What would it take...

...to make a lens with infinite focal length?

I don't mean amount of glass and fluorite needed for it but how would it work like and would it be infinitely long? My master degree is in automation so I'm not that good in physics but I'd assume it'd have perfectly flat front surface on the front lens. So if there is a lense that lets only light coming directly at it in, would that be infinite focal length?

How would work a piece of film on one end of 1km long pipe with non-reflective interior? Would you really need a lens at all in such objective. I'm really confusing myself here

It'd be essentially an ultimate macro lens since it would show a 35mm area where ever you pointed it at (of course scatter and haze and such would block view). It wouldn't be that great for astronomical puropses though because anything larger than that wouldn't fit on image

Well, another gin and tonic!

PekkaM, go read some college physics textbooks. Read the parts on optics. It gets very complicated in a hurry.

---Bob Gross---

PekkaM,

The thin lens equation (good enough for this argument)
is

1/f = 1/s + 1/s',
where
f = focal length of lens,
s = distance of object from plane of lens,
and
s= = distance of image from plane of lens.
For instance, for a converging lens (the only kind you want for a camera lens, since it forms a real image that can fall on a chip or piece of film) with focal length 10 cm, you find that an object placed s=50 cm away would create an image at a distance s' = 50/4 = 12.5 cm from the lens.

In general, in order to form a REAL image, the object has to be placed outside the focal length of the lens. Hence an infinite focal length lens would not produce a real image on your film plane unless the object was more than infinitely far away. This is beyond even Canon's enginerring prowess.

-paul