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Old June 6th, 2004, 04:05 PM   #901
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the bosscreen did reduce the light that much, but I think I can reduce this an the vignetting by usung a fresnel or a condensor behind it.
The groundglass was a mediumformat gg from Mamyia, the markers on it where removeable with aceton. It isn't as fine as your self grided may be, but at this big size it really doesn't matter. Remember the gg is 6x6cm!

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Old June 6th, 2004, 04:25 PM   #902
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<<<-- Originally posted by Bob Hart : Obin

Sanding off the top layer. That option works fine except for the disk having a green tint which has to be manually white-balanced out. Not a big deal but it is there.

Pat.


it's not green it's clear
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Old June 6th, 2004, 06:33 PM   #903
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Hello all...

I'm just popping my head in here to note a finished adapter that works very well.

I did a tutorial on building the Agus35 originally, and then followed it up with an Aldu35 tutorial shortly thereafter, and then got submerged in life's distractions and demands. I finished the adapter recently and took it to NYC for some random tests.

Here are the images. I'm currently working on encoding some footage as well.

This adapter is built from instructions in this tutorial, for the GL-1. It's not the cheapest adapter, but it is simple and effective. The only notable exception to the instructions -- as they currently stand -- is that in the most recent iteration I ground one side of the GG by moving the GG ontop of the grinding glass, instead of the other way around. By doing so, it was easier to ensure that the side that was to remain free of scratches would do so.

The main problems I'm having to overcome now surround keeping dust off the elements, and keeping my shots steady while pulling focus handheld. I sometimes worry that spray paint might be flaking off the interior and settling on the glass -- in the future, I'm going to see if I can dye the plastic of the PVC tube instead of painting it.

When proper time and care is given to callibrating the adapter I've built, there's no vingetting/blooming of the image. I don't use any condensor lenses, and attribute the image quality of my adapter largely to the use of the Century Optics achromat.

I've taken note of James Ball's acid-etching instructions and have ordered etching cream -- thanks for the extensive notes, James. As it stands I'm not sure if I do or don't like the grain I see in footage, but there's always room for improvement and it's nice to have a choice.

Thanks for everyone's who has been contributing!

Anyone know what's up with Agus these days? He had stills back in the day from a short using his adapter that I've wanted to see...

- jim
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Old June 6th, 2004, 09:09 PM   #904
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Obin

Good news if there are clear ones around. Could you advise which brand of CD-R it was.
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Old June 7th, 2004, 08:04 AM   #905
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not sure jsut some random disk at work :) I will look today and tell you
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Old June 7th, 2004, 08:44 AM   #906
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Bob the color in many CDR discs is from the foil on the top. You are grinding the top off anyway; therefore you will end up with a clear disc.
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Old June 7th, 2004, 09:02 AM   #907
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Yes, John that is/was the deal with mine
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Old June 8th, 2004, 12:52 AM   #908
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<<<-- Originally posted by Obin Olson : FYI I just took a normal cd-r NOT the spacer type and sanded the top layer off...now it's a better then spacer-type GG because the cd-r is a higher quality then the cd-r spacer! I took my small orbit sander and did away with the top coating..works great -->>>

I got a cheap obital sander some time ago for this pupose too (only about $10US) but have yet to get around to it. How much better is it than hand grinding, or chemical etching?
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Old June 8th, 2004, 12:02 PM   #909
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It's not better than grinding with AOx -- I don't understand why people are regressing to the original project -- the results simply aren't there.

I've got an original Agus35 that hasn't left the shelf since I made the static solution pioneered by Aldus -- and while it's a nice relic, and has some aesthetic potential, the investment needed to get it working properly outweighs the benefits.

- jim
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Old June 8th, 2004, 07:53 PM   #910
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Jim you really have to change your tutorial to say "grind only ONE side of the UV filter's glass" not BOTH. If they do both they are sure to have very soft images. Not to mention it take twice as long to gind both sides. I only mention it because I know you agree but havent changed the tutorial yet.
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Old June 8th, 2004, 11:55 PM   #911
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Actually, Brett, quoting the tutorial:

Quote:
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.
I've since placed some emphasis on this part of the tutorial, but it's been there for a while now.

In the future, once I get the time to grab more shots of myself grinding, I will change this area of the tutorial to demonstrate grinding by moving the GG over the grinding glass, rather than how it's currently demonstrated...in this way I found it easy to keep one side free of accidental scratches.

Still waiting on my etching cream :D

- jim
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Old June 10th, 2004, 05:08 AM   #912
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Hi all,

I've been reading this forum with great interest for the past few weeks. While I start putting together the components for an Aldu35, I'm also waiting for the design to "settle".

I'd like to help out, so I've done a little more research into optics and home-made cameras and have dug up some useful stuff, I believe. (It's hard to read every post, so apologies for repeating and any wheel re-inventing).

