Can an afocal lens system project a sharp image onto a screen?
Asked 10/8/2022
9 views
2 answers
0
I’m trying to understand afocal optical systems. My understanding is that an afocal setup is focused at infinity and has no finite focal plane. If that’s true, can it form a sharp real image on a screen at any distance behind the lenses, or does it not project an image at all?
As an example, suppose I build a simple afocal system from two positive meniscus lenses, each with a focal length of 4000 mm. I’d like to know:
- How do I calculate the effective focal length of the two-lens combination?
- If the lenses are arranged as an afocal system, is there any back focal distance where a screen would show a sharp image?
- For two identical 4000 mm lenses, what spacing would make the system afocal?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
2
An afocal lens does not produce an image on a screen. Diverging rays from an object or screen point keep diverging on the other side of the lens, just not in a straight continuation. The point of an afocal lens is to change the geometry of the virtual image you are seeing. To get a screen image, you'll still need a convex imaging lens.
Originally by user108787. Source · Licensed CC BY-SA 4.0
user108787
3y ago
0
Generated from our catalog & community — verify before relying on it.
An afocal system does not form a real image on a screen by itself. Its defining property is that parallel rays enter and parallel rays leave, so its effective focal length is infinite and there is no finite image plane or back focal distance for projection.
For a two-lens system, the usual combined focal-length formula is:
1/F = 1/f1 + 1/f2 - d/(f1 f2)
which is equivalent to:
F = (f1 f2) / (f1 + f2 - d)
For an afocal arrangement, the lens spacing must be:
d = f1 + f2
At that spacing, the denominator becomes zero, so F = infinity. That means no sharp image can be formed on a screen with just that afocal pair.
With two 4000 mm lenses, the afocal spacing would be 8000 mm. In that configuration, rays from a distant subject remain parallel after the second lens. The system can change angular magnification, but to project onto a screen you would need an additional imaging lens to bring those rays to focus.
Recommended products
UniqueBot
AI3y ago
Your Answer
Related Questions
How do I calculate the lens-to-slide and lens-to-screen distances for a small slide projector?
How can I estimate an object's real size from pixel size, focal length, and subject distance?
Does image size on the sensor depend on sensor size or 35mm-equivalent focal length?
Why doesn’t simple focal-length geometry predict magnification in macro photography?
How can I estimate a lens objective’s numerical aperture from focal length and aperture?