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Old February 21st, 2004, 09:57 PM   #256
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<<<-- Originally posted by Tavis Shaver : Okay, but does this solution correct chromatic aberrration?

So let me get this straight, we don't need any achromats, but instead it would go:

(SLR) - (GG-Fresnel-Condenser) - (Macro) - (DV Cam)

So if this is the case please explain the specifics of the fresnel and condenser required, ie. do they need to have differing values depending on what SLR lense used? Any good source to buy these? -->>>

Any 'good' lens is an achromat lens. Brett covered this very thoroughly in his posts. Your macro should be an achromat. Is there a suitable acromat for a field lens? I don't know, sorry. You don't want Fresnel for the field lens because of it's inferior quality.

There is no dependency: SLR lens projects onto the GG. This image becomes an object for the second part of your system. Both parts can be treated independently.

Another consideration: if you have a really good diffuser (like a holographic one), you may not need a field lens at all.
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Old February 21st, 2004, 10:07 PM   #257
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Roman you keep using fresnel in your examples. They work fine in SLR's but you need to replace them with a achromatic condenser (field lens) for our purpose here because the ridges of a fresnel lens are visable to the camera's lens.

Also, as I understand it, SLR viewfinders use both a fresnel AND a condenser with one on one side of the GG and the other one on the other side. The reason for both is the same reason that achromats need two lenses to correct for chroma abberation.

So if this is all true you would need two condensers (field lenses) so they can work together on this problem.

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Old February 21st, 2004, 10:17 PM   #258
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Halo diffusers

When you say a holographic diffuser I guessing you mean the ones found at EdmundOptics.com. What has your experience been with them? Sounds like your saying they have a much wider viewable angle than normal ground glass but even if thats true I would think they would be brighter if viewed straight on. Besides if they have the characteristics of being viewable at the same brightness at very wide angles that mean they have incredibly high diffusing properties. The more something diffuses a light source the more light you need to pump thru it. In other words they would suck up a hell of alot of f/stops worth of light.
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Old February 21st, 2004, 10:35 PM   #259
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Seems we need to figure out the proper focal length for our two element achromat condenser. What I cant seem to figure out is what distance to measure for it.

A)The distance between the gg and the CCD of the camera

or

B)The distance between the gg and the front element of the achromat macro lens in front of our camera

or

C)?
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Old February 21st, 2004, 10:45 PM   #260
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doesn't the power of the achromat correspond to the focal length of the primary lense (the SLR)?
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Old February 21st, 2004, 11:01 PM   #261
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As I understand it there is a relationship between the focal length of the SLR lens and the condenser because the angle of light coming from the SLR lens changes as you zoom in and out with your lens. This is why the hot spot on the ground glass might be even worse when you use one SLR lens verses another. Apparently though there is a bit of wiggle room for the condenser otherwise we would be seeing hot spots in our still camera viewfinders as we zoom in and out. So lets say for example we want to use a Nikon 28-135mm lens most of the time with our adapter...what would be the ideal focal length of our achromat condenser?
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Old February 21st, 2004, 11:12 PM   #262
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that is a very good question that i am in absolutely no way qualified to answer...
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Old February 21st, 2004, 11:15 PM   #263
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Ah... this is getting fun. (And extremely addictive... I stayed up until 8am reading about optics). Anyone else feel like starting an 'Aldu35 Anonymous' group?

[jonathon] "Hi, I'm Jonathon"

[group] "Hi Jonathon"

Anyways... I'm just going to try some things... too much reading and narrative speculation for me. Need some hands-on so I have a better feel for it.

For any interested, http://www.sciplus.com is an interesting resource for really inexpensive optics. I'm not looking at these things as my final optics - but as a means of practical experimentation.

I ordered 6 2x2 ground glass squares, about 20 various sized lenses, a set of 3 various achromats, 3 credit card fresnels for like $18.00 including shipping. (!). They also have prisms, etc... I just wanted to start with the optics first and worry about inversion later.

They acknowledge that the quality isn't great, but I'm not all that interested yet. For <$20 I'll have lots of things to experiment with and once I feel like I know what the (explitive deleted) I'm doing, I'll invest in better optics.

