Frank, I have a hunch that along with "condenser lenses" you can also look for "PCX (Plano Convex) lenses", which are flat on one side and curve out on the other... The Edmunds site describes them as follows:

"Plano-Convex (PCX) Lenses have a positive focal length that makes them ideal for collecting and focusing light for many imaging applications."

http://www.edmundoptics.com/IOD/Disp...productid=1747

I'm still waiting for my batch of cheap experimental lenses (which includes some of these) to arrive, so I can gauge the effects of various focal lengths and diameters.

From what I've heard, you can go finer and finer with your grit over the coarser stuff. You should be good using your existing glass with finer grit. (That's how polishing telescope mirrors apparently works). Be sure not to have any stray particles of the 'rough stuff' in your work area.

Jonathon: Thanks! I haven't heard of the name 'plano convex', but it makes sense. Seems like the technical name for a condenser lens, but I guess there is some difference.

I'd like to see what you come up with when you get those other lenses in.

Thanks!

,Frank

Condensers and Plano-Convex lenses
 

You'll need to have tubes like that custom fabricated. I searched everywhere as well.

Thanks for the info about the Plano-Convex lenses. Your right. Plano-convex and Condenser lenses are one and the same. Condensers are the name they choose to use for SLR viewfinder optics. Its important to note that the condenser in a SLR viewfinder has a particular focal length which would need to be measured and matched when ordering a Plano-Convex. So everyones question, including mine is what is the focal length of a SLR condenser? Anyone out there have one to check?

-B

I do - from my dismantled SLR. I'll try and measure the FL on the sucker when I get home from work (cough, cough... you can tell what I spend my time doing while i'm 'working')...

Jonathon: Ha ha ha!

Besides the condenser, can someone out there that's building one of these for a GL2 tell me what achromatic diopter they're using? I'm managing to get a decent picture without one, but I haven't tested it with Brett's chart yet, so I may find that I require one.

Thanks!

Jonathon: I'm also using PVC tubing. Painting both matching surfaces does reduce the amount of play. I also drilled a hole in the end of the female adapter, and tapped it so that it takes an 8/32 nylon screw. After adjusting backfocus for the SLR lens, I just tightened the screw.

Brett: I was looking at condensers used in 35mm projectors. Their condenser consists of two plano-convex lenses that are arranged like this:

[) (]

I wonder if this would make the lighting more uniform than a single lens, and also help with aberrations. Does anyone have a 35mm projector to try this out? Mind you, a 'true' field lens is normally a single (as opposed to spaced, like in a condenser)achromat lens. As for the focal length of a condenser lens: since you're putting it right against the GG, how does 50mm sound? OK, how about 70mm? Should make no difference. Yeah, I know, easy for me to say, I don't even have a condenser to try it out with...

Ah - that's a good idea, Roman... I'll try that tonight (after I measure my condenser's FL)...

Condensers/Plano-Convex lenses
 

I really feel that the focal length of the condenser is important even though its resting right on the ground glass. Dont forget the curved side od the condenser is still forced to be few millmeters away from the ground glass though.

About the focal length info I previously post. I just remembered that its not going to exactly match the focal length of a SLR's condenser afterall because after the condenser the gap between it and the achromat macro filter is diferent for each video camera. I can say that the focal length for a SLR condenser should be in the ball park of what we need to end up using.

I can say one good thing though. When you have a diffused source behind the condenser (ie GG) you dont have to be exactly perfect on the focal length for the condenser. The closer the better, sure, but get it close and it shoud be fine.

-B

Forgive me if I'm being redundant... the months of ongoing 35mm adapter posts are getting hard to keep track of. Anyway, knowing the focal length of the condenser lens will help a lot, but the focal length of the fresnel should also be taken into account. On my SLR, the fresnel is very hard to see, but is located on the back side of the "ground glass" piece, with a mini condenser lens in the very center. That's why the center is so much clearer to focus than the surrounding area. I've been searching for weeks trying to find any reference to their focal lengths, but have always come up empty handed. I'm certainly looking forward to Jonathon's calculations.

Surprising Condenser FL Results
 

Well, I measured it - and the focal length: 155mm (!)

