Roman: I am using After Effects' native transform options to rotate the image 180 degrees. I'm sure you can do this in Premiere or Vegas.

I'm not sure about what kind of condenser. I was assuming they were all pretty much the same. I do not know for sure, though.

I just did a test, I put the fresnel on the camera side of the gg and it did smooth out the hotspot by making it about 20% larger, so it moved the light outward.

This is good for theory just proving that we need a good acromat. A fresnel has rings which are visible, not good but did well for experimenting.

I will have some acromats in a few days, to solidify the theory.

Thanks, Frank! John, just for the fun of it, could you try to put the Fresnel on the SLR lens side of the GG? I wonder what the results would be...

<<<-- Originally posted by Roman Shafro : So, my order would be:
SLR lens -> spacer -> GG -> Condenser -> spacer -> Macro (if needed) -> DV -->>>

But the probleme remains ... what's the best deal for the condenser part...

a field lens is a good choice but it gives a lot of distortion....2 identical field lenses is much better...but it's bigger...this is the reason why the first field lens is replaced by a fresnel just after the GG in all the SLR viewing system...

we have a lot of options
1- single field lens
2- 2 field lenses
3- 1 Fresnel (but a lot of ghosting effetcs)
4- 1 fresnel and a field lens
4- an achromatic

think that now, experimentation will be the key...

heres some pics of my fresnel placement test, as you can see the fresnel does its job best on the camera side of the gg, however the rings are visible, thus I will replace the fresnel with an acromat later this week.

http://aequantum.com/fresnel.jpg

cool john, that's great! can't wait to see what it looks like when you get your achromats.

Thanks, John! I guess the question of which side the condenser should be on is over... Olivier, I'd say a thin achromat field lens is your best choice, and the focal distance shouldn't matter...

The reason why I thought you needed two corrective lens (one on each side of the ground glass) is because I was under the understanding that in a traditional SLR viewfinder system the order was as follows:

35mm lens>mirror>FRESNEL>GROUND GLASS (sometimes the fresnel and ground glass was one piece with the fresnel on the 35mm lens side and the ground glass on the other)>followed by a condenser>and then the prism and eyepiece optics of coarse.

I'm going to call a local guy that repairs used cameras tomorrow so find out and finally put this issue to rest. In the mean time if someone wants to open their old SLR I would be very interested in what they find.

-B

I have done this, and the one I have is a glass whith fresnel on each side then the pantaprisme and eyepiece optics .

The mark on the fresnel, normaly a circle ,is a small circle on one side and a biger one on the oher side, so it look like when you are perfectly in focus the image that correspond to the film plane are in between the two fresnel .

Alain

Alain let me make sure I understand what your saying. The SLR you opened up had a piece of glass that had one fresnel pattern on one side and another on the other side and seperate from that was the ground glass?

No ,It's only one piece whit a fresnel on each side,apparently no ground glass.But it's realy hard to see.I have take a look at it whit my microscope and it look that they make a( ground fresnel )or maybe it's just the quality of the fresnel that look grainy.This is made from plastic also. Probably a cheap way to make this thing.

Alain

totally forgot about holographic diffusers in laptop LCD screens! that's VERY similiar or even the real a real holographic diffuser, this could very well be another key to improving high quality low-light transmition. possibly (yet another and possibly better?) alternative to ground glass (!). just a hunch, but great idea Giroud!

My old dusty Canon AE-1 which had previously been just sitting on a shelf collecting dust (since the purchase of my digital camera) contained ( in this order from behind the lens):

nice mirror: 35mm film size
real condenser - not fresnel right up against: 35mm size
ground glass/focusing screen (reticle on glass): 35mm size
very nice pentaprism 35mm entry - smaller exit
viewfinder had small real lens to magnify - size of small exit

And of course, the real question - was the condenser on the lens or viewfinder side of the ground glass... answer:

"I DON'T KNOW!!" (it popped out as I removed the last screw and I didn't see which way it was. C'est la vie).

I didn't have any fresnels in there.

Thats alright Johnathon. I just wanted to know if it had both. Besides the condenser goes between the gg and the prism. Take a real close look at the profile of that condenser's glass. Is it two lenses cemented right on top of each other or a simple single lens. It will be hard to tell so look closely. If its two they are using a two element achromat condenser if not its a simple single element condenser.

