An enlarger head with a built in ND (Neutral Density) filter, can be very handy and is a great darkroom tool for both B&W and colour.
My De Vere 504 free standing De Vere runs a MK IV single lamp head, it does not have a built in ND filter. On the left of the head it has a filter dial, this moves all of the filters out or into the light beam, nothing else.
With most, if not all enlargers I have used with an ND filter, all had an ND filter with 60 units of ND. Every 30 units is 1 stop, meaning you have 2 stops of ND.
In practical usage I find it a most versatile tool when doing multiple sizes from the same image. Theoretically, if you do an 8x10” print from a 4x5” negative with the ND feature at 30 units at say f/8 for 20 seconds. You then decide to use 16x20” paper and frame the same. You then remove the ND filter to zero, gaining a full stop of light and make your next (larger) print at f/8 for 20 seconds. If you wish to make smaller enlargements from whatever original size you have been successful with, you would be adding ND to keep the exposing time the same as the larger print.
The theory is great, but in practice, the larger or smaller you go, there are almost always very slight differences in density control on a print. To add to the complexity of the process you may find you could be adding a ¼ of a grade of contrast, or maybe slightly less, say an eighth of a grade difference to make the larger print look the same as the smaller print.
This keeping your exposure time the same, as you move up and down the enlarger column was reasonably important with certain colour printing processes, having an ND filter was the best possible thing, but it was often over emphasised as to how important it was. There was a difference, but really very, very slight unless the print size difference was huge. I’m talking of going from a 1m wide print to say 4m to 5m wide. Then the time difference is vastly different; the differences we do in our home darkrooms is almost impossible to detect.
If you don’t have an ND filter on your enlarger, it is really simple to add your own ND by adding say 30 units of each (all three) colour into the head before you start enlarging. In practice it works like this.
Grade 3 on my enlarger is Y30 M90 C00. To add ND I simply move all the dials up by 30 units, which would now read Y60 M120 C30. If I was using f/11 at 20 seconds and moved up a paper size I can change the filtration to read Y30 M90 C00 and still use f/11 at 20 seconds.
Hope that helps.
In regard to obtaining the smaller light mixing chamber for your head, unless you were going to do screamingly big enlargements, I don’t think it is normally necessary. I worked in a De Vere heaven at one stage, we had around 5 free standing 504 units a couple of 507 units and two 10” x 10” horizontal De Vere units for mural enlarging on rails.
The only time we switched to really correct mixing chambers was when doing extreme enlargements with 35mm film. This was when one was literally on your hands and knees focusing with the front wheels and with the baseboard scraping the floor while the enlarger head was literally making marks on the darkroom ceiling. In other words using a glass negative holder and doing slightly wider than 1.2m wide prints. If my memory is correct, the difference is slightly more than a third of a stop more light, with the smaller mixing chamber. I measured this difference on my own enlarger using the exposure mode of my Lici colour analyser, which is extremely accurate.
I’ve just been into my darkroom to see just where my smaller mixing chamber is, still where I left it many years ago. I run between 35mm and 4x5” enlarging all of the time, sometimes from one neg to another I do a lens change then into it. It would be nice to have for extremes, but if you are thinking of moving your De Vere on in a year, I don’t see why you should get one.
By the way, if you move your De Vere enlarger on, unless you get something extremely delicious, I think you may regret that decision.
Mick.