Optics and Close-up Focus

[Miles Guidon]

I am not too strong when it comes to optics / lenses, but I am trying to figure out a way to get a person to be able to focus on something substantially closer than his or her point of focus. I understand this technology has been around for centuries (eyeglasses), but I am looking for a way to be able to focus on an image at a distance of about 1-2 inches from the eye. I think it has something to do with focusing the image at infinity, so the viewer doesn't have to constantly break his or her focus on the things around them to focus on the image. I think I may be able to do this by collimating the light using a fresnel lens, but I am still not sure.

 

Can anybody offer advice? I'd love to discuss this.

Edited November 23, 2010 by Miles Guidon

[lesolee]

I am not too strong when it comes to optics / lenses, but I am trying to figure out a way to get a person to be able to focus on something substantially closer than his or her point of focus. I understand this technology has been around for centuries (eyeglasses), but I am looking for a way to be able to focus on an image at a distance of about 1-2 inches from the eye. I think it has something to do with focusing the image at infinity, so the viewer doesn't have to constantly break his or her focus on the things around them to focus on the image. I think I may be able to do this by collimating the light using a fresnel lens, but I am still not sure.

 

Can anybody offer advice? I'd love to discuss this.

 

Well I'm not too strong on optics either, but am willing to try

You use a Fresnel lens as a cheaper way of getting a large lens. Rather than have all the material in the middle of the lens you just use the curvature that you would get at various points on the lens.

 

Wikipedia

 

When you say you want to focus on an image at 2 inches from the eye I am reading that as wanting to see an object 2 inches from the eye. In this case it is too close to focus on as the rays are diverging too much. You therefore need a converging lens with a short focal length.

 

Hopefully the attached ray diagram makes sense. (FP = focal point) My free-hand lens shape didn't quite work out as well as I might have hoped

 

[John Cuthber]

"I am not too strong when it comes to optics / lenses, but I am trying to figure out a way to get a person to be able to focus on something substantially closer than his or her point of focus"

Use a magnifying glass.

[lesolee]

I think it has something to do with focusing the image at infinity, so the viewer doesn't have to constantly break his or her focus on the things around them to focus on the image.

 

In telescopes and microscopes you use an eyepiece. This ideally produces a virtual image at infinity so your eye is relaxed (unaccommodated).

 

John's answer of a magnifying glass is in the right direction but will probably not have enough strength (short enough focal length). This wiki article explains it nicely.

 

magnifying glass

[John Cuthber]

If you get a positive lens with a focal length of an inch or two it will do the job.

The magnifying glass on my Swiss Army penknife would do just fine (though it's a rather expensive way to buy one).

There are plenty of suitable magnifying glasses on eBay.

So, my reply was " in the right direction" in that it got to the destination.