Hmmmm... upon closer inspection - my ground glass/focusing screen must be a fresnel... because it magnifies as I move it closer and farther away from a subject... but its just very very very fine. I can't see the rings at all, even under magnification. It appears (can hardly tell - even under magnification) that the back of the fresnel has a sort of 'ground glass' finish. (plastic)

And as I look at the shapes of these things, I'm almost positive the condenser was on the lens side. with the 'ground glass' pressed against it, leaving the fresnel side of the focus screen pressed right up against the entrace plate of the prism. (It all seems to 'fit' together this way)... When I hold it all together and put my 35mm slr lens in front of it at the right distance... even with no light baffling, it completely and evenly fills the viewfinder with no obvious vignetting.

The condenser to the best of my magnified attempts appears to be a single element. Must not be an achromat.

(heh, Brett, I see you're awake on the CML threads too...)

See now we are back to what I was talking about before - the apparent fact that SLR's use BOTH a condenser AND a fresnel. We need to confirm the proper order of them including the GG. Once we have that its a matter of replacing the fresnel with the same power condenser. From what I've seen SLR viewfinders have a single piece of plastic (or glass) that has a very fine fresnel pattern pointing towards the 35mm lens and the ground glass texture on the otherside. Following that is a condenser lens.

A field lens with sufficient power to bend the edge rays into the DV cam lens results in a lens with great curvature.

Not only will a lens with such a short radius of curvature produce:
- a viewed image with unacceptable pincushion distortion
- a large amount of chromic aberrations.

Two thin lenses in series will give the same power as the thick lens with great curvature.
But each of the two lenses have less curvature, thereby greatly reducing the pincushion distortion and chromic aberrations.
But for a camera it has the disadvantage of taking up space, making the camera larger.
Camera designers realized that what they needed was a very thin lens with reasonable power.
Fortunately, such a device was available: The Fresnel Lens.


But are we really concerned by space problems here with this adapor...i don't think so...



http://topcontechnotes.home.att.net/viewingsystem/page5.html

http://www-optics.unine.ch/education/optics_tutorials/field_lens.html

What about using white frost lamp effect filter material. The stuff they hang in front of film lights to defuse it? This material is cheap in small amounts, durable and grainless.

Just an idea.

Great ideas here.

DK

a freasnel would be fine to use in the adapter, there are fresnels available that have rings fine enough that there's no way the dv's ccd would pick them up, but you need to combine a condenser WITH a fresnel, like this http://topcontechnotes.home.att.net/viewingsystem/page6.html

<<<-- Originally posted by Daniel Kohl : What about using white frost lamp effect filter material. -->>>

I tryed with that. when you zoom in to find appropriate picture - the grain is visible. i checked different types, but the grain is always visible. the good side is that is very equally placed. so, maybe someone will like it.

filip

Tavis Shaver,

What a great link. For anyone interested in this subject the whole site should be required reading.

Starting at: http://topcontechnotes.home.att.net/viewingsystem/page3.html

all the way to page 16.

Paul
dvdof.com

<<<-- A fresnel would be fine to use in the adapter, there are fresnels available that have rings fine enough that there's no way the dv's ccd would pick them up, but you need to combine a condenser WITH a fresnel, ... -->>>

This solution was chosen only for a matter of size...
2 low power condensers are better then one of high power to avoid spherical distortion.
And when u REALLY need space u can replace one condenser by a fresnel...but optically the fresnel MUST not be used in a system dedicated to record a sharp image...

is space really a limiting parameter in our equation?

what about coating some stuf on a regular glass.
For example if you already have used cyanoacrylate (instant glue) on glass, there is always a white haze around the glued parts.
You can try to heat some of this glue under a glass.
They do this for looking at fingerprint on glass.
<b>Caution: cyanoacrylate contains cyanure (lethal gaz), so it is good to take care of doing this outside and not let the gaz reach your skin, eyes or lungs.</b>

Giroud-
Your talking about a similar technique as a product called a "Bosscreen". They are 4X5 and 8X10 focusing screens that are grainless. They achieve this buy melting and then hardening a very thin layer of wax sandwiched between to flat clear pieces of glass. The results are amazing!

Heres the bad news. Its going to be nearly impossible to not only find the ideal thickness of the layer of wax to use but having it lay down completely flat and even is a real feat that may only be possible by a commercially machined process. The other thing that there is a chance that Bosscreens will melt and be destroyed on a hot day. And dont even think about cutting down a commercially made one to size for the same reason. They are expensive too.

One interesting note though. Bosscreens advertise their product as having a microcrystaline layer (the wax). I know another company that says the same about their product...Movietube.

Good luck.

Just got off the phone with a highly recomended camera repair man. He confirmed that there is both a fresnel AND a condenser lens in a SLR as shown before here:

http://topcontechnotes.home.att.net/viewingsystem/page6.html

He also confirmed that without one of the two you will have chromatic abberation problems.

