Evenness of illumination

[markbau]

I've been playing around with my new RH analyser Pro. In starting to do some calibration I noticed some unevenness in illumination on my baseboard. Here are the readings from 6 points on my baseboard. Is this about normal or do I have a serious problem?
Enlarger is LPL 4500, Lens was a Componon S 100mm @ f8. 6x7 neg holder.

 

[chris77]

pretty ok !

 

[Ozxplorer]

Yes... I too find some light fall off from the centre toward the edge of the baseboard illuminated area. Generally I add up to a1/4 stop extra edge burn to balance the print density. The variation is to be expected: the distance of the lens to the middle baseboard read is shorter than the distance from the lens to the edge of the illuminated area... hence, in my situation, the lesser light intensity at the edge.

 

[ic-racer]

What are your units for those numbers? I'm guessing log D, where 0.3 is one stop and 0.1 is one-third stop?

That will change with magnification and aperture, so even if you were to correct it with a special graded filter, it would only be perfect at that single magnification and aperture. Another way to do the test is to expose a sheet of your highest contrast paper, that will put those numbers in perspective.

Here prints made with old 8x10 light source before and after I made a special center filter:

 

[ic-racer]

Here is a typical enlarging lens with data on how light falloff changes with aperture and magnification. So, a 12 power magnification will only pass 30% of the light to the corners at f5.6

 

[ic-racer]

In your quest for great enlargements, you can also measure flatness of field. Your focus track probably does not have marks but the center column probably does. In my case I record the millimeter difference needed to focus the head at various points on the baseboard. This is a 'before alignment' test showing lens-to-negative mis-alignment and curvature of field.

 

[AgX]

As indicated light fall-off even occurs if the illumination of the film-stage is perfectly even.
However, taking lenses have light fall-off too. So in some cases both fall-offs might equalize each other in negative-printing.

 

[markbau]

As indicated light fall-off even occurs if the illumination of the film-stage is perfectly even.
However, taking lenses have light fall-off too. So in some cases both fall-offs might equalize each other in negative-printing.

How would you test to see if a taking lens had light fall off? Photograph an even tone and measure the density across the neg?

 

[ic-racer]

How would you test to see if a taking lens had light fall off? Photograph an even tone and measure the density across the neg?

Yes that would work, or look up the datasheet. This is the datasheet for my worst 35mm lens in terms of light falloff. It is even a retrofocus (which sometimes has less light falloff than non-retrofocus). This is the 18mm Zeiss Distagon F4:

 

[ic-racer]

An interesting exercise, which might provide some meaningful information would be to make a print of your camera negative taken of an even target. That would give you a visual reverence for the combined effects mentioned by AGX above.

 

[DREW WILEY]

There are a several issues. First, you have to be aware of cosine error related to angle of light incidence with most easel reading devices, which tend to read accurately only in the center of the projected light. Second, many light sources need a diffuser ground thinner at the edges than at the center to compensate for illumination falloff toward the corners, accentuated by falloff with most enlarger lenses unless they are longer than "normal" focal length relative to the film format. But it looks like your results already are pretty darn good if the discrepancy is only around .10 density per corner, easily managed by simple burning-in.

 

[Doremus Scudder]

Drew's answer is right on. IM-HO, edge-burning is something I find nearly every print needs (except for those that are from negatives made with very short lenses and no center filter, then the opposite may be needed). It's simple physics; the farther from center, the more fall-off. This applies to both taking and enlarging lenses.

I edge-burn 10% for starters, even when making the first straight print for evalutaion. I use a card and move from the edge in to expose at least one-third of the image and back out again over the burning time. Burning all four sides gives the corners extra exposure to compensate for the extra fall-off there. This works for me as a standard starting point; other manipulations, including more edge/corner burning, are determined from there.

Best,

Doremus

 

[logan2z]

Interesting thread. I wonder how well the falloff of the taking lens compensates for the falloff during enlarging on average?

Maybe it's the nature of the things I photograph but I can't say I've ever found the need to edge-burn my prints in order to compensate for the falloff of the enlarging lens.