Vexing Alignment, Easel Issue
I am getting really annoyed with alignment. I have a Beseler 45 VXL enlarger. I verified that the lens stage, negative stage and baseboard are in alignment. As I drilled down through possibilities, I figured out that my easel is bowed. In fact, they all are. I have 3 Saunders Pro 4 blade easels:11x14, 16x20, and 20x24. All of them are convex on the surface with the center of the easel slightly closer to the lens than the corners of the easel.
For 8x10 and 11x14 prints the problem is small. But for 16x20 the problem is unacceptable. I'm not sure what to do about it. I do have a vacuum easel that I have never used. I pulled that out but it has 4 leveling feet which I can never seem to get right but the center of it (steel plate) seems slightly bowed as well. Is it just me? Any advice is appreciated.
Would closing the lens down to a smaller aperture help?
For the vacuum easel, it may well have some bow built-in so that it is flat when operating.
On the others you may be able to tweak them flatter by carefully applying some force in strategic places. Though it seems odd that all of them are distorted.
Do you have an accurate straightedge to judge the flatness?
What is exactly the problem? (blurry on one corner, two corners, all corners?) What format? What negative carrier? What lens?
Something to consider is that bigger the magnification the less your baseboard/easel flatness matters. Also, as magnification increases, the more your negative to lens alignment matters.
For a 16x20 from 35mm negative, one side of the easel can be up in the air 2 inches and it should still be in focus.
So, it is possible that the easel is not the culprit.
Are you sure the negative is held flat?
Is the negative centered on the image circle (flatness of field will diminish at the periphery of the image circle)?
Last edited by ic-racer; 03-31-2009 at 05:40 PM. Click to view previous post history.
Perhaps. You can use the view camera focusing method with an enlarger. Jeff Conrad derived some equations that allow you to figure the maximum tolerable F number based on the MTF (personal correspondence).
Originally Posted by Stew
N_max ~ 20 / (1 + m) sqrt(dv)
N_max = maximum tolerable F number
m = magnification
dv = distance on the enlarger column between good focus on the highest and lowest portions of the curved or askew negative. (if 'dv' is zero then your negative is not curved or enlarger/baseboard/easel is not askew)
I'm curious as to what your 'dv' is. In terms of 4x5, I find that it is just a few millimeters, as the 4x5 film tends to hold pretty flat even in a glassless carrier. If you are using 4x5 and you get more than a few millimeters for 'dv' then something is amiss, though stopping down may help.
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My vacuum easel is "crowned" when not in use, but is as flat as I can measure when in use and a sheet of paper is in place. A very small rubber machinist's mallet, a dial indicator and a surface plate may be what you need to deal with your easels. After reading this: http://www.horolezec.cz/blog-engl/in...=item&itemid=5, I realize that I am a rank amateur when it comes to flatness......
Have you mistaken distorted surfaces for curvature of field? Dan
Thanks for the input.
I use a Beseler alignment tool, the one that has two bubble levels at right angles. The lens board, negative stage and baseboard are all in alignment. Then I place the easel on the baseboard. The center of the easel is closer to the lens than the 4 corners so it is crowned slightly. I would not have noticed this by looking. Likewise the effect is modest at smaller enlargement ratios. However, on a 16x20 print (4x5 negative) the 4 corners are unsharp. The top corners of the easel are worse than the others.
That is interesting that the vacuum easel might even out when the vacuum is activated. But I really don't want to use it (even though I bought it). Why would someone prefer a vacuum easel for baseboard enlarging anyway?
I don't think it is lens field curvature because I can measure it on the easel with the levels. The lens is a Schneider Apo-Componon 150 mm. I think I have had this issue all along but to a minor degree. Must be something in the are this time of year.
For the reasons mentioned, stopping the lens down does not help much.
OK, so you are in a 'low magnification' range (4x to be exact), so paper/easel/baseboard problems could possibly be evident. We can still use that equation to find the smallest aperture to bring everything in focus (just like focusing a view camera). The 'dv' (which I neglected to mention is in millimeters) would be the amount of 'crowning' evident in your easel (assuming, as you said, your negative is perfectly flat and everything is in appropriate alignment).
So, if you have about 4mm of crowning the equation would work out like this:
dv = 4mm
M = 4 (16x20 print from 4x5 negative)
So the minimum aperture needed (taking into account both diffraction bullseyes and out of focus circles of confusion) would be:
N_max = (20/5) * sqrt(4)
N_max = 4 * 2
N_max = f8
Likewise, if you can't get it all in focus at say f22, your easel must be crowned up 30 millimeters from the corners.
Can you confirm that when the easel is removed everything is correctly focused on the baseboard? Something other than your easel seems amiss (based on the info at hand).
Originally Posted by ic-racer
Thank you very much. I will try to confirm on the baseboard tonight and will try stopping down to f22 to check out when I print the next 16x20. I went back to 11x14 this afternoon with a typical working aperture for f11 1/2.