Enlarger Drop Off - what is acceptable?

I am still fooling around with my Durst 138s, trying to get the right light source for it. I am having a lot of success with LED lamps but I am still getting light drop off from about 1/3 to 2/3 stop, and in some case, a whole stop.

What is acceptable drop off? Is 1/3 - 2/3 not that much? Am I being a fussy old fart?

Ooops. I should have added that I am using a 210mm lens with a 5x7 negative.

I have not used anything above 5x4 and that was some time ago, but the lens that was used was a 150mm, so you may be border line using a 210mm for 5x7. That is also depending on what size of negative the lens was designed to cover. What is the next size (focal length) up the chain available for you

BMbikerider said:

I have not used anything above 5x4 and that was some time ago, but the lens that was used was a 150mm, so you may be border line using a 210mm for 5x7. That is also depending on what size of negative the lens was designed to cover. What is the next size (focal length) up the chain available for you

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The largest negative I can use is 5x7. I wonder if I am going to need 240mm.

Is the light fall-off on the easel or at the negative stage? If the easel then potentially it is the lens. If at the negative stage then you may have uneven lighting. JMTCW

I'm guessing that you mean light "fall-off" rather than "drop-off".
When I saw the title, I was worried we were going to be learning about a nasty darkroom accident!
Sometimes fall-off at the negative stage helps offset uneven illumination in the camera - especially with wide angle lenses.

I have about a third stop drop-off at the edges of my Omega D-II with the Aristo Grid / Zone VI lamp.

You can see it clearly in my media "Solo Art on a Crowded Beach - Pacifica, CA".

Lately, I have been simply using a giant dodger in the middle and giving the edges 1/3 stop burn.

Bill Burk said:

I have about a third stop drop-off at the edges of my Omega D-II with the Aristo Grid / Zone VI lamp.

You can see it clearly in my media "Solo Art on a Crowded Beach - Pacifica, CA".

Lately, I have been simply using a giant dodger in the middle and giving the edges 1/3 stop burn.

Click to expand...

Bill, I have known of photographers, some in Europe who wrote articles on printing for Linhof's Photo-Techniques magazine, who burned in corners about 1.3 stop as "Standard Operating Procegure" on every B&W they printed. This very gently caused the viewer to concentrate on what was important in the photograph (my opinion). I don't remember if this was done by "burning-in" or "flashing". (Not the raincoat kind).........Regards!

You should experiment with different lenses if you can, it can make a difference!

When printing 4x5 I've observed that the corner falloff of my Beseler 45MXII is worse with my 135mm Nikkor-EL than it is with my 150mm Nikkor-EL, even though both are specified by Nikon as suitable for 4x5. I believe the root of the issue is the finite size of my cold light source relative to the size of the negative. The 135's wider field "sees" further, out to where the light source starts to end, which means the corners get less effective illumination. I'm probably not explaining this very clearly, but it was easy to observe simply by swapping lenses and adjusting the enlarger height to print the same size image.

I don't know if this same issue comes into play with a condenser system.

Are you measuring light falloff on the projected image? If so, have you optimized your lens and magnification?
(4x5" format example; I don't have the falloff data for the 210mm) Top most dotted line (best case possible 1/4 stop falloff) = f11 at 3x magnificaiton. Lowest line (worst case about 2 stops falloff) is 12x magnification wide open. On the graph 50% relative illumination = 1 stop.

You can generally just burn in the edges and corners. But if you want a better solution, you have to grind a convex sheet of translucent white plastic with the corners and edges thinner than the middle, to create a custom diffuser for even illumination. Or you could go back to the drawing board and make a bigger LED panel with a reflective mixing box below, plus a plastic diffuser or two, as needed. Mockup mixing boxes can be made by taping up pieces of white fomecore board, but don't use that material for the final component, just for testing.

If you look at an Omega D5500 mixing box diffuser, like Drew mentions above, it is convex. Also, you can project and print an exact negative center filter with a sheet of negative film.
This is the one I made for my first 8x10 light source which had a pretty severe hot spot in the middle.

1/3rd or 2/3rd drop-off is fine. It compensates the drop-off that was created in the negative during exposure. In case there is too much vignetting that you don't want you can do some burning around the edges, maybe even creating some dark vignetting, either very subtly, hardly visible, or more pronounced.
Stopping down the lens wil reduce the drop-off as well. Which condensors are you using? For 5x7 it should be 2 latico 240's.

Bill Burk said:

I have about a third stop drop-off at the edges of my Omega D-II with the Aristo Grid / Zone VI lamp.

You can see it clearly in my media "Solo Art on a Crowded Beach - Pacifica, CA".

Lately, I have been simply using a giant dodger in the middle and giving the edges 1/3 stop burn.

Click to expand...

Long ago when contact duping 30x40 halftones on diazo film I used a giant dodger to efficiently correct for the uneven illumination on the contact frame. In an enlarger with certain enlarging lenses, the dodging should be done close to the easel to prevent some loss of sharpness.