[Elphel-support] Reasonable motorized lenses for 50-60 meters zooming for Elphel camera

[Andrey Filippov]

>
> My question is not about face recognition. Its another big problem. My
> question is about zooming till possibility to recognize face (recognize by
> human).
>
>

> Experimenting with telelenses and professional photo cameras I come to
> conclusion what it would be enough to recognize face if it will be
> minimum 150 pixel height. So if face is about 20cm height, lenses must
> provide zooming for 150pixels/20cm at 60 meters. Dont know how to
> convert it to lenses units :) (x-y, f...)
>

That's not that difficult. With that data and knowing that the pixel size is
0.0022mm you can calculate focal length of an ideal lens:
http://www.google.com/search?hl=en&q=60m%2F((200mm%2F150)%2F0.0022mm)+in+mm-
that would be ~100mm.

The lens for the 2.2um senors should have power not less than 3-4, if it has
less (or iris is closed more) you'll have resolution degraded by diffraction
(pixel size is just about 3 times the wavelength) And 100 mm is for _ideal_
lens, if one has just 150lp/mm, taking 2 pixels/lp would result in some
150mm focal length.

>
> Actually, I thought about 2 lenses, but its not nice and general solution.
> But its real right now.
>

I would not agree - sensors are cheaper than lenses. And you are looking for
a top performance lens, not just the zoom one for low-res video. The
calculations above show that it is a challenge to get the results even with
a 120mm fixed-focus lens.
I

> I supposed if I have such a wide picture from camera I can zoom into
> certain regions (for example with your built in Point of View).
> Without rotating or tilting.
> I need only about 20-30 degrees of view.
>

With the 120mm lens you will get less than 3 degrees horizontal
If you need 150 pixels on 200 mm than 5 MPix sensor will give you 2.5m x 3.5
m field of view (100mm lens). With 120mm - it will be even less. Zoom/no
zoom lens, but without rotation platform you will not be able to see
anything with full resolution (1.33mm/pixel) outside of that area. to be
able to see 30 degrees with resolution of 1.3mm at 60 meters without
mechanical rotation you'll need to have 0.5gigapixel sensor. Or about
hundred 5 megapixel sensors with individual lenses (still probably cheaper)

Andrey
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