Bottom line is we are back to my idea of finding a replacement condenser lens for the fresnel and having two condensers working together to make a achromat field lens. This way we wont have any "pincushion" (barrel) distortion or chromatic abberation. Damn I wish we could of K.I.S.S. but that would be S.

-B

Hey you glass grinders!

Those of you that chose to go with the 1000 grit - did you like the results? I've heard of people using 320, 600 and 1000. There was some speculation at some point that there was a point where you've gone 'too' fine and would start to lose the diffusive properties... You can go finer that 1000 grit Aluminum Oxide as well, wondering what the pros and cons of finer and finer grit are...

Any comments from the polishing among you?

I have mine done whit 9Y (9 micro bryte capsule) that is a bit smaller than 1000 and it's ok ,but after that ,there is something call Red pollishing that I have try ,but then I start losing the diffuse properties and I start to see thrue it , so ,no good.In fact it can reduce the grain more but I was not able to control the grinding enought so I start having some tranparent spot and this apend just in couple of second.

Alain

I did mine w/ 1000 alum ox, I just did a quick 1hr grind which I should do another 1hr final grind.

here is a pic, look at the top pic- notice a very small amount of grain in the edges where the light is less strong. I will do a final grind shortly to clean it up but overall I am pleased with the results of 1000.

http://aequantum.com/fresnel.jpg

Alright, here's a dumb question for Alain (or anyone with the answer):

I've got a Tiffen UV filter here and it has on both sides a raised ridge at the edge. How, if I'm supposed to have a piece of glass as my grinding implement, do I make contact with the UV filter's surface with the grit and the grinding tool?

My piece of grinding glass exceeds the diameter of the filter, and so the ridge prevents the two pieces of glass from touching. Did you use a smaller piece of glass to do your grinding? Or, did you remove the filter from its mounting ring? (and if so, how?)

Thanks...

- jim

yet another question about ground glass...

Is there anybody who really tried an holographic diffuser....
Theoretically they make the post condenser useless...

anybody with real tests?

thanks

1-Shure you have to remove the glass from the ring.

2-That's why you have to start whit bigger grain , to make the surface of the glass even .Sometime you could put a bit of pressure in the center but not much.

That's the reason why you need two pieces of glass plus your filter, you grind on one for a time and then you change for the other one .Otherwise the surface of the glass you'r using for grinding start to become uneven .(concave)

Hope you understand, I am missing some english word to explain it more clearly.

By the way I whil send you some new image in 1 hrs.

Alain

Jim I was thinking of using the same thing - uv filter on a ring.

Here is what I was thinking of doing:
Get a small round mirror from a craft store, a small suction cup like one from a child’s toy dart, attach a shaft to the suction cup, the cup to the backside of the mirror and the shaft to a dremel, then buff away.

I have not done this yet, but that is my plan. Although I may have a hard time keeping it even.

Hmmmm - I've always understood the grinding process as this (and I think this is what Alain is describing).

Take a large piece of glass - say 8x10. Put it down flat on a clean work area. Put your grit mixture onto this and smooth it out a bit. Then you take the glass from the UV filter (or whatever your real ground glass is going to be) and put it down on this 8x10. Space your fingers out evenly across the back of the filter and work it in circular motions on the much larger piece of glass. So you're moving the UV filter across some larger piece of glass... not putting some tool to the stationary uv filter.

This is what Alain means... after a while, the area of the 8x10 will get ground in under the pressure from your filter, and you either have to move to a different spot or get a different piece of glass

Alain: how did you remove the filter from the ring?

In your original image of the ground glass -- the close-up -- it's still in the ring after being ground. I'm confused :/

- jim

In order to remove it, you must have one of the filters that has a 'retaining ring' (not all do... my Hoya is completely unopenable). If it has a retaining ring, you'll usually see two slots across from one another on one side of the filter. You use something called a 'span wrench' which is like a double-headed screwdriver which stretches across the filter and when its in the slots, you turn it, lifting out the retaining ring.

Yes ,because I put it back on the ring so I can use it to screw on my other parts.
My adapter is made from aluminium tubing that screw on each other

Normaly the tiffen filter have a ring that you unscrew to remove the glass, but take care they are somtime hard to remove.Take care to not scracth you glass and not to scrap the screw or (filets in french)


Jonathon
That's it . But I normaly change from one glass to the other every
5 to 10 min.

Alain

Jim, most of the filters I've seen have an inner ring that needs to be unscrewed from the outer ring, thus releasing the filter. Professionals use a spanner wrench to do it:

http://www.edmundoptics.com/IOD/DisplayProduct.cfm?productid=1457

Expensive stuff... Look at their Instruction Sheet )PDF file), you may want to make one...

