Aristo VCL 4500 contrast

I’d never had or used an Aristo VCL4500 head but bought one used on ebay. I wanted to check for maximum contrast on the new Ilford MGFB Classic paper using a 4x5 Stouffer 21 step projection wedge. Both blue lamp only, and blue/green mix dialed to 5.0, gave almost identical contrast of 8 and 9 steps respectively when projected. I counted the dry discernable shades between but not including pure black and pure white and remembered to leave off the duplicate step in the middle. So I’m only getting about a grade 2. I did get up to a grade 4 (4 to 5 steps discernable) when contact printed with blue only. The lens is a Schneider Componon S 150mm at F8. Does this mean that the blue lamp needs to be replaced, or have I missed something? I was hoping for grade 5 projected. I see the lamps are available for $260 each but hope not to spend the money if I don’t need to.

Thanks!

You have a lot of flare in you system. Things to check would be lens elements, stray light from the enlarger head, light bouncing from somewhere to the paper, imporoper masking of stray light when projecting the step, etc.

I have the newer reengineered set of blue and green tubes installed in my VCL4500. My unit does not allow the separate dimming of the green tube, so when I set '5' on the control box, it's with both blue and green input active. Even so, I am able to achieve a full grade 4 contrast using a step wedge on Ilford MGIV FB & RC.

I've never tried it, but I suspect that if I manually filtered out the green component, equivalent to turning off the green tube, that I might be able to reach grade 5 with the blue tube by itself.

As I recall, the original tubes in the unit would actually reach a full grade 5 without manual filtering. But unfortunately they were so old that they had dimmed significantly, and regular exposure times were in the several minutes range and unworkably long.

One thing to note for reference. My testing showed the effective speed of the green tube by itself to be approximately 10 times slower than the max blue/green combination. In other words, setting the control to '0' and testing for threshold print exposure gave a value 10 times longer than doing the same with the control set to '5'.

This relationship graphed out relatively linearly, so that it was then possible to create an exposure time conversion factor table for quickly switching between grades while matching the highlight values. Quite a time-saver in the darkroom.

Ken

Yeah I was expecting a minor, if any, contrast reduction by enlarging vs. contact but that's ridiculous. Time to check other factors...but even contact printing I'd hoped for an honest grade 5.

Thanks...

Thanks Ken. I must have the same unit as you in that the green is never dimmable. When in normal (non split) mode, dialing down from 5.0 to 0.0 just dims the blue. Even at 5.0 the green is still full intensity. There's little difference between that 5.0 max blue/green setting and max blue only (in split mode). I got one less step (8 instead of 9) in blue only split mode...usable but not mind blowing.

With my head there is nothing like the kind of speed difference you get between the two lights! It's quite consistent actually.