# Lens focal length question

Discussion in 'Photography' started by Bruce W.1, Dec 21, 2003.

1. ### Bruce W.1Guest

I want to attach a small video camera (surveillance type) to my existing
door peephole. I do not want to replace the peephole. So I'm left
wondering what sort of lens the video camera should have.

The peephole lens is 1-1/2" from the rear flange. The peephole lens is
1/4" diameter. And the inside diameter of the hole is 3/8".

So the camera's field of view should be about ten degrees if the camera
lens is 1-1/2" from the peephole lens. What then should be the focal
length of the camera lens?

Any other suggestions would also be appreciated, like a particular
camera. I'm not sure if it should have a pinhole lens? It should
output NTSC video. Monochrome or color is fine. I will build the
camera mount.

Bruce W.1, Dec 21, 2003

2. ### mgGuest

Yes (assuming you did the geometry right) if you want the camera to look AT
the peephole lens. NO, if you want the camera to look THROUGH the lens.
The peephole lens has an output pupil where your eye is behind it. Your
camera needs to have its input pupil at the same place. The camera FOV
needs to mathch you eye's FOV, or the part of it used when you look though
the peephole. The aperture diameter of the camera pupil should be the in
the same ballpark as the size of the peephole output pupil and the pupil of
your eye. The camera focal length is found from the FOV and the sensor size
using a popular formula. The camera lens will be focused at infinity if it
is placed right.

Good luck !

mg, Dec 21, 2003

3. ### Bruce W.1Guest

===================================================

Guess a focal length is pointless without knowing the vidicon chip size.

That's good information and corresponds to what I've seen. It also I
think shoots down another idea I had, i.e. to find a camera that
actually fit in to the peephole, like maybe this one:
http://www.jonescam.tv/
Could this focus if very close to the peephole lens? If so the field of
view would be no problem, just slide it in or out.

From spec's. I've seen it looks like a 50 mm lens has a ten degree FOV.

I want the camera to see the peephole in its entirety, rather than look
thru it (which would restrict the FOV).

Thanks.

Bruce W.1, Dec 22, 2003
4. ### ?Guest

Why not just replace the peephole with one of the peephole cameras tha tseem
so prevalent now days they have a large angle of view and are not much
larger than the peephole and look just like the peephole.?

Ed

?, Dec 22, 2003
5. ### James R \(Jim\) Lynch IIIGuest

"Lens Design" by Milton Laikin, Marcel Dekker, Inc., ISBN 0-8247-8403-0,
First Printing, Chapter 13, Afocal Optical Systems, Figure 13-7, pp 145-146
gives the optical prescription of a "Door Viewer". He says the exit pupil --
the eye -- is 0.5-inch behind the last lens. If the full FOV is 150 degrees
in object space, the full FOV in image space is 19.1 degrees by raytracing.
This is less than half the full FOV of a "normal" lens -- about 53.1
degrees. Therefore you'll want a focal length at least twice that of the
lens which would normally be supplied with a CCD camera. Since the last lens
of your door viewer (as viewed from the inside of the door) is 1.5-inches
away rather than 0.5-inches, you may have problems with vignetting.

James R (Jim) Lynch III

James R \(Jim\) Lynch III, Dec 22, 2003
6. ### Bruce W.1Guest

=================================================================

Not sure what you mean by "vignetting".

Just for grins I put my video camera up to the peephole. This worked
well except it only saw a small portion of the area you can see with
your eye. I think this is because the CCD is about 3" back in to the
camera.

Next experiment is to put my webcam up to the peephole, as soon as I get
a long enough USB cable. This webcam has 4 different lenses so one of
them should work. Based on this I may buy a small video camera to
attach to the peephole, if I can find one with the right lens.

Bruce W.1, Dec 22, 2003
7. ### James R \(Jim\) Lynch IIIGuest

James R \(Jim\) Lynch III, Dec 23, 2003