Led in an old camera body?

I just had a (maybe silly idea).. what if we mounted a led on the focal plane of an old SLR? would that make an insane thrower?



i have a 200m f/2.8 lense for example.. distance 10m
https://dl.dropboxusercontent.com/s/bsfj4qw7ou1p93y/2014-01-06 21.12.53.jpg
no idea if it got more intense.. but it got very small

50mm f/1.8 10m again
https://dl.dropboxusercontent.com/s/5cbju6ab1zmzs6k/2014-01-06 21.23.56.jpg?m=

i've not an old body i'm ready to break, but those are cheap enough to get..

from now on, i'll keep my eyes open for broken beamers to scavenge

My gut feeling is that optical-grade optics have relatively high losses compared to non-optical optics. The F/2.8 means something like "1/2.8 of the light passes through this lens"... At least in one direction. However, I have not yet checked my aspherics against my SLR lenses... Update later today!

My gut says that the lens would not capture much light coming from the back, due to the aperture being pretty small.

But is that a 35mm image at 10m? If so, that would be about a 0.2 degree beam width (FAHM), and you wouldn't have to capture a lot of light to make a good thrower.

DIWdiver said:

My gut says that the lens would not capture much light coming from the back, due to the aperture being pretty small.

But is that a 35mm image at 10m? If so, that would be about a 0.2 degree beam width (FAHM), and you wouldn't have to capture a lot of light to make a good thrower.

Click to expand...

yeah, but that kind of glass costs about 900€.. 500 used maybe. and it weights.. 1500g

Throw (max lux in the spot) is determined by the diameter of the lens only. Focal length doesn't matter.
And the f number doesn't help directly here because it's the quotient of focal length and effective diameter.
Instead it's a helpful value for calculating expoure times.

In fact camera lenses are of great quality and have very few losses.
It's just that they focus (what a word play) on image quality over the whole image area (which is a lot bigger than an LED).
Plain aspheric lenses would be completely useless here, but they're great for the tiny LEDs in flashlights.

It's so incredibly much harder to achieve this good image quality with high camera lense diameters.
But photographers do like these, too, because it means shorter exposure time.

thank you seven_m, that is very helpfull

so, my frontlense is 88mm the back one is < 40mm, but cant exatly be measured becuase its partly hidden by the lock mechanism

sven_m said:

Throw (max lux in the spot) is determined by the diameter of the lens only. Focal length doesn't matter.
And the f number doesn't help directly here because it's the quotient of focal length and effective diameter.

Click to expand...

Perhaps you could explain a bit more what you mean. I think that what you say is in conflict with these two pretty simple experiments:

1. Take a zoom lens. Change the focal length from wide end to tele (everything else is constant). The spot becomes tighter and brighter.
2. Take a manual lens that has a mechanical shutter control mechanism. Change the aperture from (let's say) 5.6 to 22 (everything else is constant). The spot becomes dimmer.

I think that someone here did a pencil beam thrower with a telescope(?) quite a few years ago. The throw was impressive but the light was big, too. Big and not very useful. I couldn't find a thread though. I also tried a led with my lenses and the spot was impressively narrow with a tele lens. Unfortunately though, only small part of the light actually hit the lens.

Esko said:

Perhaps you could explain a bit more what you mean. I think that what you say is in conflict with these two pretty simple experiments:

1. Take a zoom lens. Change the focal length from wide end to tele (everything else is constant). The spot becomes tighter and brighter.

Click to expand...

Yes, it becomes brighter. But not everything else is constant: the effective diameter of the lens changed also, and this is the important change.
(illustrated by the constant f number: double focal length means also double diameter.)

2. Take a manual lens that has a mechanical shutter control mechanism. Change the aperture from (let's say) 5.6 to 22 (everything else is constant). The spot becomes dimmer.

Click to expand...

Yes, it becomes dimmer. But changing aperture while leaving the focal length constant means changing effective lense diameter.
I can't see any contradiction here either.