Enlarger Lens Light Fall-Off Problemgreenspun.com : LUSENET : Large format photography : One Thread
Ihave an Omega 4x5 Variable Condensor enlarger that I bought used and it came with an "Elgeet 135mm f/4.5 Colorstigmat" lens. The lens worked OK but I decided I needed a better lens so I bought a 135mm f/5.6 Schneider Componon-S (on ebay) and have encountered the following problem with the Schneider. There is a 1 stop variation in the amount of light in the center of the print and the corners (8x10 print size) when the lens is set at mid aperatures (F8 or F11). This drops to about 1/2 stop variation when stopped all the way down (F45). I went back and checked the "Elgeet" and found only about a 1/4 stop variation at mid aperture settings. I noticed that the "Elgeet" has a 12 bladed diaphram which produces an almost round hole and the Schneider has a 5 bladed diaphram. Any suggestions on what I might do to resolve this problem. The light fall-off is very apparent in the prints produced with the Schneider.
-- John Randall (firstname.lastname@example.org), July 26, 2000
This sounds like some sort of condenser mismatch although why it's occuring with the same FL lens I don't know. Could be because they're different lens designs.
At any rate, I'd suggest experimenting with condenser positions or lens-cone length. But that's just a guess.
-- John Hicks (email@example.com), July 27, 2000.
Do you have the correct bulb in the light source? When I first got mine, it had a #240 which was much smaller and further away from the condensers. Illumination was good on the 135 but not so good on 80. The projected image of the bulb has to fill the enlarging lens' pupil. Are two lens' element's diameters significantly different?
Also, here's a long shot: I have an old uncoated Federal 135 mm lens that's probably quite similar in design to your Elgeet. I also have a Componon-S 80mm. Although they are not the same FL and require a different condenser setting, I've noticed that the componon is still less uniform. (I could measure a difference but I don't have to burn to compensate.) Now, since both paper and exposure meters are more sensitive to blue/UV suppose that for some reason the old lenses have a natural blue/UV center filter that would compensate for the drop off. I think this could come about in the old lenses from the thickness of the glass, the type of optical cement, any coatings or just haze. The new lens is multicoated and probably passes blue/UV better.
What I might suggest then is trying a UV or skylight filter on both lenses to see if it makes a difference. It may not improve the componon but may explain the discrepancy.
-- Duane K (firstname.lastname@example.org), July 27, 2000.
The natural fall off of light intensity with distance (inverse square law) dictates that the minimum fall-off is going to be in the region of half a stop from centre to corner. If you're only measuring 1/4 stop, there must be some magic going on somewhere. The 'centre filter effect' is a good theory, but it should change with stopping down. I don't think it's anything to do with UV if a meter is reading it as well. Meters have almost no sensitivity in the UV region.
-- Pete Andrews (email@example.com), July 28, 2000.
Boy do I feel stupid !, or what?. It occurred to me today, while driving to work, what the possible problem was. The Schneider lens came with a holder for a D5 enlarger which is dish shaped and the lens screws right into it. The "Elgeet" is mounted on a flat holder and therefor the Schneider rear element is lower and is actually below the hole opening in the mainplate that the lens holder mounts to. The fall-off is coming from vignetting caused by the hole in the mainplate. Also,I was probably wrong about the 1/4 stop fall-off with the Elgeet since it's clickstops are at whole stop intervals and the Schneider clickstops are at 1/2 stop intervals. I am going to open up the hole in mainplate and possibly bevel the inside surface. Thanks for the input though.
-- John Randall (firstname.lastname@example.org), July 28, 2000.