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.

porro prisim
 

http://www.solarobjects.com/product.asp?catID=61&productID=283
would this have a prisim we could use

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!

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.

re: no achromats before ground glass
 

Makes perfect sense, Roman... and welcome to the discussion!

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

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.

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.

Roman,

Just for clarity, when you say "behind" the ground glass, do you mean on the camcorder side?

Yes Justin, always follow the path of light rays. What I mean is this order:
Objective lens -> GG -> Field lens -> Macro Lens -> Camcorder.

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?

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?

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.]

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?

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.

<<<-- 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.

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.

"Can you dig it?"

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.

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)?

doesn't the power of the achromat correspond to the focal length of the primary lense (the SLR)?

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?

that is a very good question that i am in absolutely no way qualified to answer...

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.

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.

That would be a real shame.

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?

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.

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?"

lets review
 

so no fresnel at all? hmmmm...

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.

Thanks for summing everything up. I still think the condenser lens has to have a particular curvature (focal length) but thats what experimenting is going to have to answer. My adjustable tubes are micro threaded tubes that fit inside one another. As they unscrew they extend and then you lock them in place as you would on a rail system. Im currently looking for some new ones with a flat black finish.

-Brett

Jonathon Wilson
 

------ 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.--------

What I did to reverse the image is using a cheap Ambico V-0310 Fish-eye ,around $60, that I put on my lcd and on top of it I use a smal telescope prism that reverse the image. Now I have a view finder on the side of my Vx-2000 so I can put it on my shoulder.

I also have a lcd monitor that I can put on the side but I realy don't like those monitor ,you have to be realy in front of it and it's hard to focus on this.

Here is a example.This is not the final version but not no far.

http://ideaspora.net/aldu35/ALDU35.JPG

Alain's viewfinder
 

Good job on the viewfinder. What kind of prism(s) are you using? I know what you mean about the LCD's. Never the less Im still going to flip or buy a special LCD because Im a steadicam operator as well.

The only thing that troubles me about the add-on LCD is the need for some kind of power for it. If you're walking around with your steadycam, you're going to have more wires and more weight in your batteries, or whatever. I'm sure it will work, but it's starting to be a lot of baggage... I like Alain's prism idea... I guess that old "learn to operate your viewfinder with both eyes open" technique really has its place here :)

As usual, Alain - nice job... looks like a nice rig.

Brett Erskine
 

It's call 45 degree erecting prism and it's $39.

Alain

Prism
 

There is some here but more expensive
mine is $39 Can so in Us it should be $2 :-(

http://secure.sciencecompany.com/thesciencecompany/product1.cfm?SID=1&Product_ID=261

Prism
 

Maybe something interesting here.

http://tweaktown.dealtime.com/xPF-Bausch_and_Lomb_Bausch_Lomb_Erecting_Prism_1_25

Alain, that\'s it, if you\'re going to put it on the viewfinder, the 1.25" size is OK, and you won\'t need your fisheye lens anymore. I was hoping to find this in the 2" size, to make the camcorder see correct image. Brett, have you had any luck finding the 2" porro?

This seems like such an elegant solution. Throw one of these onto the existing eyepiece... here\'s some more of the same, but lower prices:

http://www.scopestuff.com/ss_diag7.htm

http://www.scopestuff.com/ss_diag6.htm

It\'s not that easy.
It cannot just be put on the viewfinder.We have to put another
magnification lens on top of it to replace the one already on the viewfinder, and the focal of this is a problem ,You cannot just put the actual magnifier from the camera.Because the prism change the focal lenght neede\'d.


I have see some biger but it\'s around $300.

Alain