Great! You found it.
I think a careful painting job using matte black paint under the negative stage base and maybe even inside top of lens stage would be a good move to reduce the internal reflections in the area between the negative and lens.
Some "flare" is expected. You should be able to get good Dmax without the modification. But I still recommend the paint job. Also if you were using a condenser enlarger before, this is a diffusion enlarger which requires "harder" negatives to get the same results as before.
Your 50mm lens doesn't show it up because it's angle of view doesn't cover the entire stage.
Originally Posted by kmallick
How are you determining one enlarger prints with more contrast than the other? Same lens on both enlargers and graded paper, right?
This is a stubborn problem. I put black tape over the edges of the opening in the negative stage. See pictures of the mod below. The stray lights didn't go away.
I put a paper mask on half of the negative carrier (above the negative) and the stray lines disappear on one half of the baseboard.
I also tried moving the bellows up and down and that didn't move the stray lines. Therefore it doesn't seem like the bellows causing it.
I also checked the inside of the bellows carefully. Its matt black with no real shiny edges.
Is this some sort of diffraction?
Last edited by kmallick; 06-30-2012 at 12:34 AM. Click to view previous post history.
Notice how the bellows show up in the two pictures. They would be pitch black if they didn't reflect. It's images of the bellows that you see. Light goes down through the negative and bounces off the inside of the lens stage which "lights up" the flats of the bellows.
Thats what I was suspecting too. But with the enlarger on, negative in the stage and room dark I tried poking and moving the bellows up and down from outside from different sides to see if that would move or shake the stray lines. It didn't.
Originally Posted by Bill Burk
The bellows do show up in the two pictures when there is no negative and the opening "lights up" the flats of the bellows. I don't see the parallel lines reflecting off the bellows when I have the negative in. I see just 4 stray lines in a square pattern no matter what size of negative I put in, 6x4.5, 6x6 or 6x7.
However when I mask half of the negative off, the stray lines disappear from one side. This definitely points to the fact that the light is reflecting off of some edge on the negative stage, doesn't it?
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A mask with flat black craft paper that closes the opening in the negative stage (below the carrier) by 1/8 inch on each side significantly reduced the intensity of stray lights. Making the opening of the mask any smaller encroaches into the projected image from 6x7 negative.
I think the problem you are facing is that the surface of the bellows is not dark enough and they are narrow enough for the lens to "see" the reflection off of them. Four suggestions for you that should solve your problem.
1-Make a mask like the one you have there but put it down inside the bellows inside the second or third fold under the neg.
2-Make a type of reverse lens hood by simply wrapping a length of paper around the rear of the lens forming a tube that sticks up toward the negative.
3-You may want to get some flat black fabric paint and paint the inside of the bellows with it.
4-A better alternative to the paint would be to get some telescope flocking and apply it to the edges of the inside of the bellows.
If I had your problem I would go the flocking route. I would rather absorb the stray light than simply try to block it. Telescope flocking absorbs over 99% of the light that hits it. When you hold it in your hand it is like looking at a black hole, I kid you not. I have a ton of it and i would be happy to send you some if you need it.
edit-Looking back after having written this, I think your best bet would be to do the first thing I suggested and cover the mask with flocking. I would eat my shorts if that didn't work.
This is a fascinating problem. I tried some experiments with my Omega B66XL dichroic enlarger to gain some insight. I chose this machine because it is of a similar size to the LPL/Saunders 6700 dichroic enlarger. Each test was done without any safelights on to maximize my ability to see any stray light.
First, I installed an empty 35mm negative carrier and a 50mm f/4 EL Nikkor and focused the image of the edge of the carrier onto the baseboard. I saw nothing but the image of the illuminated window of the carrier.
Next, I placed an empty 6 x 6cm carrier and an 80mm f/5.6 EL Nikkor in the enlarger and focused the image of the carrier opening as before. It took a second or two to realize that there was an extremely faint set of concentric squares, possibly 3 or 4, surrounding the main image. Clearly, I was seeing the image of light reflected by the inner folds of the topmost portion of the bellows. The 80mm lens throws a large enough circle towards the film to “see” the highlights reflected from the bellows.
I generally reserve this machine for 35mm enlarging, but I have used it occasionally for 6 x 6cm work. I hadn’t previously noticed the projection of the squares.
Next, I put an average-density 6 x 6cm negative into the carrier. With the aperture wide open on the 80mm lens at f/5.6 I didn’t initially see anything outside of the projected image. However, after 10 seconds or so my dark vision gained enough sensitivity to barely make out the brightest line nearest the front. I could no longer distinguish full squares. This was so dim as to be close to the limit of my ability to perceive it. Closing the aperture to f/8 made it much harder to find. At f/11 it was almost imperceptible. On my B66, the effect was only noticeable without a negative in the carrier. When I put the negative in, it was quite hard to locate the spurious light at the baseboard.
It’s obvious that on my B66 this is caused by the longer lens seeing the illuminated edges of the folds in the bellows due to the size of the pickup circle towards the negative. The 50mm lens can’t see the folds due to its smaller pickup circle (even though both lenses have similar angles of view).
The only way to totally eliminate this is to choose an enlarger with bellows sufficiently large enough that the lens can’t see the bellows or the edges of the opening in the negative stage. For example, an 80mm lens is unlikely to see the reflections of the bellows or negative stage of a 4” x 5” or bigger enlarger.
This is likely inescapable for enlargers with bellows just big enough for the largest format for which they were designed when using the longest lenses for that format. The upper parts of the bellows and, in some cases, the negative stage opening are visible to the lens. Bright reflections are necessarily projected to the baseboard, even if only dimly.
Regarding Post #26, I agree that replacing the temporary black paper mask with one made of the most light-absorbing black cloth, such as felt, velvet, or whatever you can find will reduce the effect as much as possible.
Ultimately, it doesn’t matter. The miniscule amount of light in the projection of the bellows folds or of the edges of the opening in the negative stage falls well outside of the enlarging paper and is, therefore, irrelevant.
The rear element lens sees everything you can see when you look up in the bellows with the lens off. The 'theoretical problem' here is flare on that rear element. You can just ignore it as it is highly unlikely to cause any effect in printing (unless one has a foggy lens). In addition to Ian's suggestion, if one wanted to eliminate any chance of flare on that rear element one could also rig a compendium shade over the rear element. But nobody does this, no manufacture recommends this and it is not needed. I'd spend my time making sure the enlarger is aligned. Enlarger alignment is an art that can take some skill to perfect.
Originally Posted by Ian C
I made a temporary mask consisting of 2 L-shaped parts that forms a square opening and that fits just above the first folds of the bellows. This pretty much got rid all of the stray lights. I just need to find the right flocking material that will fit in that space and I need to cut it square to very precise dimensions because its tricky to fit the mask in between the folds.
I feel much better now that I could identify the source of the problem, thanks to everyone's input.