Help needed with an enlarging sharp images

[Stephen Willard]

My intent is how to improve the sharpness of an enlarged photograph at the edges when making big enlargements of 40 inches or greater along one side.

I currently have one of the latest EL Nikkor 210mm lenses for enlarging photographs using 4x10 color negatives. The lens cover the format and the negatives are sharp from edge to edge. Four years ago I benched marked this lens against a new Schneider and Rodenstock enlarging lens of comparable focal length and found the Nikkor lens produced the sharpest image at the edges when making big enlargements. All of the lenses were very sharp near the center of the image, but after a side of the print reached 40 inches in length, the edges became noticeably softer for all lenses. Please note, I have not had any customer complaints yet about this problem, but I still find the soft edges unacceptable.

I suspect that part of the problem arises because I am using faster-wider aperture settings (f8, f11) to keep the exposure times reasonable short or under 30 seconds. I am also trying to use the least dense CC adjustments as possible to further reduce the exposure times. My color head uses 2000 watts consisting of eight 250W halogen lamps. Long exposures can burn out lamps in my color head real quick. I have resorted to using two shorter exposures with some cool down time between each exposure instead of one long exposure. This seem to help cut down on lamp burning out.

I am also considering buying a 300mm enlarging lens. I am hoping because of the greater coverage of this lens, this means the edges of the 4x10 negative will now be closer to the center of the lens and there will be less blurring at the edges. Can anyone speak about this approach of using a larger focal length lens to get greater sharpness at the edges of big enlargements? Is my logic plausible?

[Bob Salomon]

Did you try this with a 150mm Apo Rodagon-N or a 120mm Rodagon WA lens? You also need a glass carrier and a critically aligned enlarger and be using the optimal aperture of the lens and be within the optimazation range of the lens.

You don't mention any of these very critical requirements for a high quality very large print. The longer the focal length the less the lens is corrected for larger magnification. You need a lens of the correct focal length corrected for the amount of magnification that you want.

[Ari]

Bob is exactly right.
I just wanted to add, from my own experience, that simply changing focal lengths will not improve your coverage; that depends on how much your particular lens will cover. Some shorter focal lengths cover more than longer focal lengths, to wit, some of the lenses Bob listed
And if you're interested in keeping your exposure times down, stay with a shorter focal length lens, as that will reduce your lens-to-paper distance.
Good luck.

[Dave Dawson]

"And if you're interested in keeping your exposure times down, stay with a shorter focal length lens, as that will reduce your lens-to-paper distance."

But it won't increase the illumination level

Same amount of light spread over the same area...Same level of light.

Cheers Dave

[Drew Wiley]

I routinely do 30x40 prints with 305 and 360 Apo-Nikkors, which are very sharp from f/11 down. When I need more speed I use a 360 El Nikkor at f/8. This is with
8x10 film of course.

[Arthur Nichols]

I cannot address the lens sharpness concerns that is left to more knowledgeable people than I, but I also have a 2000 Watt Devere color head that I use regularly for VC b&w printing. I purchased long life bulbs for my color head and have never had any problems with lamps burning out, even when leaving the enlarger on for long exposures. I assume that you have a fan powered cooling system for the head. I cannot imagine a 2000 watt head without one. Some further investigations into why lamps are buring out may be in order. Possibly there is a problem with insufficient air movement.

[ic-racer]

That lens does not cover the format at f11 at that magnification; and you know it because you just told us its blurry at the edges at that aperture and magnification.

Just like a view camera lens covers better at closeup range, an enlarger lens covers better at small enlargements.

Doing a big enlargement is like going to infinity on your view camera, so coverage will get worse.

[Stephen Willard]

In response to some of the thoughtful considerations noted:

• I use a negative glass carrier.
  
• The 210mm lens is the shortest focal length lens I can use to cover a 4x10 negative. A 150mm lens will not cover the 4x10 negative.
  
• All of the longer focal length lenses have considerably greater coverage than my 210mm lens.
  
• I have aligned my enlarger with my own laser alignment tool.
  
• All of the prints are razor sharp in the middle and soft on both edges 40 inches or greater.
  
• All of the lenses I have tested range from 210mm to 240mm show the same characteristics of being sharp in the middle and soft at the edges. The Nikkor lens was the sharpest at the edges, but it to was still soft.

[Bob Salomon]

In response to some of the thoughtful considerations noted:

• I use a negative glass carrier.
  
• The 210mm lens is the shortest focal length lens I can use to cover a 4x10 negative. A 150mm lens will not cover the 4x10 negative.
  
• All of the longer focal length lenses have considerably greater coverage than my 210mm lens.
  
• I have aligned my enlarger with my own laser alignment tool.
  
• All of the prints are razor sharp in the middle and soft on both edges 40 inches or greater.
  
• All of the lenses I have tested range from 210mm to 240mm show the same characteristics of being sharp in the middle and soft at the edges. The Nikkor lens was the sharpest at the edges, but it to was still soft.

After selling enlarging lenses to professional, high end labs, for over 20 years and not having had youir complaints I would have to question your technique or your testing. Just what is your alignment system?

And you need a lens equal to the diagonal. That would be something like a 300mm Rodagon since you have a diagonal over 12" long on 4 x 10".

[Stephen Willard]

After selling enlarging lenses to professional, high end labs, for over 20 years and not having had youir complaints I would have to question your technique or your testing. Just what is your alignment system?

I use the Parallel by Versalab laser alignment tool to align my 10x10 Fotar enlarger. The enlarger has lots of adjustments that can be made to facilitate alignment. A 16x40 print enlarged from a 4x10 negative is will be very sharp in the middle, but both sides of the print will be soft. Either the lens is soft at the edges or the enlarger becomes noticeably out of alignment at the 40 inch length or 20 inches from the center. Here is a test I could do that just occurred to me:

• If I am able to use my 10x focusing loupe and bring either end of the projected 16x40 image into focus by raising or lowering the colorhead then the problem is an alignment problem.
  
• If I cannot bring either edge of the projected 16x40 image into focus then the problem is not an alignment problem, but rather an inherent shortcoming of the enlarger lens.

And you need a lens equal to the diagonal. That would be something like a 300mm Rodagon since you have a diagonal over 12" long on 4 x 10".

From my calculations the digaonal of a 4x10 negative is 10.77" or 273.558mm which is barely within the 270mm coverage of the lens. Perhaps I should consider getting a 240mm lens just to make sure!

Thanks Bob for your persistence. I will get back to you about the results of my little test outlined above. In any case, please recommend 240mm lens that would allow at least a 8x enlargement or greater. I want to get the best lens possible.