Cure for light fall off

[Erik L]

Hi folks, I have recently cobbled together an 8x10 enlarger with an Arista w31 10x12 cold light. I use a roscoe cc40y to enable vc printing (still not spot on with the contrast grades, but I can work with it). The problem I am having is light fall off at the corners. 1 to 1 2/3rds stop difference from center to corners. It's not my lens (240 rodagon) because even without the lens I get the same readings. It is a major pain doing mural sizes (40x50 inch horizontal) because the exposures are really long (5 or 6 minutes) and then I have to burn in the corners another 7 or 8 minutes to compensate for the fall off. Is there such a thing as an inexpensive center filter I could attach to even out the exposure. I tried to make an 8x10 blank print and develop it and then scan the print and try to make a negative on ohp transparency stock and use it as a filter but I'm not having good luck with that. What I was hoping is that there is some kind of center filter I could attach to the lens and be done with it. any thought out there? i would appreciate the help.
regards
Erik

 

[Christopher Walrath]

Wirelessly posted (BlackBerry9000/4.6.0.167 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102 UP.Link/6.3.0.0.0)

Just a quicky idea. Cokin makes center spot filters. Might be able to make something work.

 

[Erik L]

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

 

[epatsellis]

Erik,

I've had an idea that might work, if you use a piece of dupe film the same size as your light source, expose it to get a medium to light gray and then after processing, tape it to your light source (sort of how plume uses a scrim to correct for light falloff.


erie

 

[archphoto]

I agree with epatsellis, it should work.
In that way you hold the center part back in exposure in the same ratio as the light-fall-off you have at the corners right now.

Peter

 

[Erik L]

Thanks guys, that's kind of what I did when I made a blank grey print and made an ohp transparency with my inkjet and put it above the negative. I guess my skills at making a decent ohp are not very good. In theory it should be the best way to go but I haven't had good luck so far. Thanks for the suggestions.
regards
Erik

 

[ic-racer]

http://www.largeformatphotography.info/forum/showthread.php?t=40384


Also, Posts #104-106 in this thread http://www.largeformatphotography.info/forum/showthread.php?t=32852

 

[Erik L]

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

 

[ic-racer]

How many stops did you lose because of the added density with your filter set up? I appreciate the help.
regards
Erik

Actually none at the edges

I suspect it is about a stop at the center.

I had read about people who use gray paint on the lamp housing, behind the central portion of the light grid. With any of the methods, I don't think there is any getting round a lot of trial-and-error to get it right.

 

[RJS]

The Arista 10X12 should cover 8X10 just fine. I wonder if the problem is not in the lamp house - you don't mention what the housing is. Age can cause discoloration and changes of various sorts; I would be looking there very carefully before trying to balance with a filter.

 

[ic-racer]

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).

 

[gainer]

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.

 

[Erik L]

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

 

[ic-racer]

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).

 

[RJS]

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.

 

[Martin Aislabie]

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

 

[Erik L]

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

 

[richard ide]

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.

 

[Daniel Ferri]

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

 

[ic-racer]

Reflective material around the perimeter of the housing is sounds like a good idea.

My Aristo 1414 W45 tube runs on 480V.

 

[RJS]

You might try contacting Aristo; if the tube you are using is made by them for photographic use it certainly should cover 8X10 very nicely. If it is made for some other purpose you are in uncharted water! Given it is the correct tube, all those I am familiar with wind back and forth to make a pattern to cover the required area, then perhaps it is not working correctly somehow - blackened portions internally? That size MUST cover 8X10 reasonably well if it is working properly, pretty much regardless of what you have it sitting on (an old camera or whatever would not effect the light striking the negative. Something very strange here!

 

[BetterSense]

I built a very big enlarger from a process camera and used 10 48" flourescent tubes as a light source.

Wow! What size negatives were you enlarging?

And the OP, with a 50-inch pegboard vacuum easel, how are you developing the exposed paper? In a kiddie pool? Just kidding, but I'm honestly wondering.

 

[richard ide]

I could enlarge negatives up to 30 x 48. My vacuum easel was 78 x 192. I did a lot of separation positive film for silk screen printing. Anything up to 48" wide went through a processor. Over 48" was hand processed.

 

[Erik L]

Thanks folks,
there is plenty of coverage of the negative - an inch extra on all sides. I contacted aristo and they told me that the bulb I have was "sold to individuals who wanted to experiment with a warm light for b/w prints" so I assume it should at least attempt to be even? The bulb is pretty tightly packed coils that cover all the way to the edges of the housing. What type of material would one use as reflective to put in the housing? chrome tape, or similar? I'll give a shot with using film to make a center filter if the reflective material in the housing doesn't help. Bettersense, I use the scroll method for processing prints in troughs about 52"x10"x5", it's a piece of cake.
thanks again for thinking this problem through!
regards
Erik

 

[jp80874]

Guys, thanks for putting the thinking caps on ....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. ...regards
erik

Erik,

If you do consider buying a new or used bulb this may help. Several years ago Michael Mutmansky took a normal 5x7 Durst 138S and converted it to 8x10 using an Aristo 12x12 cold light. He was concerned about fall off of the 8x10 light and felt any fall off on the 1212 would be beyond the 8x10 negative. When he changed formats I bought the enlarger from him and have been very happy with the even light obtained. There are a couple of pictures of the unit in my gallery. I use a 300mm Rodagon and only enlarge to 20x24 though there is room to go much larger either by lowering the table or switching to the 240mm Rodagon. I haven’t wanted to wrestle with larger paper in a high sink with a 7' 2" ceiling. The floor awaits that effort, but I am making 7x17 contact prints instead now.

John Powers