fixed focus -> distance?

[jspeybro]

Hi guys,

Since this post got lost due to a backup restore, I'll post it again.

I'm working on a Mag mod using an aspheric, and while doing some calculations, I noticed that I was missing some piece of information that I never encountered on this forum, and also never really thought about.

If you know the specifications of the aspheric (like (back) focal length), you can quite easily calculate how far away the light will be focussed for a certain distance between the LED and the lens, or you can calculate where you need to put the LED for the light to be focussed at a certain distance (using the lens formula).

What I never thought about or found on this forum, is at which distance a divelight should be focused when using a fixed focus design :confused:

This question doesn't make much sense for reflectors or TIR optics which have an angle that makes the light spot bigger as the distance becomes greater, but when using lenses this can be quite a difference if you move the LED only a little closer.

If you position the LED, do you use a certain distance for which the LED die is clearly and sharply imaged? do you use whatever surface is near? or do you base the distance on what you expect to do with the light?
Or is this something nobody cares about?

I'm usually diving in waters where there are quite a lot of particles in the water with visibilities ranging from about half a meter to a few meters, sometimes up to about 10m if I get lucky. My buddy is usually not more than a few meters away due to the low visibility and I usually stay close to the bottom to look between rocks and things. Therefore I was thinking that focussing the light at about 3-5 meters would be a good setting.

what is your experience with fixed focus aspheric dive lights and the distance at which the light is focussed?

Thanks,

Johan

 

[Packhorse]

I dont think it makes that much difference does it?
I think you need very fine adjustments to change from a focal point or say 1 meter to 5 meters and it does not really change the beam size that much just the cleanness of the beam. To clean a beam and you get the individual lines of the die projected. Slightly defocused and its a more even beam.

 

[jspeybro]

makes sens. it's indeed a very precise adjustments to change the focus quite a lot.
If the LED die is clearly imaged on a wall, do you still see it when under water at a similar distance?

I think I'll just try out with a focus of 4m and see how it goes.

It's a long way from finished anyway since I don't have a canister yet...

 

[Packhorse]

I think above water and below water focusing would be different.
I will not hazard a guess which way it goes but remember that a in water beam is tighter than an out of water beam.

OK I will hazard a guess. A 4 meter focus above water will give you a 5.33 meter focus in water for fresh water at a temp of 16 deg C.

 

[DIWdiver]

makes sens. it's indeed a very precise adjustments to change the focus quite a lot.
If the LED die is clearly imaged on a wall, do you still see it when under water at a similar distance?

I think I'll just try out with a focus of 4m and see how it goes.

It's a long way from finished anyway since I don't have a canister yet...

If the convex outer surface of the aspheric is in contact with the water, it will change the focus a lot.

In order to keep the focus the same, you need a flat lens in front of the aspheric.

 

[jspeybro]

If the convex outer surface of the aspheric is in contact with the water, it will change the focus a lot.

In order to keep the focus the same, you need a flat lens in front of the aspheric.

I'm aware of that and is already taken care off. I did a simulation of this once somewhere in thread of packhorse.
I'm only talking about things behind the aspheric. a little change there can also change the focus a lot.

Johan

 

[DIWdiver]

I'm aware of that and is already taken care off. I did a simulation of this once somewhere in thread of packhorse.
I'm only talking about things behind the aspheric. a little change there can also change the focus a lot.

Johan

Sorry, misunderstood the question.