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Old May 4th, 2004, 11:40 AM   #781
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extension tube

Nick - the extension tube I have has a threaded lens mount and a threaded camera mount separated by three threaded removable rings (all are 52 mm). I simply unscrewed the camera mount piece and screwed the tube into a 55-52 step down ring.

- Damion
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Old May 5th, 2004, 12:41 AM   #782
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Nick-
Once you get your back focus set with one lens when your using a condenser you shouldnt have to ever reset your back focus distance when you swaping between lenses.

Never the less you said you did. Very strange. I wonder...does it appear slightly out of focus or are you getting more/less of a hot spot when you change lenses?

If its out of focus then somehow your backfocus is changing on you. I dont think you have to "match" the backfocal length of your lens with that of the condensers'. They are working indepedently. I would double check and make sure that your back focus isnt shifting on you because thats the only reason I can think of that it would go out of focus.

If the hot spot is changing when you change lenses then this is due to the fact that while condensers do a great job of correcting for hotspots any given ONE will do it at a set degree. When you change between a ultra wide lens to a long telephoto the same condenser is unlikely to work perfectly in both situations. The easiest way to deal with this is to get a condenser that favors wider lenses (if you shoot mostly in this range) and when it comes time to use a telephoto len your going to want to zoom in just a bit more on your video camera's lens to hide any hot spot problems. Medium format photographers have always had this same problem. Thats why some of them carry two condensers (they call them fresnels) in their bags - one for wide angle range lenses and another for telephoto range lenses.

-Brett Erskine
Freelance Cine/Videographer
www.CinematographerReels.com
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Old May 5th, 2004, 09:46 AM   #783
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Thanks for all the help Brett,

When I have the condenser in and I switch between lenses everything becomes out of focus, but I'm going to test more and I'll let you know how its going. There wasn't really too much hotspot change when I switched over to telephoto but I did notice I would have to zoom in more as well.

Maybe I'm off with the distance from slr lens to ground glass or not properly focused on the ground glass, although I feel like I'm not off on those parts. Can you name any companies that make internal focus lenses? This breathing has got to go! Thanks.

Nick
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Old May 5th, 2004, 11:18 AM   #784
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Hey Nick,
I don't know if you mentioned this or not, but what is the physical orientation of your condenser lens inside the adapter? So far everyone has the curved surface further from the 35mm lens than the ground surface. Is your's the opposite (flat side closer to the camcorder)?
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Old May 5th, 2004, 11:26 AM   #785
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Hi Nicholi,

I actually had the flat side of my condenser right up against my ground glass. I'll definitely flip it around and point the convex side towards my ground glass and shoot some more footage. Thanks for the help!

Nick
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Old May 5th, 2004, 12:02 PM   #786
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By the way, Nick - I *was* able to view your pics and footage (I had to change an adlu to aldu in the url...). Looks like you're making great progress! That condensor really helps the hotspot - your 'before/after' makes it very easy to see the difference. Thanks for posting.
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Old May 5th, 2004, 02:11 PM   #787
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Hey Jonathan - I'm glad someone was able to take a look at that footage. I'm excited to play with the condenser more to see if I can work out my problems and get this thing on a real shoot. Did you happen to notice the almost radial blur look to objects out of focus in the background? If anyone can check out the video to see what I'm talking about, I'd like to hear comments and see if anyone is experiencing the same thing. I'm not sure why that is occurring and I'm gonna have to figure out how to correct it, but I do know I was seeing it with and without my condenser in place. Anyone have any ideas? I know when I ground my UV filter I was using circular motions similar to the brushed look of the out of focus points in the footage, but that shouldn't show up in the grain at all right? I guess every step forward makes you take two back.

http://www.themotherofallshows.com/Mini_35/Nick_Conti_aldu35_tests.mp4

Nick
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Old May 5th, 2004, 02:33 PM   #788
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Yeah, I noticed the radial effect (more in certain areas than others... like when your subtitles drew attention to it :).

I actually had a similar problem with my first adapter because the condensor I was using was not PCX, but DCX (curved both sides) and this along with my mounting apparatus kept the condensor a fair distance from the ground glass (maybe 10mm). I found that if I zoomed way out (to where I could see the entire condensor in frame) I had some serious spherical abberation out on the edges and as I moved around this abberation causes the edges of the sphere to 'move faster' than the inside. The edges were also out of focus. This was likely due to the fairly short focal length (43mm) which means there's quite a bit of curve to the lens. It also means that the focal plane out on the edges of the condensor is in a slightly different place than in the center due to the curvature.

