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Old March 4th, 2004, 10:19 PM   #1
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35mm adapter with +7 diopter

I've been following along with everyone and really like the progress of the 35mm adapters being made. The latest uses a Century Optics +7 achromatic diopter mounted to the camcorder to focus on the tiny projected image. This is an excellent method of increasing image size, but unfortunately also increases the visibility of the ground glass grain (magnifying the projected image on the ground glass).

Adding two condenser lenses will provide a much more usable picture by avoiding vignetting. That means you don't have to zoom in as far to fill the video frame, capture a more concentrated area of ground glass, and record a higher quality picture. I would like to take it even further by increasing the image even more, thus filling the frame with a higher concentration of grain and preserving a much better picture.

If you place a magnifying glass between the 35mm lens and the ground glass (as opposed to between ground glass and camcorder), the image will increase in size before hitting the ground glass. This results in the same, magnified image, but contains a higher number of "grains" in the recorded video frame and thus less degredation of the picture.

Has anyone with an achromatic diopter experimented with this? or would be willing to? According to my experiments, if the diopter is placed right up against the ground glass (screwed directly into the 58mm ground glass filters that people have made) it will increase the projected 35mm image to the size of the diopter (58mm). Therefore, filling the entire frame with a much higher concentration of grain.

Just in case I'm not making sense, I'll draw the adapter configuration.
(( = achromatic diopter, | = ground glass

current configuration: 35mm lens.......|..((camcorder
configuration to test: 35mm lens.....((|camcorder

Although the flange focal length will probably change a bit, I feel that increasing the image BEFORE the ground glass is going to be key to a high quality (maybe usable for HDV?) image and will even make the entire adapter shorter. Unfortunately I can't afford an achromatic diopter yet, but if someone can try this experiment and post the results, everyone will benefit.
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Old March 4th, 2004, 10:30 PM   #2
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That sounds pretty slick. I would like it if that were doable. Seems like all the designs try to keep from interfering with the relationship between the 35mm lens and ground glass (since we want to capture the original DOF characteristics of the lens), but I'd like to see what Brett or one of the other optics guys says about this.

I think that's a good idea, Nicholi. I completely agree that if we can keep the DOF we're after, while projecting the image over a bigger area (spreading it over the grain), we'd have a nice, sharp image, likely less chromatic abberations, and easier focusing on the screen.
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Old March 4th, 2004, 11:20 PM   #3
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The +7 diopter may turn out to be too strong. If the image is magnified so much that only a small portion of the entire 35mm image is seen, the shallow depth-of-field will be sacrificed which is totally opposite of what we're going for here. I think that might be what Frank was referring to. I do believe the diopter will work wonderfully. It will just be a matter of figuring out the most usable power... or we could just make the ground glass bigger and further from the camcorder.
I'm getting a little ahead of the experiment here, but I wanted to mention that before it slipped my mind.
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Old March 5th, 2004, 12:16 AM   #4
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Nichola your reasoning for doing this is sound BUTT by putting a magnifying lens between the 35mm still lens and the GG your changing the DOF characteristics of the image being projected. This is the same reason why you cant maintain the same DOF if you take a lens with negative magnification properties and use it to shrink down the image instead to fit directly on the 1/3" CCD (no need for ground glass).

Now the interesting part about this idea is that (as I understand it) if you shrink the image down you eventually get back to DV DOF but it you make it larger....well you might just make the DOF more shallow than 35mm. Maybe too much. This would have to be tested. I can tell you one thing for sure. As you make the image larger it also becomes dimmer. For example if you make it twice as big then it will be 1/4 as bright (2 stop loss).

On a interesting side note. If you were to say take a XL1 and mount a 35mm still lens directly to the camera body with a negative lens in between the two to reduce the image size down to 1/3" you wouldnt have 35mm DOF anymore but you would maintain the same FOV. And, if Im not mistaken, the reduction will make your lens a hell of alot faster. All that light will be focused on a smaller area and should really make night shooting alot easier. This seems too good to be true so I'd run it by a optical designer first before trying it but "I think it just might work!" (I love saying that.)

-Brett Erskine
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Old March 5th, 2004, 01:19 AM   #5
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Well, there goes that idea. Unfortunately, my severely limited knowledge of lenses is based only on personal experiments and research. During my tests, I only took into account the size and brightness of the image and didn't even consider DOF being influenced. After setting up the lenses again, I can now see how drastically the DOF is increased with just a little magnification and understand why it is necessary to do any magnification to the image AFTER it is projected onto the ground glass... hence the diopter. That basically puts us back to square one: bigger image = more visible grain. GRAIN BAAAAD!!
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Old March 5th, 2004, 01:34 AM   #6
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Nicholi-
The ground glass being made with 1000+ grit aluminum oxide is pretty much grain free. It does have a slight too diffused/soft look though.