P.S. You guys type too fast, I'm always late with my replies :).

Alain, what kind (size, source?) of tubing are you using? I'm still playing with PVC...

I have remove it whit a small screwdriver but whit precaution.
hold your filter whit something realy steady ,like a vice ,and that's it

Alain

Damn, this is an annoying detail I wish was included earlier.

My filter has these two notches seated opposite of eachother, so I'm assuming this is a retainer ring filter. The thing doesn't want to budge, though :/

edit: nevermind

- jim

---Alain, what kind (size, source?) of tubing are you using? I'm still playing with PVC...


I have use part's of len's that I have unmount to make my adapter.

I keep everything,and turn it to something else. :-)

Alain

i'm just amazed at how addictive this project is for everyone. we are discussing lot's of different issues at once, it's crazy and great at the same time...

so here is what i've figured out so far... ideally you WILL affect the image quality using a FRESNEL lens, so optimally you want to go with a cheap and better CONDENSOR LENS like here, http://www.edmundoptics.com//IOD/DisplayProduct.cfm?productid=2032

secondly, you can use a GROUND GLASS or as a better quality alternative a HOLOGRAPHIC DIFFUSER-but they are expensive, unless your on the Euro...

so far the K.I.S.S. (keep it simple stupid) seems to be the ideal configuration, which goes as such for a standard DV cam:

PRIMARY LENS+spacer+GROUND GLASS or HOLOGRAPHIC DIFFUSER + CONDENSOR LENS+spacer+[if you need a +7 or greater macro]=mounted to the DV lens

at this moment i'm crawling up the walls trying to figure out a better method for xl1/xl1s version as it seems a bit tricker-you can almost put a lens right up to the CCDs themselves-so this creates much more complex optical arrangement that needs to be optimized.

comments, thoughts, quips?

back to work...
-D

Hey All,
This thread is amazing! I stumbled onto it accidentally and haven’t been able to leave it alone. I started building my Agus 35 when I found the separate Alain 35 thread and so I’m building both simultaneously. I’m at the point where I now need to install a condenser lens to help with the hot spot in the middle of the image. My first question is, what are the condenser lens specs others have used? Does it depend on my choice of SLR lens or are all of you using the same condenser lens? I followed the link to the Edmunds optics condensers but I have no idea which lens is right. I read in a thread earlier that a condenser from an old slide projector might work. Has anyone tried that?
Second, does the condenser lens really go between the GG and the camcorder’s lens? You are still focusing on the GG with the camcorder lens, right? So you focus through the condenser lens? This doesn’t make sense to me as I guessed that the condenser would spread the light more evenly across the GG as it does with a slide projector. Can anyone set me straight on this? You all are amazing. Thanks for a totally entertaining project.
Oh, BTW, can anyone provide a link on more info about the Hollographic Diffuser mentioned by Dino in the previous post?
Joe Holt

http://www.edmundoptics.com/IOD/DisplayProduct.cfm?productid=1363

That's Edmund's holographic diffusers - pretty expensive. In general, I've found most things available at Edmunds for up to 75% less in other places. Holographic diffusers, unfortunately, are not one of those items I've found anywhere else :-(

I've learned some interesting things messing around with the optics removed from my 35mm SLR.

All of the optics involved (other than the objective lens) are rectangular and exactly 36mm x 24mm -- the size of a 35mm film negative.

However, the round objective lens definitely 'sees' a circle - which is significantly larger than the 36mmx24mm footprint.

One thing I've noticed about my rectangular optics is that they seem to be completely covered, evenly, with image. I can't help but wonder if some of the corner distortion others have seen comes from not zooming in quite enough. With a circular image on a ground glass, I would think it would be hard to gauge where the 'true' 36x24 frame lies...

An Agus builder could theoretically draw a 36mmx24mm rectangle right onto the ground glass - and focus their camcorder on this rectangle. I'd bet they would find significantly less distortion within the boundaries of this frame (especially with a condensor or two). Alternatively (if you didn't want to mark up your hard-earned ground glass), you could insert another piece of glass with the marked frame.

Here's an (unfinished) article on the cropping I'm talking about... be sure to read pages one and two - the cropping is shown on the second page...

http://www.digitaldingus.com/articles/fov/fov.html

Nice article, Jonathon! Pardon my ignorance, but is having a 36X24mm GG image a kind of a 'Holy Grail' concept that cannot be altered? What if it's slightly smaller? Just wondering...

