Uneven light, please help!

[Ash]

Right,

I did seem to have a fairly even spread of light before painting the inside of the adapter box, but I may be wrong.

My problem lays with having a fairly odd spread of light here.

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC01976.jpg

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC01979.jpg

Everything so far is experimental and non-permanent, hence the white paper tube and corners.


I've tried multiple diffusion setups and nothing seems to work:

I place the opal diffuser directly above the neg, and I get hotspots along the side of the print from the lamp placement.

I put the opal diffuser at the top (by the lamps where it originated), and I get really bad vignetting directly onto the print, it looks like this:

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC01985.jpg

So with the neg carrier:

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC01986.jpg


With a lower diffuser as well, this time a piece of ground glass (my only other means of diffusion right now, I get not so sharp vignetting but vignetting none-the-less.



I'm keen on getting a proper-sized flashed opal diffuser for directly above the neg, but I'm not going to spend out on a custom sheet if it's not gonna solve the problem.

Either way the light is very powerful and I'm losing at least 2 or 3 stops with the top opal diffuser, with a lower one as well I may as well install a coffee machine thanks to exposure times I'd have!


I'm keen to stick it out with this head because it's Dichroic, powerful, and has built in filters!


Any useful advice would really be appreciated.

[CG]

Try adding one of two things: a second gg diffuser separated from the original, or, put some neutral density in the middle of your image area by marking the second gg diffuser with a pencil till the lighting is even - think of it as permanent dodging the center.

C

[Eric Woodbury]

(I'm assuming that the negative is at the top of all this, as in an 8x10 conversion.)

I'm guessing that since the round image has a square hot spot, it is the adapter that is acting as a secondary source. This part of the enlarger needs to be black: as black as black can make. A flat black paint such as UFB (ultra flat black) by Krylon is the blackest I've found on the consumer market. Of course, at an angle such as you have, even black is reflective, so a series of baffles would be good too. Think about how a bellows works. Flocked paper is very black and makes good baffles. (Edmund Scientific)

If you take the lens out and look up at the negative, all you should see is the negative and everything else should be as black as you can get it. Very difficult. The vertical walls of your adapter are contributing to the reflections that the lens is 'seeing'. If the walls could be 45 degrees, then the light reflecting from them would have to take a few more bounces before going through the lens. Every bounce attenuates the brightness of stray light.

Think about how what happens to all the light from your light source. Each dot on your neg is illuminated from all directions, but only from one angle does the light then go to the lens. All that other light is just bouncing around in the box below the neg and mucking up the contrast of your image. You need to trap that stray light.

What you have is like painting the inside of your camera white. You wouldn't want this. Baffles, my friend, baffles. Think baffles.

[Ash]

Thanks for the ideas CG.

Eric, you raise some good points. I was hoping the white would reflect more light. As you say it should be directed *away* from the lens, but is acting as a mirror. i wanted 45degree walls but the construction would have been too difficult.

So it turns out I did the totally wrong thing eh?!



Right, down to the shed with something black! :)

[Ash]

Sorted!

I tried with the black card. No joy. Regardless of diffuser placement - rather than a square within a square - I got a circle within, well, nothing really.

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC01997.jpg

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC02002.jpg

It's a very evenly lit circle though....

So however light works, I have no clue. It's all magic to me.



With this decision that it's magic, I thought again about returning the condensers to the enlarger. Then I took note of what Chauncey Walden said:


Ash, the Nikor/Honeywell/Rollei etc. 6x7 color heads adapt easily to replace the lamp housing over the condensers of many enlargers. I have used them on 4x5 and 5x7. And, they are cheap
Ash, the 6x7 head replaces the light source over the condensers. The condensers take care of the rest. The diffusion sheet on the 6x7 head is larger in area than the cross section of the original lamp so better than new. These heads are compact, light and well balanced making it easy. I used two sheets of plywood fastened together to adapt it to the condenser housing - one cut out to fit the head and one cut to fit the housing.

So I took this to heart by taking away the front panel of my adapter box to accommodate for the original condensers inside of it. The adapter box fits perfectly around it, only the front needs replacing. I've currently covered it with the black sugar paper seen above.

