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Old February 21st, 2004, 08:05 AM   #241
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Two achromats??

I'm new to this film-look 'hobby', but not entirely new to optics and such. Here's a couple of points:

1. By putting an achromat or ANY other lens between the photo lens and GG, you're altering an arrangement that's already perfect, i.e. a photo lens positioned at the flange length from the film in a 35mm camera (GG in our case) will do it's job better if you don't mess with that part of the system, unless you think that you can beat lens manufacturers at their game, and design a better lens. If so - good luck!

2. The image formed on the GG IS the object for whatever imaging system will follow. If you get vignetting/halos, you may want to put a so-called 'field lens' (already discussed in this thread?) right AFTER (ideally, at zero distance) the GG. A Fresnel, when used as described elsewhere in SLR viewfinder systems, is a cheap version of field lens. The trick is to find a thin field lens with little chromatic aberration.

3. Using an achromat as a macro makes perfect sense unless your DVcam can focus close enough without one (my Sony PC100 can). Using a 35mm projector lens for that purpose is another great idea.

Brett, what kind of quick change mounts are you referring to? I'd be very interested. Do you have a link? The only solution I see so far is purchasing a c-mount for $30 and removing the lens adapter part, but this is hardly universal. A reverse t-mount would do the job, but I cannot find any.

A note on inverting and reverting the image. Looking at Pro35, I believe they're using a porro prism, the same one used in binoculars and spotting scopes. I can't figure out the proper size. The one widely available for telescopes is 1.25", and would be too small for a 35mm image. The next 'standard' diameter is 2", which would be ideal, but I can't find any manufacturers that produce it. The cost of a 2" porro would be pretty high. Another alternative would be to use a diagonal upright (Amici prism, I believe). These do exist in 2" size, but are expensive as well. Also, you'd have to put the camcorder at a 45degree angle.
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Old February 21st, 2004, 09:38 AM   #242
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porro prisim

http://www.solarobjects.com/product.asp?catID=61&productID=283
would this have a prisim we could use
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Old February 21st, 2004, 10:07 AM   #243
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Rhoman Shafro,
I have been in a search for such a mount myself well i found a reverse t mount if you are interested.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2989251818&category=30059
I am too trying to figure out what adapter to get for my Minolta Md f/1.7 lense. I looked at a few c mounts and since i never had one i asked the guys if the bolex thread mount comes off, didn't get an answer yet.
good progress everyone!
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Old February 21st, 2004, 10:13 AM   #244
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2 Richard Mellor: Yes, that's the kind I'm talking about. I'm guessing, however, that the one inside 16X50 binoculars may be too small. Having a porro that's used in telescopes, with 1.25" OD, will be better, but may also not be enough. You could get a Swift or Bausch&Lomb 1.25" porro for about $50-60, and a TeleVue or Astro-Physics for just over $100, but I've yet to find a 2" porro. Here's an example of 1.25":

http://www.bhphotovideo.com/bnh/controller/home?A=ShowProduct&kw=SWEP864&Q=&O=&sku=219216

Overall, a porro gives a better image than a diagonal prism, so that would be my preferred choice.

2 Dmitri Henry: This is EXACTLY what I was looking for! Thank you so much! However, the one I need is for the Nikon F-mount. Since there's plenty of T extension rings available, you won't have to make your own adapter out of PVC or what not. You're guaranteed perfect lens alignment, and it'll look better.

I have a Nikon brand F-to-C. This is a $160 item, but someone gave it to me ages ago. The bayonet mount part comes off, there are 4 tiny #0 philips screws. I'm trying to adapt that to a 1.5" female PVC connector, but would much prefer to save the F-to-C and get a reverse T-mount instead. Like most of us, I don't have a shop to manufacture mounts, cut threads, etc.

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Old February 21st, 2004, 03:29 PM   #245
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re: no achromats before ground glass

Makes perfect sense, Roman... and welcome to the discussion!
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Old February 21st, 2004, 03:52 PM   #246
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Roman-
Welcome to the thread. Its great to have you. Let me answer some of your questions.

1)Reverse mounts normally are a problem because you loose the ablity to focus at infinity but my adapter has a adjustable flange tube so the issue would be resolved and multiple mounts would be possible. Never the less I decided not to go with reverse mounts. Im using straight PL and Nikon mounts (but just about any mount you want will work). Each is attached flush against the face of a filter stacker with a hole in the center. Simply twist on/off any given mount.

2)These guys will fabricate just any adapter piece to your specs.
http://www.srbfilm.co.uk/index1.html

3)Field lenses are the same as condensers.

4)The second lens is probably a good idea to have because the image on the ground glass is not the same as say for example the same image were printed out and viewed by your camera. The reason being is that when the camera is viewing a image projected on ground glass, the image becomes increasingly brighter the more inline you are to the incoming light. Thats why everything from rear projection tvs to the hot spots on our adapter are brighter when your looking straight at it. Your example about film planes/condensers is not valid. Film (or CCDs)dont need any type of condenser because its much much more forgiving on this issue. This is why you dont see a condenser in front of the film gate of a SLR camera.

