Diy 6x17 enlarger

[gunner]

I,m in the process of converting an old Devere 5x7 monorail camera into an enlarger for 6x17. I've now got to the stage where I'm working on the light source, I started off with an led site lamp which worked o.k, but I then went onto making an enclosure with 2 x 100 watt equivalent led lamps. Using a lux app and my sekonic L508 I'm getting a difference of 1/2 stop between the centre and the edge of the projected area on the baseboard. Just as an experiment I took readings from my LPL 7450, with the head raised high enough to fill the baseboard and there was a stop and a half difference between the centre and the edge, going on that basis should I be content with a 1/2 stop difference with my DIY enlarger?. I've used my LPL 7450 successfully for many years and without having taken those readings I would have said the illumination was very even so I'm guessing a1/2 stop on my self build is quite acceptable?

 

[xkaes]

You don't give any details about where or how your are measuring the light. You mention the lens board, but is there a lens in there somewhere????

 

[gunner]

You don't give any details about where or how your are measuring the light. You mention the lens board, but is there a lens in there somewhere????

I used a 180mm lens for the 6x17 focused a negative to give a image 2ft across on the baseboard removed the negative and then took measurements at the baseboard, just for reference I used a 150mm lens on the LPL and did the same thing with a negative focused to give a 2ft image removed negative and took readings

 

[xkaes]

Thanks. Did you stop down the lens three f-stops to take measures? If used wide-open you would expect fall-off.

 

[blacksquare]

There will be some error in the way of measuring light or working with the LPL enlarger. Stop and a half difference between the centre and the edge it's too much.

To give you an idea, it's like an exposure of 10 seconds in the middle and only 3.5 seconds on the edges.

 

[gunner]

Thanks. Did you stop down the lens three f-stops to take measures? If used wide-open you would expect fall-off.

No I didn't, I always do when printing, but just assumed I should take light measurements with the lens wide open, stopped it down 3 stops and i am getting a lux reading of 40 in the centre of the image and 39 on the edges, I think thats pretty good. I tried the same with the LPL, when stopped down 3 stops there was still a slight variation, but stopped down 4 stops and the reading from the centre to the edge was the same.
Now that I know the light source is o.k I can go on with the rest of the build

 

[xkaes]

SOUNDS GREAT. I'm really, really surprised that I'm the only one who suggested stopping down. Light fall-off is one of many reasons why lenses -- on enlargers or cameras -- should be used (or tested) stopped down -- when possible.