How to get even illumination with Focomat 1C ?

[M.Hilo]

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Hello, does anyone know the procedure how to get even illumination with the Focomat 1C ?

 

In Apug there is a thread where this is coming up: Double condenser modification for Focomat V35 (even though this looks to be about the V35, the issue is about the Focomat 1C)

 

In the past I have kind of solved light falling off, but have to admit I don't really know what I am doing. This what I did:

 

1) made sure the right bulbs are in the enlarger (shorter 150 W for the rounded 1C head, longer 150 W for the eg shaped 1C head)

2) I brought the enlarger up to plus minus 30X40 printing

3) closed the lens (when it is closed I see better the unevenness)

4) then I raised and lowered the bulb (3 adjusting screws !) and made sure the bulb was centered,

 

Anyone? Thanks

 

PS, this is a question about solving a problem within the system itself, not by changing the enlarger head.

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[M.Hilo]

Answering my own question: via the Apug thread and a Photonet post of almost 10 years ago by E. Zimmermann. Thanks in retrospect !!

 

Bulb Alignment

 

1. Raise the head all the way up so that the entire baseboard is illuminated by light when the bulb is on.
   
2. Center the bulb using the screws so that the holder looks "to your eyes" as centered.
   
3. Pull the bulb all the way (as high as it goes).
   
4. Place a spot probe on the middle of baseboard.
   
5. Measure the light.
   
6. Slowly lower the bulb watching the meter. Stop at the highest position with the highest light level.
   
7. Measure the four corners.
   
8. If they are not all aprox. equal then shift the screws moving the bulbs center slightly.
   
9. check the center and raise/lower the bulb slightly.
   
10. check the corners.
    
11. confirm that the difference between corners and center is what one would expect (this depends upon the probe and if it has "cosine" correction or not).
    
12. Repeat steps as needed untill one is satisfied.
    
13. place some black leader or a gray negative in the film holder and make an enlargement (with the time adjusted). Measure the reflective density and confirm that there are NO differences between the corners and center. NOTE: To save paper one can use some strips for the corners and center and develop all to completion.
    

[tobey bilek]

Tip the meter probe toward the lens when measuring the corners. The corners will read less because of lens vignetting and cosine error. Just make them all the same.

 

In my experience, long neck bulbs are used full up, short full down.

[M.Hilo]

In my experience, long neck bulbs are used full up, short full down.

 

Yes, I came to this conclusion too.

 

Strangely enough I came to the best result using a short neck 150watt bulb in the later "eg shaped" head of the 1C. Which is ok for now as I have some more of those bulbs. However, I would prefer to get it right with the long neck bulb, as I have plenty of those . . .

 

All in all, I find the older versions 1C (rounded head, no filter drawer) easier to get even in terms of illuminaton. Could be just coincidence.

 

And, I have no such problems with the 2C. That one has been fine for years, from 135mm to 6X9 . . .

[tobey bilek]

Yes full up or down.

 

Put the enlarging meter in the corners and make them all equal.

 

Tilt the cell to max reading, ie straight toward the lens.