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  1. #11
    ic-racer's Avatar
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    Quote Originally Posted by Erik L View Post
    Ic-racer, thanks! I read your posts and that is where I got the idea to try and make my own filter on ohp. I just wasn't very successful with my ohp printing. How many stops did you lose because of the added density with your filter set up? I appreciate the help.
    regards
    Erik
    I didn't go into a lot of detail on the specifics of printing the filter, but I made my Photoshop image pretty light and I ran the transparency film through the printer 3 or 4 times. Each time I would 'test' the transparency in the enlarger. If it was not dense enough, I'd run it through the printer again.

    Also, I thought that when the filter was sitting right on top of the diffuser, some irregularities from the printing process would show through. So, I put the center filter up in the filter holder, about an inch from the diffuser. (My filter holder is above the diffuser, which I think is unique to my setup).

  2. #12
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    I know a bit about optics but nothing about that cold light head. How far is it from the negative plain? The symptoms are like what happens in a condenser head when the light source is not properly focussed on the negative plane. If the cold light head is too far above the negative plane, the corners of the light field could be outside the field of the projection lens. You might be able to test this theory if you can move the cold light up and down with the light on while watchung to see if the corner illumination changes.
    Gadget Gainer

  3. #13

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    Thanks folks for the suggestions. Gainer, my light is about 1/2 inch from the negative. I can't move it any closer but I can move it farther away. It is a 10x12 inch light so it covers with an inch to spare all around. In a previous post by RJS he mentioned something about the age of the bulb which I do not know because I received it used and that may have an impact on my problems, who knows? I'll give your theory a shot and see if it improves and if not I will do as Ic racer says and monkey with a filter until I get it close enough for government work
    regards
    Erik

  4. #14
    ic-racer's Avatar
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    Gainer makes a good point. Since the light is NOT collumated, if the diffuse source is the same size as the negative and, say two inches from the negative, then the edge of the negative receives darkness on one side and light on the other. A point in the center receives light from all directions. In my case I was able to move the diffuser to about 1/2 inch from the negative and it overhangs by one inch on all sides (12x12).

  5. #15
    RJS
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    My cold light (4X5) is as close to the negative as the enlarger - Beseler - allows. There is a diffusing screen or whatever between the light tube and the negative. Is the diffuser leaking light around the edges? Mine (Aristo) is in a circular housing that is closed around the edges. Could light leaking at the edges be a problem? I was thinking (age) about the reflective surface surrounding your light. Ctein writes abiut a problem similar to yours caused by age etc. in a diffuser box on, I think, an Omega color head. But the idea is he same.

  6. #16
    Martin Aislabie's Avatar
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    Quote Originally Posted by Erik L View Post
    Thanks Christopher, I need something like that but in reverse - I need density in the center of the filter and clear towards the edges. Maybe there is hope with an inexpensive filter. Those filters were only about 15 bucks

    regards
    Erik
    Thats what Centre Filters look like - dense in the centre and fading to nothing at the edges.

    They compensate for the cos^4 effect

    Martin

  7. #17

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    Guys, thanks for putting the thinking caps on I've been out playing with the trout on the green river this weekend so I apologize for being absent. There is no light leaking around the edges of the light. The diffusion screen is a pretty tight fit with a flange to hold it in. The inside is in pretty good shape, paint wise (white) but I do not know how efficient it is. The housing is rectangular which might have something to do with the way it throws light, I'm not sure? I can't move the light any closer to the negative without some serious planing which I hope to avoid. It has an inch extra coverage all around the negative. This enlarger is not "within factory specs" but rather something I have cobbled together. It works fine and is easy to align with my 48x60 inch pegboard vacuum frame. It is portable and easy to stash when not in use which helps with my double duty darkroom/office. I enjoy it, but would like to figure out how to get more even light to make printing simpler and waste less paper if I can help it. I'm trying to avoid buying a new bulb if there is a fix. I would hate to buy a new bulb and have the same situation with fall off. Here's a couple pics so you know what I'm dealing with.
    regards
    erik
    Attached Thumbnails Attached Thumbnails enlarger.jpg   enlarger2.jpg   enlarger3.jpg  

  8. #18
    richard ide's Avatar
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    I have made vignetting filters to even out an enlarger light source. Set your enlarger to 1 : 1 and make an exposure on a piece of film so that you get a density of .02 to .05 in the lightest areas. Place this behind the light diffuser and you will have very even illumination. I built a very big enlarger from a process camera and used 10 48" flourescent tubes as a light source. The vignetting filters for various lenses I made allowed illumination evenness to 1/10 stop.
    Richard

    Why are there no speaker jacks on a stereo camera?

  9. #19

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    Quote Originally Posted by Erik L View Post
    Ic-racer, thanks! I read your posts and that is where I got the idea to try and make my own filter on ohp. I just wasn't very successful with my ohp printing. How many stops did you lose because of the added density with your filter set up? I appreciate the help.
    regards
    Erik
    Erik,

    a filter should do the job, but you will loose precious light.
    Check your light source first. From your photos it looks as if your cold cathode is perhaps not large enough on the width.

    Is your fall-off on the 10inch side of the neg?

    If you can gently take off the diffuser cover, look for signs of yellowing of the lining material, blackening, dust deposit on the tube?

    Careful with the tube, it is a single piece and fragile.

    I have rebuilt some in the past, and added a reflector all the way around the inner box, and also inside the frame, just above the neg carrier. this helps spread the light to the edges.

    The material used was called ASTRALUX card and coated with mirror finish plastic surface. DO NOT USE METAL.

    MAKE SURE nothing gets near the terminals.
    If you have a proper cold cathode, as opposed to a normal fluorescent tube,
    the voltage to the tube is in the region of 2500 Volts.

    Danny

  10. #20
    ic-racer's Avatar
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    Reflective material around the perimeter of the housing is sounds like a good idea.

    My Aristo 1414 W45 tube runs on 480V.

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