Try printing a middle gray. The slope is steepest at the middle of the curve. This will show the abnormality best.
Usually small changes in focus have no effect with a diffusion light source (unlike condenser), however, if the lens is way out of place (closer than the focal length) you will see falloff.
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I'm curious whether this is not just a natural phenomenon of all camera's & enlargers and whether it would be at all perceptible in a print?
It's "natural" to an extent and yes it does pop up in prints in problematic situations, and no it's not something that magically corrects itself. In problem cases it's often to do with light beam shaping issues above the negative stage as @DREW WILEY also alludes to; problems with diffusing chambers being wrongly configured, wrong condenser choice for the lens used, wrong lens - condenser distance etc.
So, does anyone happen to know, what an acceptable tolerance or deviation would be of enlarger light fall-off between the centre vs the edges/corners, expressed in f-stops?
And should the standard be zero?
When I did this with the aperture set to f/8, my Sekonic showed the same in corner as it did in the center. There probably was a difference, but it was less than the resolution of the display.
For example the common Schneider Componon-S 150mm loses about 2 stops at the edge at large magnifications when wide open.
The lens no problem, I checked another lens(nikkor el 150mm) at same result
This is principally aimed at the questions posed in post #33. It addresses the light falloff due to the natural characteristics of lenses. Light sources can introduce problems of their own, of course.
I did the following informal test to measure the falloff from center to corners in a projection sized to cover an 8” x 10” sheet of paper. I used an Omega D5XL with the Dichroic II diffusion head and Componon-S 5.6/150 lens. I disengaged the detent mechanism so that I could smoothy position the aperture ring in fine increments.
I first sized and focused a 4” x 5” negative Then I removed the negative from the carrier and replaced the carrier, since I want to measure the light intensity delivered by the lens, not influenced the varying density of the various parts of the negative.
With the aperture wide open at f/5.6, I placed the small sensor of my Ilford EM-10 in the corner of the projection and adjusted the sensitivity dial to get the green LED. Then I moved the EM-10 to place its sensor in the center of the projection. I slowly closed the aperture until I once again got the green LED. In room light I examined the position of the aperture ring relative to the index line. It appears to be closed about 0.7 stops from f/5.6 towards f/8.
Next, I started with the aperture set at f/8 and repeated the above steps. This time I could move the aperture ring in only a tiny increment with the sensor in the center of the projection to get the green LED again. The total movement of the aperture ring was less than 0.1 stop closed (likely something like 1/20th of a stop) from the f/8 setting I used for the corner.
Next, I used the spot metering function of my Sekonic L508. Leaving the aperture wide open at the f/5.6 setting, I got readings of f/8 + 0.8 stops in the corner and f/11 + 0.4 stops in the center, a difference of 0.6 stops. (Note: The resolution of most digital displays is to the nearest 0.1 stop).
When I did this with the aperture set to f/8, my Sekonic showed the same in corner as it did in the center. There probably was a difference, but it was less than the resolution of the display.
Both instruments gave similar center-to-corner differentials. This is consistent with center-to-corner readings I’ve measured with other enlargers. I usually get something like 0.6 to 0.7 stops difference from corner to center with the aperture wide open. The difference is much smaller with the aperture closed one or more stops.
My condenser enlargers usually show about 0.3 to 0.2 stops difference from corner to center with the aperture closed one or more stops, while my diffusion enlargers show a smaller difference, usually 0.1 stop or less.
A badly designed, badly made, or badly adjusted light system, especially a condenser light source, can give worse falloff than what I’ve usually measured.
By closing the aperture at least one stop, we’ve done about all we can to reduce the falloff to an acceptable value insofar as the falloff is due to the lens.
It makes good sense to learn how to give just enough of a burn to the corners/edges to make our prints look good. Done with the skill gained through experience, most viewers will never know that you did so. It’s easy to learn.
I think it would be fair to assume that the centre-to-corner/edge fall-off, would be exacerbated by printing larger say from 8x10" to 16x20"?
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The bigger you enlarge, the closer the lens comes to the negative (in order to focus). This increases light falloff as, one reaches the limits of coverage.
xkaes - I've personally had to use wide f-stops for certain applications, such as highly masked (up to .90 density) chromes for Ciba prints - a slow medium to begin with; or for actual enlarging onto slow contact papers like Azo. But in ordinary black and white silver printing, or RA4 work, my light sources are way too powerful for that, and I too consistently stop down.
The bigger you enlarge, the closer the lens comes to the negative (in order to focus). This increases light falloff as, one reaches the limits of coverage.
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