Lenses

[Ho Pei Jiun]

I've some questions which need your advice:

1) If the object is distant (ie quite far away) from the 35mm f2.5 lens, is it correct that we have to set the 35mm lens to focus as infinity in order to capture clear image ?

2) Has anyone heard of the Model ICS 42 Cooled CCD camera? I'm searching all over the websites but couldn't find the technical specs of this ICS 42 cooled CCD camera.

Sincerely,
Ho Pei Jiun

[Ernest Purdum]

1) Your assumption is correct.



2) I'm not familiar with this camera.

[James E Galvin]

Infinity focus: The subject is so far away that it might as well be infinitly far away. Stars are that far away, but we include all things far enough away that it doesn't make any difference. If the lens is focussed on a star, and then used to image a closer subject, if the out of focus blur is smaller than the resolution of the lens or the film, then that subject is for practical purposes at infinity. Also a lens is fucussed at infinity when it is placed its focal length away from the film.

(If you get technical, its rear principal plane one focal length from the film)