Re: lens choice for high res video camera

In article <1122137544.274959.323150@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"buster" <buster_googlegroups@xxxxxxxxx> wrote:

> Currently I'm working on an job where a lightfield is projected on a
> foil. On the other side of this foil a ccd video camera looks to this
> projected image. From this image the dimensions will be determined
> using IMAQ- Vision from National Instruments. I've not much experience
> with optics and can't get clear which lens to use.
>
> The camera used is an Adimec 4020m with a resolution of 2048*2048 pix.
> The CCD sensor size is 15.15 *15.15 mm. The field of view (FOV) is 400
> *400 mm (the area inside the light field is projected) The working
> distance (WD) is fixed and should be approx. in the range of 500-700mm.
>
> Some simple calculations lead me to a focal length of 22.7
> [focal length = (sensor size * working distance)/FOV = (15.15 *
> 600)/400)]
> Can I now simply choose a lens with focal length of for example 25
> (leading to a WD of 660mm)? Or is this just too simple?
>
> Also the depth of field (DOF) should be as small as possible to avoid
> problems with "seeing thru" the projection foil (seen in experiments
> with older test environment). I know that a small lens diameter in
> combination with a large sensor size and a large aperture (diaphragm)
> gives a lot of distortion. What is the impact of this for the lens
> selection?
>
> The camera is supplied with an adapter from c to f-mount. If I'm
> correct informed this gives me more choice for lenses with a larger
> diameter?
>
> Thanks for any suggestion!
>
> Regards,
> Buster
>

Well, your focal length calculations are close. The actual theoretical
focal length for the desired reduction at 600mm WD is 21.9mm because you
have to extend the lens to focus. But a 25mm will work fine; the front
focal distance (which is generally going to be different from the
working distance) is actually 685mm.

You may have a better chance of getting a 25mm in C-mount than F-mount
since the latter is a 35mm standard and most of the lenses are made to
cover a 43mm circle. There are a lot of 24mm F-mount lenses but they're
wide angles and commonly do have some distortion. The distortion over
the area of your sensor, though, should be very low.

You can get very fast (low f/number) 25mm C-mount lenses but a lot of
them are low quality and designed for small sensors. You need a good
lens that will resolve the 7 micron pixel pitch of your sensor and cover
a 21mm circle. The only one that comes to mind is the Rodenstock f/4
enlarging lens but there should be others.

What concerns me is that you stated:

"I know that a small lens diameter in combination with a large sensor
size and a large aperture (diaphragm) gives a lot of distortion."

This is, if I parsed it correctly, not true or even self-consistent and
implies that you indeed do not have much experience with optics. It
would probably be worth your while to have someone with the appropriate
experience check out your setup in detail because there may be other
issues that would affect your results.
.

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