I need a lens to perform flat copy work. I will photographing very sharp prints about 32 x 40" down to 4x5 film. So about 1:8 . I am trying to pick a lens (and a lens fl) to perform this task which will yield the sharpest images to the entire film area. I tried researching flat field lenses, as this task is surely a flat field task. After reading this article by Ron Wisner, I am starting to beleive there is no real benefit of a flat field lens over a traditional lens. (except maybe for wide angle flat copy, which can be avoided by choosing a longer fl lens)
Here is my thinking so far, please feel free to correct or add to any of my thoughts.... I have no experience at doing repro work.
1. Lens fl - First I take the image diagnol of the film, 150mm, and desire a fl lens which is at least equal to this. My theory is, the larger the angle of view of the lens, the more obstacles a lens must overcome, which prevents the lens from being as sharp as a normal fl lens. (normal fl being defined as, lens fl = film diag.) Of course, the same applies to camera lenses.
2. MTF values - Next, I looked at the MTF curves of lenses within this "normal fl", and within the 1:8 magnfication working area.
3. The obvious items, such as image circle, light fall off, distortion, etc.
Now, my first thought was that a darkroom lens (not camera lenses) would be most suited for this task, and it's possible they are. But after reviewing the MTF values of Schneiders enlarging lenses, I began to second guess this. I found the Schneider 150 mm Digitar lens (camera lens) to be superior to the enlarging lenses in just about every aspect at this magnfication ratio??
Then, I began to think it was the "flat field issue" that made the darkroom lenses superior for this task, but Ron Wisner article convinced me otherwise. (despite Bob Solomons small mention of this a few weeks back) Then I began to think, possibly the latest digital lenses simply trumped the previous lens technologies, whereas in the past, darkroom lenses would produce superior results vs. camera lenses, but possibly the latest lens designs and glass types may have given the digitar camera lenses an advantage, even in reproduction / darkroom applications? Of course, this only applies when the digitars specs can accomplish the reproduction requirements, which is not always the case.
Or, I am missing something completely? Maybe when the digitars are suitable for reproduction, they are the best option available, even though they were designed for camera use? What criteria would you use to pick such a lens?
If the digitar becomes the lens of choice, I am considering a vacuum back, as the 150mm digitar is optimized for this magnfication ratio at f8, which can make the ol "depth of focus" an un-welcome deal breaker.