Thanks for the info about the prisms. I wasnt aware that B&H had something like that. The two prisms you mentioned and a few more that will work can be found in a link I posted here awhile back. You shoud check it out because it has diagrams explaining just how each prism effects light. Heres the link:
http://www.dvinfo.net/conf/showthread.php?s=&threadid=17195&perpage=15&pagenumber=58
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Old February 21st, 2004, 06:36 PM   #247
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Roman-
When your talking about the measurement "1.25" for the porro prism is that dimension representative of the viewable distance across a image circle, square or rectangle?

The reason why I ask is because we need to fit a 36mm by 24mm image within the above image area. 1.25" comes out to just about 31mm.

If it wont fit at least the motion picture film gate at 24mm by 18mm will.
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Old February 21st, 2004, 08:01 PM   #248
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2 Brett: I believe that the 1.25" reference is for the outside diameter of the eyepiece, that's why this porro prism (or 1.25" diagonal prism) will be too small, and the next 'standard' size as far as scopes go is 2".

Reverse mounts are not a problem unless they extend the overall flange distance beyond the standard one (like 46.5mm for Nikon, 42.1 for Canon FD, etc.), that's when you can no longer focus at infinity. I'm trying to avoid manufacturing connections as much as possible, not just for aestetics, but also to have a standard solution anyone could use. Thanks for the link, I haven't studied it yet, but it looks like a goldmine!

The difference between a GG and film is this: any incident light that falls onto film will cause the desired photochemical reaction. The GG scatters light in all directions, and that's why you get a hotspot in the center and vignetting on the periphery due to off-axis light beams. The field lens corrects that. In SLR viewfinders, the Fresnel right before or right after the GG IS the field lens. Fresnel is very thin, and that allows it to be placed right next to the GG, i.e. in the focal plane of the objective. Having the field lens in the focal plane is the key: this placement does not change the principal planes of the overall system, it only helps to collect off-axis light beams.

The difference between a field lens and a condensor is (mostly) semantics, but usually condensors are used to create a collimated light beam (like between a light bulb and the slide in a slide projector), and field lens is used to condition the intermediate image.

The bottom line, IMHO, is that you do need a field lens right behind the GG, but you do not need anything between the objective lens and GG.
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Old February 21st, 2004, 08:57 PM   #249
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Roman,

Just for clarity, when you say "behind" the ground glass, do you mean on the camcorder side?
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Old February 21st, 2004, 08:59 PM   #250
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Yes Justin, always follow the path of light rays. What I mean is this order:
Objective lens -> GG -> Field lens -> Macro Lens -> Camcorder.
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Old February 21st, 2004, 09:05 PM   #251
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I apologize for my deep ignorance of optics, but...

So, unlike in an SLR camera where you might have a fresnel or condenser before the ground glass, you are saying that a better choice, in this instance, is the field lens after the ground glass. Why is that?
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Old February 21st, 2004, 09:10 PM   #252
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It looks like we are on the same page here because your basically saying the same thing I just wrote. Its too bad gg reacts that way. BTW the 2" prisms exist. Im going to have to hunt down where I last saw them. I wonder though how DVX100 users will ever get ANY type of prism(s) in the shallow gap between the camera's lens and the ground glass. With one +10 diopter the minimum focus is 10cm which, if Im not mistaken, becomes 5cm with the use of a second +10 diopter to make a total +20. The DVX is going to need the help of a +16 or greater power diopter to beable to focus on the intended target on the ground glass. With the P+S Technik being used on the DVX100 already we known that it is possible...but how?
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Old February 21st, 2004, 09:24 PM   #253
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Because the Fresnel is so thin, it forms a single unit with the GG. You can put the Fresnel either before or after the GG, although I still think that putting Fresnel after the ground glass is better: you're collecting diffused off-axis light rays & make them follow the optical path of your system, thus increasing image brightness. AFAIK, most SLRs have a GG in front of the Fresnel. Have a look at this link, it'll help:

http://topcontechnotes.home.att.net/viewingsystem/page4.html

The article also goes on to describe chromatic aberrations and pincushion correction, and the use of a Fresnel.

2 Brett: I'm totally confident that you'll find those 2" suckers, judging by tons of useful reference material you've dug up so far! If 2" porro is not available, how do you feel about a 2" 45degree diagonal prism? I think that's what MovieTube is using, but having your camcorder at 45 degrees feels awkward to me.

Brett, please disregard this paragraph, I misread your post...
[I haven't given much thought to the prism placement, but I'd say it has to go after the GG, not before. You just don't have the room between the objective lens & GG. The additional path through the prism should also be considered when selecting the macro. The distance added is equal to the light path through the prism times prism's refraction index.]
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Old February 21st, 2004, 09:34 PM   #254
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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?
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Old February 21st, 2004, 09:48 PM   #255
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It does if your field lens is an achromat. Or, if your field lens is a thin plano-convex, but I'm not 100% sure on that.
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