The holographic diffusers are interesting - $100 for a 2" square - but it would replace the ground glass while keeping 85% of the incoming light. The only question is the image on the diffuser good? Hard to know without seeing and hard to just drop $100 to check it out.
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Old February 22nd, 2004, 12:46 AM   #264
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Nothing like a good old trial and error but I wonder, are the lenses in that kit labeled? I hope so otherwise when you find a element that works you wont know what it is and what you need to go out and buy.
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Old February 22nd, 2004, 12:48 AM   #265
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That would be a real shame.
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Old February 22nd, 2004, 03:10 AM   #266
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Roman,

Louis Feng was researching using holographic diffusers for this purpose on the other Static35 thread, but the middle of last month, he sort of disappeared...

Maybe he was on to something and they found out.

Sorry, too many conspiracy theory books.

Anyway, so you also think that a holographic diffuser would probably remove the need for some type of condenser/fresnel lens?
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Old February 22nd, 2004, 07:41 AM   #267
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Drifting away...

When I did my original post yesterday, I didn't mean to stir up a contraversy, I simply felt that the discussion was getting off track when achromats started appearing before and after GG. I did follow this thread from its inception when Alain introduced his amazing results. Let's just get back to reality here: Alain had created a simple, elegant system that works. He took time with his GG, and got excellent results with just one additional macro lens. Jonathon is right on the money: we have to experiment more, and then report our results. Thanks for the link, Jonathon, just lay off those Fresnels :), they're no good for imaging applications. I only used the word Fresnel as an example of a field lens, I didn't mean to use them as part of our system.

I have no real experience with holographic diffusers. All I know is that circular diffusers are available with angle of diffusion from 5 to 80 degrees. Lower angles mean brighter image and (potentially) a hotspot. True, at $100 a pop it's an unkown commodity.

What is the focal length of an achromat condenser? I'm gonna stick my neck out and say: in theory, it doesn't matter. As I said before, if that lens is positioned in the focal plane of the SLR lens, i.e. right after the GG, it does not introduce changes to the focal length of the system. Take any thin lens you have, put it flat on a page of text. You should see no magnification.

Since you're discussing the SLR lens / condenser lens relationship, let me say this again: any optical system can be broken into its sequential elements to analyze it. In our case, the element 1 (SLR lens) makes an image I1 of the object O1 (whatever you're filming). The I1 is visible on the GG, but the GG is not necessary for the analysis: knowing the object distance & focal length of the SLR lens, you get the image I1 distance from that lens. This I1 becomes the object O2 for the second optical element - the achromat macro. The macro forms an image I2 that becomes an object O3 for camcorder lens. Notice I'm not mentioning the GG itself or the condenser lens, they're not relevant for calculating distances, since both are positioned in the focal plane of the SLR lens.
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Old February 22nd, 2004, 08:15 AM   #268
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There certainly won't be fresnels in my final adapter. I also think my kids'll get a bang out of some of this stuff, so I just got a batch of things to mess around with. Yeah - they're likely *not* labeled, but then again, I'm not really looking for that kind of information... I'm more interested in answering simple questions like, "does the focal length of the condenser even matter?"
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Old February 22nd, 2004, 12:56 PM   #269
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lets review

so no fresnel at all? hmmmm...
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Old February 22nd, 2004, 01:31 PM   #270
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No... instead of a Fresnel, I'll have a 'real' lens.

It's been commonplace to use the term 'Fresnel' in this discussion to mean the thing which increases light and reduces distortion behind the GG, because many SLR camera viewfinders actually use a real Fresnel lens to do this. In SLR's there's never a Fresnel in the light path to the film - just in the light path to the viewfinder.

So, witholding more detail, I'd say the diagram has gotten pretty simple recently:

SLR Lens
Ground Glass at correct focal distance
Condenser (not a fresnel - a real lens) as close to GG as possible
Macro (if needed)
DVCam

details, still discussing:

Theoretically the condenser focal length doesn't matter... it should just be large enough to cover the image on the ground glass. It seems like one would want the condenser to be the inverse of the spherical effects of the macro... but now I'm outside of my skills/knowledge.

Best way to 'house' all of this:
Many have successfully used PVC.

There are rumours (Brett) of making the SLR-GG distance adjustable so that various types of SLR lenses/mounts could be used on the same adapter. Haven't seen designs for this yet.

Some talk of a T-Mount extension tube.

Also - in current form the image that reaches the DVCam is still rotated 180 degrees. Makes it a little hard to use - would be nice if there were some way to view the image right-side-up while composing.
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