Anyway, to zoom in to the correct size (let's say 36X24), you can print a 36X24 square in a circle on a transparency sheet, and stick it right after the GG. Remove after zooming.

About Holo diffusers: well, I've tried them last night. Don't ask how I got them, that's a whole other story... I had 3 samples to play with, 5deg, 20deg, and 60deg diffusion angle:

1. 5deg is the brightest, but exibits a strong halo, and the texture is too rough for the DV lens not to notice. With the SLR lens fully open at 2.8, the out-of-focus objects looked like they have 'crawling ants' all around their edges. This could be also a 'feature' of my crap Sony PC100.

2. 20deg is OK, but just OK. Somewhat smallish halo is visible, but if you don't mind zooming in a bit, could be OK. Still, I think it would require a field lens. The results are NOTHING compared to what Alain had produced with his finely ground GG. With my camera (1/4.7 CCD size, what a joke!), you get a lot of digital noise.

3. 60deg is way too dark to be of any use. So dark, in fact, I couldn't even get a focus (manual, of course!).

After this fiasco, I started thinking: if Halo diffusers are so great, how come no manufacturer uses them for typical photo applications as a GG replacement? I'm sure they could spare $100 or so for a quick test :D

Bottom line: I think Alain's solution (lots of elbow grease) is the one. Unless, of course, we get hold of the supplier of this magic 'crystalline wax'. Since I'm lazy, I had high hopes for Holo diffusers, but...

Wow - good info on the holos... we'll all just assume that they "fell off the back of a truck." You make a good point about the photo applications not using them - and sticking with the tried-and-true ground glass. Once again, Alain finds the best solution first shot out of the gate, and then we all run around with our heads cut off trying other things only to learn he had it from the beginning :)

I certainly don't think that the 36x24 gg image is a holy grail. You can certainly focus your entire image on any size ground glass you can get and focus in to wherever you'd like. I just thought it was an interesting point of reference as 35mm cameras certainly are aimed at this size, so their focusing systems (mainly the viewfinder focusing system) would be aimed at minimizing optical distortion only to this size... not the entire circle. It would seem like we'd be pushing our luck to expect no optical distortion outside of this frame.

Speaking of focusing - this is another area of concern for me. Since our cams will be hard-focused to the ground glass, all of the actual image focusing will be done manually with the 35mm lens. I, for one, put little faith in the ability to consistently get good focus on my crappy (and at the moment, inverted) viewfinder. It is common practice in medium format photography to have a ground-glass 'focusing screen' which the photographer actually scopes with a magnified loupe to inspect for focus. There has also been specialized focusing screens that either line up two lines or their color evens out when in focus - a number of tricks which make focusing much more quantitative - less subjective. Given our manual focus situation, it'd be interesting to see what ideas might pop up for some kind of magnified or non-visual focusing systems.

I certainly noticed that much of the example footage I saw on the original thread had some pretty serious focus issues. If anyone ever intends to blow any of these shots up for projection, they'd better figure out how to have some hard focus :)

I agree with the focus issue and the first thing that poped in my head was the idea of taking the focusing aid circle, that you see often in the middle SLR viewfinders, and off setting it to the very top or bottom of the screen where it would hide behind the area where black letterbox lines normally are. You cant have the circle in the middle because it will ruin all your shots and since alot of us already shoot 4:3 and add our letterboxing in post anyways we would have the best of both worlds. Just a quick idea. I'll give it more thought.

Excellent idea... I wonder if we could stretch your idea even slightly more and place the focusing circle (I've got one from the SLR I just trashed for parts...) just outside the full 4:3 frame. One could zoom out with the camcorder and set the focus, then zoom back into the full frame. If you were good, you could use this technique multiple times to set a few focus marks somewhere on the 35mm lens for a follow focus shot... Of course, if the circle were in the shot the whole time, you could just keep your eye on it. Who needs 4:3 anyway ;)

http://www.stabitech.nl/Bosscreen.htm#top

The manufacturer was not easy to find, it took a bit of Sherlock Holmes-ing, but got there in the end!

They're expensive, (120 euro for a 6x6) - but hey, they have all the right properties,

- excellent light tranmission
- no hot spots
- good dynamic range
- no grain

You can specify no markings, however I would recommend asking for a 36x24mm frame guide to be printed in the middle.
They take around a week to make, and the company is based in Holland.

Ive persoanlly tried commercial fine GG, (6 micron), Holo diff, Opal Diff. The best out of these is by far the Hol diff, but as someone else noted, they don't transmit nearly as much light as GG.
Not tried Bosscreen yet, but contemplating it...