesn't 40 ppf look pretty good? ... ?

Chris Dearing

•Dec 18, 2013

In reply to Horace Lasell

Thanks for the awesome shorcut for long focal lengths!

Cohu claims it can spot a person at 30km at a cost of 30k....

Perhaps John will consider you for an IPVMU Honarary DO (Doctor of Optics) in the selection of the yet-to-be-conceived IPVMU Honorary Degree Program.

P.S. Is that Tamron lens calculator thing just wrong or am I crazy?

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