Depth-of-focus problem, enlarging lenses

[Ulophot]

Just out of curiosity, I looked up depth-of-focus calculation today. Leaving aside the confusion, of many, with depth-of-field, I am trying to understand how to apply the formula, which is not as exotic as I imagined. What I found is:

t = 2Nc(v/f)

In this equation, t equals the total depth of focus, N equals the lens f-number, c is the circle of confusion, v equals the image distance, and f represents the lens focal length.

Questions:

1. What circle of confusion would I be using for my 135mm (4x5 neg)? would it be the same as my favorite online depth-of-field calculator's, i.e., 0.108 mm?

2. Is v the distance from the middle of the lens to to film as i sits in the neg carrier?

Thanks.

[Michael R]

I would have thought in the case of an enlarger depth of field is at the negative stage and depth of focus would be at the paper plane.

[Drew Wiley]

Circle of confusion? Why make it confusing by even going down that rabbit hole? It's easy enough just to make experimental test strips and see if the image sharpness is acceptable to you. You can even surmise that wearing a pair of reading glasses when projecting the image onto the easel. But everything depends on the specific image itself and the degree of magnification.

[Ulophot]

Drew, I think you may have missed the beginning of he thread. I'm trying to avoid a texturing pattern on a diffusing material above the negative stage. Eric Woodbury, see above, offered one solution based on his use do the same material.

[Maris Rusis]

I use an Omega D2 enlarger with a 135mm lens to enlarge roll film negatives to an 8x10 size and occasionally larger. The negative carriers are home made and feature a top glass made out of common anti-glare picture frame glass.
This glass has a fine wavy texture in it and it lies directly on the negatives and keeps them flat.

The texture of the glass does not show on the prints even in large areas of smooth tone. Why? I think it is because the Omega D2 enlarger is not really a "condenser" enlarger even though it has three condenser lenses in the head. The key thing is the big white light bulb in the top which is a long way removed from an actual point light source, more a "semi-condenser" or "semi-diffusion" optical system.
Stopping the enlarger lens right down does not cause the glass texture to appear, at least in my set up. What does appear at f45 are vague out of focus images of dust specks on the condenser lenses. Don't use f45.

I've tried similar anti-glare picture frame class in home made negative carriers for a Durst 138 enlarger and the texture does show. The original light bulb and condenser set up for the Durst 138 delivers a much more collimated light beam.

A famous member here, Drew Wiley, reports using fine texture anti-newton glass top and bottom of the negative for perfect flatness. Again, no reports of the texture coming through.

I'd guess a highly diffused enlarger light source tends not to print texture in the light path but only practical testing is truly decisive.

[Ulophot]

Maris, thank you for your contribution, which is very helpful indeed. In fact, my 4x5 carrier has a custom inset of white plexi in the top window, for use with a hand-penciled or -marked masking technique, which I employ occasionally. Therefore, the texture issue with that carrier is not an issue for me. However, Bob Solomon, as you may have seen above, said that the 135 lens has greater depth-of-focus than shorter focal lengths. Since I occasionally use the 135 for enlarging 645 negs, in a standard carrier with open "windows", like my 35 neg carrier, I thought I should pursue the matter, also because my builder friend has toyed with the idea of making more of these heads for D2s, should there be any interest, assuming this pilot model works out well.

(By the way, this head is to replace my aging coldlight head, which, of course, was already a diffusion source. It replaced my condensers and variable condenser long ago.) Again, thank you for sharing your directly relevant experience. Best wishes.

[Drew Wiley]

It's all relative. I ordinary do not get any texture replication. However, in extreme cases of needing to salvage some badly underdeveloped neg by resorting to some very high VC contrast setting, in conjunction with an uber-crisp multicoated apo enlarging lens, I might indeed get a bit of visible texture in an open sky. I don't think that would be too big a risk with a 135/5.6 Rodagon, a lens I once owned too. Wish I still had it for that very reason. I normally enlarge 6X9 negs with a 150/4.5 Apo Rodagon - deluxe, but quite contrasty.