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC02005.jpg

http://i2.photobucket.com/albums/y43/second-belated/beselerconv/DSC02007.jpg

The corners must only be a half-stop darker, if that. This gives me an almost perfectly lit rectangle! (no pic of this I'm afraid!)

[Donald Miller]

The thing that I notice is that your adapter is seemingly increasing the distance of the lamp from the negative. If this is the case I would consider that the problem seems to be that the lamp source is presenting itself as too small for the area to be covered. Remember that as light source distance increases, the apparent size of the light source/lamp decreases. This is exactly opposite of what you want in this case.

I am unsure from what I read if you are using condensers. If you are, this will only amplify your problem. Uneven coverage when focused is still uneven coverage.

Baffles, in this case, would serve seemingly little purpose in this case since black should be black...baffles make sense only if the baffles are reflective/white...something that you don't want at this portion of the light path.

So, based on my experience, in designing a new light source for a Durst I would recommend having the lamp be as large as possible and as near to the diffusion material in the light path as possible.

Good luck.

[Ash]

Hi Donald.

Originally the Beseler used an incandescent above condensers.
I've tried to modify the enlarger to use a dichroic diffusion-type head instead.

As you've noted, the light source is too high up to adequately and evenly reach the negative, so it's vignetting. I tried to overcome that (hence all the question and experimentation).

However, no amount of experimentation gave an even illumination after the added distance from the adapter box. I got close using multiple diffusers. I believe a fresnel plate or two may have worked, but not by much. Of course the issue regardless of height was hotspots.


So my solution was to compromise:

Rather than use incandescent/condenser or dichroic/diffuser, I'm using the dichroic/diffuser head in lieu of the incandescent, to create a diffuser/condenser enlarger.

I believe you'll remember that some enlargers used an opal bulb source to provide somewhat diffused light through condensers, as opposed to point light source.

In effect I'm using an oversized diffused light box, and having the condensers concentrate that light over the film plane. The condensers don't care what the light source is, so long as it's bright enough or big enough. So the light source is high enough and evenly lit and consistent enough to be magnified upon :)

[Chauncey Walden]

Ash, here's an image of the 6x7 head sitting on the condenser housing on my Omega E.

[Chauncey Walden]

Whoops! I'll try again.

[Eric Woodbury]

Ash

so let me understand: this is not an 8x10 conversion? You are trying to make a diffused light source and it just happens to be big. Is this right? If so, then the neg is down below still?

First, a few words about condensers. In this case, the condenser takes light from the light bulb, of a specific diameter in a specific location and turns it into a lightsource of parallel light rays. If the light bulb is evenly illuminated on its surface, then the condensed light is now even. So the condenser re-images the lightbulb onto the surface of the negative.

If you put a large diffuse source above the condenser, then most of the light doesn't even make it into the diffusor and some of what does, does not make it to the negative and instead hits the inner walls of the condenser or something else. Basically, if you are going to use the diffusor that came with the enlarger, use a lightbulb of equal diameter in the same location.

Concerning diffusion material: Some are better than others. White plexiglass is pretty good. Flash opal glass varies. Put the material on a newspaper or similar and see if you can read through it. The better materials are so diffuse, no comes through. Usually, the better the diffusion, the greater the attenuation This is where cold lights are so good, because they have so much light in a relatively small area and can tolerate some attenuation. Also, the spectrum of light can be specific to photo emulsions so you don't need to make the whole spectrum of light the way an incandescent bulb does. Most of a light bulbs light is unseen by the paper. Anyway, the point is the more diffuse the better. If your diffusion is poor, then this means it is somewhat clear and some of the image of what lies beyond the diffusor will come through to your print. This looks like uneven illumination.

If there is some distance between the diffusor and the lens, then that volume can also act like a light source and it will need to be controlled and even. It is best when the light source, the diffusor, and the negative are all close together. Close enough to keep the light even and far enough apart for the diffuser to work.

