condenser madness!

[Martin Lindstedt]

Hi all!

I've been messing around with condenser lenses for some time.

I just wonder if anyone has been experiencing the condenser to be working fine from a distance, ( evenly bright ) but once you put it in front of your camera, a hotspot appears, within the 36x26 area.

I tried with a smaller condenser lens, and it didn't have this problem. Nice and bright all the way, but too small to use though ..

So I was thinking, it might be the focal length it all comes down to, the shorter you get, the brighter, but after searching through the threads here at dvinfo.net, it has been said that the focal lenght isn't an issue.

Can someone help me with this? I'm kind of lost.

PS: Small lens: 30 mm dia, 40 focal length
Big lens: 50mm dia, 80 focal lenght

[Cosmin Rotaru]

I think the camcorder (not sure what part - mabe the CCD :) ) has to be in the focal plane of the condenser. So yes, the distance between the condenser and the lens DOES matter.

[Nicholi Brossia]

Hi Martin,
It looks like you're actually asking two different questions with your post.

once you put it in front of your camera, a hotspot appears, within the 36x26 area

I noticed the same thing with my experiments. My theory on that is that the camcorder is more sensitive than your eye to the brightness difference between the hotspot and surrounding area.

it might be the focal length it all comes down to, the shorter you get, the brighter

You're certainly on the right track. As far as focal length of the lens, that matters greatly. A lower focal length (using your 40mm as example) lens will diffuse more of the hotspot. Generally this would indicate that a low focal length lens is the answer. Unfortunately it isn't quite this easy. The lower focal length lens causes positive image distortion. A longer focal length (using your 80mm as an example) lens will not diffuse as much of the hotspot, but also won't distort as much as the shorter lens.

This all sounds like a rock-and-a-hard-spot situation, but there is an answer. If you stack two long focal length lenses in sequence, you will produce the less distorted image with better diffusion.

I have a feeling that my explanation isn't up to par. Page 4 and page 5 on Topcontechnotes webpage provide very good information on exactly what you're asking.

[Martin Lindstedt]

Wow, thanks alot guys!! I really appriciate your help!!

And, yes Nicholi, two condensers in sequence did the trick!! .. Nice and even! :)

Thank God for this forum! :)

[Joel Aaron]

<<<-- Originally posted by Nicholi Brossia : Hi Martin,
This all sounds like a rock-and-a-hard-spot situation, but there is an answer. If you stack two long focal length lenses in sequence, you will produce the less distorted image with better diffusion. -->>>

So something like two 50.8MM X 76 FL in sequence is a good solution?

I found that lens available here:
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2032

Before I purchase I just want to verify I'm not making a mistake. Can anyone verify this, or offer alternative condenser advice and source for purchase?

Thanks for any help you can offer.

[Nicholi Brossia]

You're on the right track.

Edmund Optics makes great, precise lenses, but are still quite expensive for experimenting. You may find that a 50.8mm diameter works great, but the 76mm focal length is too short. Having to buy all those $30+ lenses would add up fast. I recommend taking a look at SurlpluShed. They have lots of lenses that cost substantially less than Edmund, but will work just as good for prototype and experimentation. Then, once you make your decision/calculation and are ready for the final build, you can purchase the exact lens you need from Edmunds.

[Martin Lindstedt]

When I get a hold on some money, I'll buy myself a brand new condenser lens, since every lens I have gather from old projectors are either made of plastic or full of scratches. Since a new lens probably comes with a big pricetag, I just want to now if anyone can recommend buying ONE 60mm lens with 66mm FL? 66mm FL is the closest thing the optician could get me ..

Alright, this is getting old, the same old bitching about condensers .. sorry, but I need understand this before I do something stupid and expensive .. :)


So with ONE 60mm lens with 66mm FL after the GG, I can expect the following:

1. No hotspot
2. Image distortion

So, I guess my question is .. will I have image distortion, even though my FL is bigger than the lens diameter? .. Or will image distortion be introduced once you go under that value ?

[Nicholi Brossia]

will I have image distortion, even though my FL is bigger than the lens diameter?

Different folks have different theories on this. I'm sure there is a definate answer, but it seems that no 35mm adapter-makers have taken the time to perform a proper, controlled experiment to see just how much diameter vs. focal length will distort the image.

I do, however, remember one member using a 50mm diameter, 50mm focal length lens and commenting that it produced too much distortion.