Perhaps a Bosscreen is the answer for you. Its to clear pieces of glass with a ultra thin layer of hard wax between them. It works the same as GG but with a "grainless" surface texture. They are relatively expensive though at about $150. They use them for viewfinders on 4X5 and 8X10 cameras. Heres the link if you want to learn more.

http://www.stabitech.nl/Bosscreen.htm#top

BTW when you go to the site you'll notice something called a "Glasscreen". Its the same thing as a Bosscreen but with a thicker layer of wax. You wont want one of these because it will create a darker image.

-Brett Erskine
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Old March 5th, 2004, 03:28 AM   #7
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Here's my findings...

Nicholi - You hit the nail on the head with keeping the frame size as large as possible, (at least 36mmx25mm) - less frame/grain ratio.

In fact this is the arrangement I am currently experimenting with...

> (l ll (lens, cond, GG, FrameSize=36mmx24mm)

(the condenser is a 55mm, as used in photographic enlargers - paid 10 for mine) the GG is 9 micron.

The hot spot is only very slight at this size. Today I will buy a 2nd identical condenser to try and get rid of it completely, I plan on trying this...

> (l l) ll (lens, cond, GG)

From what I've found, it's best to keep the condenser lens behind the GG.

I think we're almost there guys, next step is a homebuilt FollowFocus unit & Mattebox!!!
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Old March 5th, 2004, 04:40 AM   #8
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Simon if you plan on using motion picture lenses you will need to focus on a 24mm by 18mm size image. (still lens are 36mm by 24mm).

Try this setup too:

(| || |)

<Condenser, GG, Condenser>

Its the same as a SLR viewfinder so will most likely lead the the best results. If it does you can simplify the design by doing this:

(||)

<Condenser, Condenser with flat side turned into GG>

Or even this:

|)

One stronger power Condenser with flat side turned into GG>

Let me know how things work out for you and post the focal lengths of your condenser(s).

-Brett
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Old March 5th, 2004, 06:19 AM   #9
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Brett, I will be using Nikon AI-S 35mm lenses - so I won't need to go 24x18mm at this stage.

I've tried a 50mm Planoconvex aspherical condenser, it condensed the image way too much, I ended up with an ultrawide "fisheye" like image on the GG. It's the one 2nd from the bottom, priced at 16.75

http://www.knightoptical.co.uk/acatalog/LensesGlassasphericcondenser-Fire-polishedgrade.htm

I have this weekend free to experiment with these two new condensers, (they came in a plain box with no optical specs). I'll see if I can get some images posted somewhere for you guys to check out for next week.

BTW - the GG is around 3cm in from of the DV camera lens, the 26x24 image is still square at this distance, I then zoom in to fill the frame.
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Old March 5th, 2004, 01:33 PM   #10
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Simon did you try it once on one side of the GG and then the other? Same results?
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Old March 5th, 2004, 02:54 PM   #11
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Simon, were those lenses 50 and 55mm in diameter? or focal length?
If those were the diameter, what were the focal lengths?

Twin condenser lenses seem to be a requirement for any descent quality adapter, but I don't know how to determine the proper focal length for the lenses. Too short will cause barrel (negative) distortion, too long will cause pincushion (positive) distortion, all the while trying to fully diffuse the hotspot to the largest effective size. Your experiments are going to help a lot.

As far as location... if the quality is no different, I will probably put the condensers between the 35mm lens and ground glass because it will make for a shorter adapter housing.
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Old March 8th, 2004, 04:58 AM   #12
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Nicholi/Brett

- I tried numerous combinations of the condenser and GG behind the 35mm lens over the weekend. Although the hot spot is noticeably less, it's not gone completely. The GG is also very prone to finger prints and dust specs, I'm now thinking about going with the Bosscreen. It would be much easier to keep clean, (outside surfaces are plain glass), and should totaly eliminate the hot spot problem. I'm don't know the focal length of the condenser, it wasn't avail when I bought it.
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Old March 9th, 2004, 12:21 AM   #13
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Simon you can check the focal length of the lens by holding it over say...a table with a ceiling light over head. When you see the light projected on the table in focus then measure the distance between the table and the lens and thats your condensers focal length. Let us know what you find.
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