If you have space for baffle use them. And here I disagree with DM. Black is not black at an angle. You can reflect light off of a black surface at a shallow angle. Black is only black straight on. Similarly, glass is clear straight on but reflective more and more as the angle gets shallower. Telescope makers know this the best. Everything inside a telescope is black and then it is baffled too. I'm not saying this is your problem, but beware of black surfaces if you are counting on them to be non-reflective. They need to be black and form traps where when the light reflects or bounces, it goes into a trap.

[blevblev]

RE: So however light works, I have no clue. It's all magic to me.

Check out the Richard Feynman on light...

http://vega.org.uk/video/programme/45

[Donald Miller]

Hi Donald.

Originally the Beseler used an incandescent above condensers.
I've tried to modify the enlarger to use a dichroic diffusion-type head instead.

As you've noted, the light source is too high up to adequately and evenly reach the negative, so it's vignetting. I tried to overcome that (hence all the question and experimentation).

However, no amount of experimentation gave an even illumination after the added distance from the adapter box. I got close using multiple diffusers. I believe a fresnel plate or two may have worked, but not by much. Of course the issue regardless of height was hotspots.


So my solution was to compromise:

Rather than use incandescent/condenser or dichroic/diffuser, I'm using the dichroic/diffuser head in lieu of the incandescent, to create a diffuser/condenser enlarger.

I believe you'll remember that some enlargers used an opal bulb source to provide somewhat diffused light through condensers, as opposed to point light source.

In effect I'm using an oversized diffused light box, and having the condensers concentrate that light over the film plane. The condensers don't care what the light source is, so long as it's bright enough or big enough. So the light source is high enough and evenly lit and consistent enough to be magnified upon :)



Thanks for your amplification. I understand now.

[Nick_3536]

Be careful with those condensor you won't replace them.

If this is still a 4x5 enlarger and your using the stock negative stage why the big spacer? Sit the head down at the same place the old one would be. If you don't the next complaint will be how you can only make real small prints.

If you want the spacer then build a slot of a negative stage up near the light source. You want to be real real close to the light source.

Rosco and I think Lee sell light diffusing sheets of gel for almost nothing. I'd try that if you want something low cost to test with.

[Ash]

Chauncey,
If Only I saw your pics BEFORE buying the monster! :D

Eric,
I was planning on making this an 8x10 conversion, then I had too many 'issues' to face. I decided for the moment to get the head working as a light source for 4x5 only and keep the 8x10 conversion for later.

Blev,
Thanks for the link.

Donald,
No worries :)

Nick,
I'm being very careful indeed.
Not sure of your comments. The big space is in fact because the huge head can't sit low enough (see this thread: http://www.largeformatphotography.info/forum/showthread.php?t=34276 ) because of its size.
I can make any size negatives so long as the unit travels up the main column - this is a motorised enlarger, the whole system moves at once. I can make 1:1, 1:2 1:4 etc. At the moment it's set up for 12x16 paper I think, but every part moves.

The location of the light head in relation to the condensers is similar if not identical to the location of the original incandescent bulb to the condensers.

This image from Glennview:

http://glennview.com/jpgs/beseler/cb7/big_3.jpg

http://glennview.com/jpgs/beseler/cb7/big_4.jpg

[Ash]

#3 is now the adapter
#5 is still in place
#2 is now the new head

[Nick_3536]

With a diffusion head you need the light source very close to the negative stage.

But ignoring that your spacer is like extending the lens bellows. The CB-7 already has fairly long bellows adding extra space will limit the sizes you can print. Unless you want to use a real long lens.

[Ash]

Nick, I think you're mistaken.


Forget the original method of using a diffusion head on a negative. It's a diffused light source (like an opal bulb) to be used in conjunction with the condenser.

The negative and the lens bellows and the lens stage are all the same. The projection of the negative is not affected in any way other than the *type* of light hitting it.

[John Cahill]

To even out the light, put a sheet of film in place and give it a slight flashing exposure. Develop it and use it with the diffuser.

[Donald Miller]

I have found this to be a good depiction of illumination as it applies to photographic enlarging.

http://www.jensen-optical.us/PDF_FILES_TANK/COLIMATED%20LIGHT%20VS%20DIFFUSED%20LIGHT.pdf