Your footage looks a little this way - the very edges (particularly in that last shot where you can actually see the radial edge in the upper-right-corner) seem to be slightly abberated and slightly out-of-focus related to the center. Did you say your condensor is 50mm diam., 50mm f.l.?

My current adapter (nearly done) uses a moving GG and the rectangular condensor cannibalized from my AE-1 SLR camera... its very very thin - 2mm or so at its thickest. I haven't measured the f.l., but I suspect its quite a bit longer than 50. This also means that the hotspotting is much more prominent... it's a balance.

How big is rear opening on your lense? My 35mm lens only goes down to f-stop of 2.8 and has a relatively small hole (19mm) compared to the 1.2 and 1.4 lenses I've seen, which seem to fill the entire tube. This likely has a huge impact.
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Old May 5th, 2004, 02:45 PM   #789
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My rectangular condensor from my AE-1 (36mm by 24mm) has a focal length of around 130 mm.
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Old May 5th, 2004, 03:03 PM   #790
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The focal length of my pcx condenser is 50mm. I'm at work right now and I don't have my equipment with me and I don't remember the diameter of the rear elements of my lenses, although i believe all of them are different considering the range i have at the moment: (19,28,50,85,105,200mm) I'll let you know when I'm back at home and can check them out later. They all are Nikon lenses though, and I'm assuming they are all going to have the same breathing effect that my 50 and 105 had, so that's a little bit of a bummer not being able to use those puppies.

So are you saying that a condenser with a longer focal length might help with this issue? But at the same time brings out the hotspot more? I did buy two of the same condenser when I picked them up, both 50x50mm...Has anyone tried using two of them at once and have the convex sides facing each other? I think I saw that configuration in a diagram somewhere, but I'm not sure what its purposes/benefits/disadvantages would be.
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Old May 5th, 2004, 04:07 PM   #791
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PCX orientation

Brett,

Correct me if I'm wrong, but I thought that having the flat side against the ground glass was a perfectly adequite way of correcting for the hot spot. If I remember right from a recent post on this or another board, grinding the flat side of your PCX condenser is a good way of eliminating one extra piece of glass and improving resolution and light loss. I haven't ground (actually going with acid etching) the flat side of my PCX yet, but the tests I've done by sticking a piece of diffussion a flat against the flat side of a PCX has worked well for eliminating the hotspot.

John
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Old May 5th, 2004, 04:28 PM   #792
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(not Brett but can confirm...)

Yes that's correct. Flat side towards (or is) ground glass. Curved side out toward camcorder. Magnification is the same in both directions, but the flat side allows you to get the condensor closer to the surface which is the goal. And grinding the flat side is about as close as you can get! (Careful not to scratch the curved side, though)
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Old May 5th, 2004, 07:49 PM   #793
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^
|

Thats right.

-Brett
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Old May 5th, 2004, 08:00 PM   #794
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As far as the pattern your seeing in the out of focus shots it something to do with your GG. Before you even mentioned that you used circular motion to grind the GG I was wondering that you had and that it was the reason why you were having problems. Sounds like you better do another pass that more random so you end up with a more even texture. Also if you arent using the finest AO (that still grinds instead of polishes) you might want to.

Thanks for the info on the focal length of the AE-1 condenser. Im starting to think that maybe we should be looking at condensers that have focal lengths even longer than I originally mentioned. In previous post I said that the shortest focal length you would want to get for a condenser would be no shorter than its diameter but we still seem to have a slight bit of spherical distortion happening. Perhaps we should take what we learned from that AE-1 condenser and be use a condenser that has a focal length roughly double its diameter. The longer the focal length the less spherical distortion in the picture while the shorter the focal length (generally) the better the condenser will be at reducing the hot spot. Perhaps the idea of having about double the focal length (vs. diameter) better puts us in that sweet spot were going for. Can someone else confirm this?

-Brett Erskine
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Old May 5th, 2004, 09:23 PM   #795
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PCX orientation

Wait I thought that the condenser goes between the 35mm lens and the ground glass, with the flat side towards (or is) the ground glass.

The post I remember talking about the flat side of the PCX acting as the GG showed a diagram of the condenser between the 35mm and GG not between the GG and the Camcorder.

?
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