I think it's also important to start identifying the trade-offs so we can make our own choices and learn how to better use the adapter. There are LOTS, especially with regard to:

Peripheral Illumination Loss
=================

After reading up, I'm learning that there are a lot of sources for the loss around the edges. Please correct/add on to these.

a) angle of view
Your choice of lens might contribute to this problem. The wider the angle of lens you mount, the greater the light loss on the periphery relative to the center. No escaping this because it's physics. Unfortunately, choosing a longer lens means losing a few stops.

b) aperture setting
At f2.8, the illumination at the edge of a 35mm image is 40% less bright than the center. At f8, the image edges are 90% as bright. By keeping our lenses wide open, we'll get more light onto the GG but we'll contribute to periphery loss. (I'm guessing this is also why people have reduced peripheral vision at night.)

c) coarseness of GG
Hand-made camera makers seem to find that, all other things being equal, the coarser the GG (ie, less scattering) the brighter it is; however, the more the edges of it will be dimmer compared with the center. Everyone here seems to be going for a fine grain, which helps keeps the GG evenly but not as bright overall, which causes us to want to open up the 35mm lens as much as possible, which contributes to light uneveness! Ugghh!

d) off-axis condenser mounting
This is the principle behind those traffic lights you can only see straight on. If you mount a condenser and it's not right on the optical axis of the adapter, you'll make a real mess around the edges of your images.

e) mis-specified condenser lens
Condensers can sometimes contribute to peripheral illumination loss. There are at least three ways: the wrong focal length, wrong position and/or the wrong orientation. More in the next section.

Condenser Lens Issues
==============

Whether or not to put in a condenser has been debated quite a bit. I'm putting myself in the must-have-condenser camp. I found this to be a really good discussion of condenser/fresnel issues for home-made medium-format camera makers: http://medfmt.8k.com/bronfresnel.html

a) focal length of condenser
Hand-made box-camera makers (who place a GG as a focus screen on the film plane before replacing with film and exposing) suggest a focal length for the condenser that is a little longer than the focal length of the prime lens assembly. This makes sense, because that's the only way to get the condenser to focus on the same point that the front lens is working on (aka, the lens's "rear principal point"). Note, though, it's not possible to get it perfect since that point shifts (and the focal length of any 35mm lens changes from its specified focal length) as you move it away from infinity focus.
The worse thing you can do, I'm reading, is use a condenser lens whose focal length is much longer than your 35mm lens, ie "overshoot" the rear principal point. In other words, if you've got a condenser whose FL matches your standard lens (say ~50mm) and you use your 28mm WA lens insteads.
But it seems to me that it should also be bad if you "undershoot" the rear principal point. Think of a Fresnel light: when you move the bulb away from the lens, it goes from flood (wide, less light in center, gentle falloff) to spot (narrow, more light in center, hard fall off). Flooding the light is like the focal point has "overshot" the light bulb. Spotting the light is like the focal point has "undershot" the light. It leads to a spotlight effect and hard edges.
Both types of peripheral illumination falloff show up in some of the test images I've seen so far. Using the wrong focal length for the condenser can sometimes be worse than having no condenser at all. This might explain why some people have ignored the condenser. I think that before you give up you should try to get focal length and position right at the same time. If you can't get one with a focal length approximately the same as the 35mm lens and position it butt up against the GG, then get another condenser whose difference in focal length makes up for the difference in position.
Re an earlier post, where Jonathan measured the focal length of a condenser element from this Canon AE-1. His was 155mm; but it was actually part of a multiple thin lens element (with a GG/fresnel combo). If that fresnel's focal length was 70 or 80mm, then the combined focal length would be about 50mm, which would sound right. It's the combined focal length that counts and where the condenser is relative to the front lens.

b) orientation of condenser
If you use a planar-convex condenser, the orientation matters. The flat side should ideally face the 35mm lens, the convex should face the direction of convergence. If you put it in backwards, you introduce spherical aberrations or over-convergence. That over-convergence might even contribute to peripheral image loss. I haven't read that specifically however. (Check out this website for a good applet that shows it: http://www-optics.unine.ch/education/optics_tutorials/plano_convex_lens_aberration.html

c) condenser position relative to GG
Putting the condenser after the GG seems most practical for us home-brewers. But, in theory, it's supposed to be better to get the light rays to converge just before they become diffuse as they strike the GG. Getting them to come in parallel helps to reduce diffusion (relative to the coarseness of the grain) and thus loss of brightness, apparently. This might be why in some SLR cameras, the designers put a condenser lens before the GG. But it also adds a little to the light path and means the GG has to be placed a little further back. (Apparently, it needs to be pushed back about 1/3 the thickness of the condenser if it's glass). Getting this right might be tricky if you're building a rigid assembly and you don't have a caliper to measure the thickness of the lens. If you've got a variable spacer between your lens and your GG, it shouldn't be a problem to make this adjustment.


Sorry for the length. Hope it's useful, especially to the people who are trying to catch up.
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Old June 10th, 2004, 08:55 AM   #913
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thank you kaushik for your informations on lens .


I have a link to my finished aldu35 . I wonder if you could look at the design and give recommendations to design changes.

I t was made with 52mm lens filters, 1500 grit ground glass, from opto sigma, and a plcx lens


www.dvinfo.net/media/mellor
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Old June 10th, 2004, 09:02 AM   #914
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Richard,

How do you like the 1500 grit ground glass form Opto Sigma? I was going to order some but they never called me back.
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Old June 10th, 2004, 09:56 AM   #915
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agus35

the agus 35 was made with 52mm lens filters the 50mm piece of ground glass fits perfectly. the filters threaded spacers allow fine tuning of focal length. as to the ground glass I,ts the best I have found or made so far. the part numer is 009-0160

http://www.optosigma.com/miva/mercha...+%26+Apertures
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