BTZS Paper Test Question

[bobbysandstrom]

Hi

I completed my paper test with kodak polymax fb / dektol 1:2 70deg for 2 min.

I'm using the stouffer 4x5 step wedge (1/2 stops) and am enlarging to 8x10 on the paper as per instructions from the book.

The step wedge contains (2) #11 steps... one on the top and one on the bottom.

My problem is there is about a 2/3 stop difference accross the (2) #11 wedges!!! If this is the case(and I'm getting falloff on one side of the wedge) how can I possibly calibrate the system.

Is this typical (that much light falloff) with enlarging? To stay under the reciprocity threshold and make the 8x10 print, I'm exposing at f/5.6 for 20 seconds.

Any help would be appreciated.

Thanks
Bob

 

[fschifano]

I use the same step wedge to make contact prints instead of enlargements. I set the enlarger head to cover approximately 8x12 with the empty 35mm negative carrier in the enlarger and place the contact "sandwich" about in the center of the projection. This way you only use 1/4th the amount of paper to do your testing and accomplishes the same thing. It still doesn't answer your problem about light fall off though.

You can check for light fall off easily, and perhaps more accurately with an incident light meter. One with a flat disk diffuser is best, but if you only have a dome, that's fine too. Something that reads out in EV's is nice, but not necessary either. Insert an empty negative carrier into the enlarger and use the lens that you'd normally use with that format. Take readings at the center of the projection and again at 1/2 way out from the center to the edge, and again at the edge. Do this for all four sides of the image. If you see more than 1/3 stop difference, you have a light fall off problem.

Sometimes this is easy to correct, requiring that you do no more than stop the enlarging lens down two or three stops from wide open. With a condenser enlarger, you may have either the wrong condensers in place for the desired film format or they mey be misaligned or dirty. Diffusion enlargers will exhibit the same sort of problems if the light mixing box is too small for the desired film format.

But often enough the problem isn't mechanical as much as it is one of perception. Section #11 section on the top half is next to section #10 which prints darker, giving the impression that #11 is lighter than reality. Reverse that for the bottom half of the wedge. The only way to measure with absolute consistency is to measure each wedge with a reflection densitometer. These are expensive and not really practical for most folks to own. A workaround would be to cut each #11 section out of the print and place them next to each other on a dark background. If they still don't look the same, then you have a falloff problem.

 

[bobbysandstrom]

Thanks for your help. It turns out my 5.6 aperature has 2/3 stops falloff. F8 took care of the problem and after having measured the illuminated area with a incident light meter, it happens to be the most consistent (within 1/10 of a stop). It's fun to really nail things down and the BTZS is helping me do just that.

Thanks again