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Old June 16th, 2005, 07:51 PM   #1
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Single element DOF adaptor (not 35mm)

After doing a bit of research into pinhole cameras, single element lenses, and various other things on optics I decided to try an experiment.

I bought a cheap reading magnifier from walmart for $5 US (2in x 2in reading magnifier +1 I think but not sure). I had some mylar contact paper and an old picture frame. Put all of those into a cardboard box. I cut a whole at the front the size of the magnifier and taped it there. Then I crawled inside and held the cheapo focusing screen and moved it around by hand. To clarify what I am saying: I am not using any 35mm lens. Its just magnifier (diopter?) and focusing screen.

I have a few screen shots but my websever isn't letting me ftp them. It looked pretty terrible actually, but that I believe is primarily due to the extremly sub-par ground glass I am using. The important thing however about this experiment, and the only reason I think its worth posting is this: My image size was greater than 5in x 7in. I would say I had at least 4x4 in. of useable image, and fairly bright considering the screen I was using.

Essentially, what I am doing is creating my own single element large format lens. I also bought a large 8"x11" magnifier. My assumption from my tests (note I know very very little about optics) is I will get DOF something akin to a 135mm to 180mm lens, but I can increase (deepen) the DOF by puting an aperture in front of the front lens (thus the reasoning behind using the biggest lens possible. I think it might also be good to put a -1 diopter behind the first lens...but honestly that's just a guess. Optics experts any suggestions?

I will pursue this futher and try to keep updating my progress, but I will probably be dividing my time since Matthew Wauhkonen's post has inspired me to give wax another try.
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Old June 16th, 2005, 07:58 PM   #2
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Here are some screen shots. Sorry for the poor quality, I held it all by hand. The ground glass is simply matte mylar paper on a picture frame glass. There is also quite a bit of problems with reflection from the inside of the box. The image is so wide it just seems to go everywhere, and also I hadn't painted the box black (though honestly that might not be enough to prevent reflections.

www.louisianaloud.com/Image0.jpg
www.louisianaloud.com/Image1.jpg
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Old June 16th, 2005, 09:53 PM   #3
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pinhole

the pinhole IS the lens...
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Old June 16th, 2005, 10:56 PM   #4
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Sorry I didn't mean to be unclear about that. I'm aware of how a pinhole camera works (now)...I was just saying that I had been searching all about various light techniques and why they work...not that my design is a pinhole camera. I tried a pinhole and it is way too little light. My design is a single element lens as the title states.
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Old June 17th, 2005, 01:44 PM   #5
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I don't think this will help with getting a shallow DOF though :). Pinhole cameras have an (effective) infinitely deep DOF!
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Old June 17th, 2005, 03:21 PM   #6
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A pinhole would not, but each magnification level via the diopter at the front involved in simulating a pinhole of a certain size, increases the circle of confusion and therefore shallows the DOF. I think with a +5 or greater diopter you would start getting a pretty shallow image. I think the bigger problem is that the fall off rate of the focus won't look 35mm at all, it will be much more gradual. It would be a unique image...hopefully unique in the 'defining a new cinematic style' way, as opposed to the looking really cheesy and weird way.
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Old June 20th, 2005, 06:54 PM   #7
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Hey, why hasnt anyone here tried this before?.
I tried it with a my Diopter(?, its an old lens from a minolta slide projector) and the images i got was actually quite good, not that far from the images i got from the 35mm slr lenses i have tried it with.

I did have some troubble focus on things from a far distans, and things closeup.

Here are some grabs from my tests.
(note. since i used the closeup lens as the actual lens that captures it all, i wasnt able to get close enough up to the gg, so i got some vignetting and hotspots, the gg is terrible made, the camera isnt that pro either[gs120])

well well, here is the link: http://arez.vestlan.com/dof/Single_lens/index.html

Last edited by Rune Hals; June 20th, 2005 at 07:11 PM.
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Old June 20th, 2005, 09:59 PM   #8
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Your already getting better results than me I can see. Do you know the strength of your diopter? Also have you tried limiting the light that enters the diopter? How big of an image are you getting? Personally I think mine is a little too big (4in x5in).
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Old June 21st, 2005, 09:35 AM   #9
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Well, im not sure how strong it is. Im no optics expert, and i have no idea how to measure the strength. But what i can say about it, is that it is a very thick positive lens. a bit under "one and a half inch" long in diameter. and its offcourse used for 35mm slides, i dont know if that has something to say about the size of the projected image. but it gives you images about the same size as a standard 35mm slr lens would give you.

About the image you got... that was a bit too big. I think "that" can give a lot of light loss since you probarly only use a small area of the projected image.

Now , i really liked the idea of using just a simple glass from a magninfier or some sort. It might be a good thing for people who dont want to invest in expensive optics to try building a DOF adapter. They might be very sceptic to the whole idea, so now they can just play around a bit with it and experience the power of shallow dof, and also. I like the idea of getting a new look that we havent seen before. Cuz thats what one of the things "alternating imaging methods" is about.

You should try your experiement with a better GG. maybe something less opaque. And keep on experimenting. Cuz this is all very interesting...
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Old June 21st, 2005, 02:39 PM   #10
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Does anyone know how I could calculate the CoC for such a single element lens? I want to see if I can use this DOF field calculator:

http://www.dofmaster.com/dofjs.html


If there is a way to calculate an at least approximate CoC for a given single element lens, then I can save alot of time and money by not using a lens that will just give me deep DOF anyway.
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