Home made aspheric questions

[tolkaze]

Pretty sure this is the place to be. I have a quick question about adding an aspheric to a stock light.
The light in question is a T20C2, the aspheric I am going to use has been harvested from a disassembled Canon 17-85mm camera lens (final focus lens before the sensor)

Now, the apeture of the light at the glass is 29.40mm, and the glass (with plastic shroud) is 27.80mm so very little, if any, light can get past the lens. Now, my question is about reflectors etc, with both the OP and smooth reflector, this lens gives a really bad ring pattern. I am guessing most aspherics do not use reflectors?

also, mounting this lens, I can get a very sharp image of the LED (including wires, etc) by holding it just above the end of the head when reversed, or just below where the head ends when the right way around. Should I mount it with the curve out? or in? does it really affect the sharpness of the focus that much?

This is purely for fun, so its not critical I get it right, I will post some images when I get a chance, and also run some tests to see how much the beam diverges at different lengths




Edit: Images

Measurement of harvested Aspheric

Close up of aspheric

Aspheric and T20C2

Beamshot with OP reflector and head assembly still attached

Beamshot with no front end of head, no reflector (hand held, so its a little bit low)

Bit of a clearer shot, it does focus better than this, but it is hard to handhold a DSLR and the aspheric at the same time

On low

How I was holding it for the shots

 

[LukeA]

Aspherics don't usually have reflectors.

The image is sharper with the convex side of the lens toward the LED, but the lens collects more light with the flat side towards the LED.

 

[tolkaze]

Okay cool, I will wait till its night time, maybe sometime over the weekend and have a play with what works best. Since this is not built for purpose, it might only be good for a limited range. At the moment, It seems sharp and bright from about 40cm (photos above are at 40cm, further away looks sharper, but i don't want to shine into other peoples offices for testing haha) up to about 8m away... after that I have no idea if it stays sharp, or if it diverges too much... will see how it goes

 

[yellow]

there might be these "rings" with the reflector,
but without it, this "additional spill" simply gets wasted inside the light.

put the reflector in, maybe use it to press the lens in place.

Enjoy additional spill exiting the light, or simply dont look for it
(in actual use it might be a bonus, at wall hunting probably not)

 

[VidPro]

from what i found putting the reflector back in, or using one there. The light from the reflector goes all over the place in nasty rings and blobs and such. it is "more" light foreward, but can deflect from the purpose of the lensing.
When making an asperical i like to have Zero spillover of any light except the main beam, because an asperical can put light WAY out where you can see something far and far away. Anytime you light up the surrounding area around yourself (i call backsplash) that light knocks your iris down so you cant see what is far and far away as easily.

so user can make any choice they desire, to have more exisiting (neer useless) lumens foreward, which can include putting reflection on ALL tubes, instead of the black anywhere, including the light head, the lens holder or anything that would absorb any of the light.
or
have less lumens on the output and the only light being the projection to the far away target.
all throw no backglow

depending on the reflector will depend on how the light from the reflector is also projected, both in and out of focus. so a orange peeled type of relflector would project the orange peel (light) at the focal point, just like it projects the emitter.

 

[Raybo]

No reflector needed to apply for the job.

You will have to get a "close guesstimate" of optimum distance from the LED to the back of the lens, hopefully you will have some kind of adjustment to go from there.

 

[tolkaze]

Okay, no reflector definately works the best when white wall hunting, but in real applications, the spill only barely lights up the immediate foreground. OP reflector is distracting, no reflector is best. I can definately see further with no spill.

The light diverges quite rapidly, i guess this is to be expected. There are other bits of glass from the lens I can pull apart, maybe make a focus system out of it hehe.

Anyways, will see if I can post up night shots soon