More information:

My SLR (Canon AE-1) had the following in its viewfinder mechanism (forgive the ASCII Art):

Side View:
(Not to scale...)

The mirror reflects the image up through the condenser onto the ground glass/fresnel. That image continues up into the pentaprism where it gets inverted and comes out through the ocular eyepiece magnifier.

When you snap the picture, the mirror flips up out of the way, so the 35mm goes straight to the film on the back wall.

The condenser is actually rectangular, exactly 36mm x 24mm (the same size as a piece of 35mm film).

It is only about 2mm thick at its thickest point, so you can image - across the 36mm width - with only a 2mm change in thickness the 'radius' of the lens would be quite large... first clue of a long focal length.

I have a piece of black construction paper (that I'm using to line/cover things I don't want to be reflective in my adapter). I set this out on the table and held the condenser above it until the image of my overhead light was clearly visible on the paper. I had a ruler measuring the height above the paper - 155mm.

I could get the sharp image at 155mm regardless of which direction the curve faced.

So.... it doesn't look to me like that 155mm has anything to do with the focal length of the camera (42.1mm in my case).

For what its worth - hope this helps.

Phew, that's a lot of info.
 

I've been reading the pages and pages, bit I've lost track of what the most recent discussion is regarding! Can someone quickly reiterate for me what the problem is with the "static design" that is currently being discussed?

on some adapters there is chromatic aberration and some blooming/hotspotting i believe.

Ari...
 

Well, I would summarize the current situation as this:

We're trying to innovate, improve. But... it works - right now.

Alain and other have already built successful models. By most measures, "Good enough." We're just looking at little things here and there (like vignetting in the corners, which has been solved in practice, by Alain with a condensor) and tube construction (tips on PVC screws from Roman, and custom fabrication from Brett are good hints for solving this)...

In short, we're running out of things to fix!

I think the majority of us are just waiting around for parts, and completing construction before we all begin the next batch of improvements, and so on, and so on. We'll probably never be done - but it's looking pretty dang good in its current state, (just ask Alain). The design as it stands works just fine. We more or less have a solution which, for somewhere between $12 and $200 (depending on what you buy) provides a similar result as that of a commercial product which *rents* for higher than our construction price and sells well into the hundreds, if not higher. Time well spent, in my book.

So lies the path of continued evolution.

Check out Jim Lafferty's tutorial (back a page or so) for a good start on the current best-built solution.

Still to be tackled, are a 'best solution' for the inverted image at the DVCam. Alain's fisheye/viewfinder combo showed promise and talk of inverting prisms here and there also interesting. Noone, to my knowledge, has inverted the image 'in the path' without need for an inverter on the viewer... might be possible, but we can't give up too much light.

I think the progress, so far, has been the result of so many people, (Those mentioned above, Tavis, John, Joe, Josh, Dino, Giroud... so many others previously), sharing information and tips so we all benefit. This forum rocks.

Sorry (sniff), just enjoying how cool this kind of exchange is. Imagine trying to do this in pre-internet days.

Peace-

PS - I can't wait to start seeing finished films made with the adapters. Its one thing to put some PVC together, its another to formulate a piece of film worth watching. Looking forward to the results.

Brett:

You highlighted this sentence in my tutorial:

"Continue grinding, both sides if you have to, until you've got a uniformly frosted surface"

And you're correct -- this is not optically ideal. As a corrective, I've now added:

"Note: Any scratches that mar the surface of your filter will show up in your camera's CCD. You may find it to your benefit to cover one side of your filter with strips of scotch tape in order to avoid scratching, while still allowing you to see through it completely. Ideally, you're going to want to frost only one side of the glass, but if you scratch both sides, grinding them until they're uniformly frosted will be your only recourse."

Incidentally, I've frosted both sides of my filter, and while it stands to the laws of optics that this isn't ideal, some eyeball checks seem to prove it OK.

Frank:

The GL1 setup I've got uses a +7 achromatic diopter from Centry Optics. It's not heap ($210), but it kills several birds (for me) with one stone:

It's zoom through, meaning I can use it with other projects,

Its diameter is perfect for pressure-fitting this adapter,

Its quality glass.

For those of you late to the show, here's my tutorial.

- jim

Jim
Your last link to the tutorial is not working.
Alain