- #1
iansmith6
- 3
- 0
This isn't homework, I really have this setup. :-)
I have an 18" mirror with an 80" focal length. The light from a point light source at twice that distance (and offset to the side slightly) bounces it's beams off the mirror and back to form a perfect image of itself next to the light if you hold up some paper.
Moving the paper back the image expands, and keeps expanding of course.
I want a lens that can sit behind that convergance point and make the light parallel so it projects the same size image no matter how far you move back from it.
This is to try and overcome issues with cameras where the diverging light is blocked inside the lens by the iris and only the center of the mirror is lit up instead of the entire thing.
Some of you might recognise this as a very similar setup to a Focolaut test.
So the question is... what kind of lens do I need? I have a number of random biconvex lenses I tried, but none have the right proprties to make the light paralell. Several work well to focus the light into a nice sharp image, but cameras can't seem to focus on that.
18" diamater mirror
Parabolic shape
f4.5
80.6" focal length
If anyone knows what properties a lens would need, does anyone know where I could get one? If it's not something "standard" I can find in a catalog somewhere that is.
Thanks!
-- www.ian.org
I have an 18" mirror with an 80" focal length. The light from a point light source at twice that distance (and offset to the side slightly) bounces it's beams off the mirror and back to form a perfect image of itself next to the light if you hold up some paper.
Moving the paper back the image expands, and keeps expanding of course.
I want a lens that can sit behind that convergance point and make the light parallel so it projects the same size image no matter how far you move back from it.
This is to try and overcome issues with cameras where the diverging light is blocked inside the lens by the iris and only the center of the mirror is lit up instead of the entire thing.
Some of you might recognise this as a very similar setup to a Focolaut test.
So the question is... what kind of lens do I need? I have a number of random biconvex lenses I tried, but none have the right proprties to make the light paralell. Several work well to focus the light into a nice sharp image, but cameras can't seem to focus on that.
18" diamater mirror
Parabolic shape
f4.5
80.6" focal length
If anyone knows what properties a lens would need, does anyone know where I could get one? If it's not something "standard" I can find in a catalog somewhere that is.
Thanks!
-- www.ian.org