[Laser] Reflex film camera on the receiving end

[Glenn Thomas]

Good idea!

I suspect the camera may not work as well as a larger Fresnel. Maybe.
The thing is that the "optical gain" is a function of the area of the
sensor and the area of your lens. The camera lens may have an area
proportional to a diameter of the few cm of the lens. The Fresnel may
have an effective diameter of many cm.

The total amount of energy available to the sensor will be proportional
to the area of the lens. The diameter of your Cannon lens may be as much
as 6 cm or so and the effective diameter of a Fresnel might be 20 cm or
so. The area of the lens, and so the amount of energy available to the
sensor, is proportional to the area of the lens and the area of the lens
is in turn proportional to the square of the diameter. Assuming both
lenses are circular, the 6 cm Cannon lens has an area of 113 cm**2 and
the 20 cm Fresnel an area of 1256 cm**2. The Fresnel will collect more
than 10x the energy that the Cannon will. In terms of "antenna gain",
the Fresnel will have 10 db more gain than the Cannon.

Another consideration is, how big (in area) is the sensor itself and how
sharp is the image of the light source formed by the lens? I don't know
how big your sensor is and how good your various lenses are. If the area
of your sensor is small and the quality of your Fresnel image is poor,
not all of the energy will fall on your sensor and the difference
between the high quality Cannon image and the lower quality Fresnel
image may not be as large as the lens area suggests.

Yet another consideration is how efficient your lenses are. The optical
glass of the Cannon lens may pass more of the signal energy than the
scribed plexiglass of the Fresnel. That's another potential loss in the
system.

As always, YMMV, depending on how big your sensor is and how good your
optics are. It sounds like you have a good experiment to do!

73 de Glenn wb6w

On 4/26/2021 8:20 AM, Paolo Cravero wrote:
> Hello!
> After playing with simple and modern hardware on 10 GHz, a group of
> Italians has started fiddling with optical communications. Three of us are
> lucky enough to live within ~10 km in LOS (not me). The philosophy of our
> experiments (games?) is to keep everything as simple and cheap as possible,
> while offering opportunities to learn new things. Our optical sensors are
> common OPTxxx or BPW34 photodiodes, nothing fancier. TX side is a matrix of
> LEDs or toy laser pointer (those that work on button batteries).
>
> We've been discussing and trying Fresnel lenses, with inherent
> complications in mounting and handling size. Then in a video of Norwegian
> HAMs using a German WW2 device for optical communication I noticed that
> their "antennas" made everything look like a large binocular (and boxes of
> equipment behind :) ).
>
> So I remembered my unused reflex film camera and imagined how comfortable
> it would be to use a compact, self-contained device that only needs the
> optical sensor to be put in place of the film frame. I browsed through the
> list archives (subjects, not all message bodies!) and I haven't found a
> similar conversion, so either it doesn't work or back-then reflex cameras
> were too expensive for this kind of game.
>
> Today with 40-50€ you can buy a film camera body and simple zoom optics, if
> you haven't got one laying around.
>
> I can mount it on a tripod right away, it doesn't look suspicious, it isn't
> bulky to carry: where's the catch, Sirs? Does a 50-80mm focal length lens
> perform as good as a 20x25cm Fresnel lens?
>
> Thank you for sharing your thoughts,
> Paolo
> ...heading down to the storage room to dig the old Canon 3000N.
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