Large Prints, Apertures, Exposure Times, ND Filters...

I have noticed in my printing posts mention of various aspects of setting the base exposure for my prints. It is a mixture of familiar and perhaps not so familiar topics.

I have evolved my darkroom practice and equipment with an eye towards steady improvement. I am new to this and have a long way to go. Being new to this I see some value is recording what I have learned in what is hopefully a clear fashion. A lot of this was learned from different forums and reading what the old-timers have to say. There is a lot of good information out there.

The Enlarger

With respect to my enlarger it has been through a number of changes which I have recorded in this blog over the years. It all started with my humble Meopta 6 enlarger I picked up on eBay for 15 quid and a day out in Norwich. My philosophy when undertaking a new interest or hobby is to start in cheaply and once I see I am sticking with it to invest further.

Big Prints

At some point I wanted to make bigger prints like 12x16 and 16x20. I initially did this by aiming the enlarger against a wall and using a magnetic white board to hold the sheets in place. This worked surprisingly well and made for a compact setup. It was a pain getting the paper in place however. Having avoided the cost for bigger easels for some time I finally relented and found a couple RR Beard easels (12x16 and 16x20). I then needed to make the baseboard larger and raise up my enlarger to get the full size of the 16x20 easel in a print. This project is outlined here.

No Enough Light!

Here I am happily printing big prints and I find a couple of inter-related problems. Bigger prints need more light as it is spread across more area. Light for an 8x10 print on 16x20 is 2 stops (1/4) less. No problem just open the aperture 2 stops or wait 4 times as long. In some cases I might be at f4 or f5.6 or 256 seconds of exposure depending on the paper speed.

Soft Focus Problems

So I first noticed my prints got softer focus with the more open aperture, especially the corners and edges. Just like a camera the enlarger lens has a sweet spot. Too open and it is softer and lacks depth of field. It is more susceptible to problems with negative flatness. Too stopped down and diffraction will soften the image. So I went out and bought better quality lenses as described in this post. I also describe how I upgraded the bulb from 75 watts to 150 watts. This would allow me to address the other problem with stopping down the lens which is exposure times. My aim is typically to be in the range of 22-45 seconds. This allows reasonable dodging and burning times. Too fast and you are forced to work too quickly and mistakes are more easily made.

What About Smaller Prints Now!

Now I have solved those problems however I still wish to make smaller prints and now have too bright a bulb unless I stop the lens down to the point I worry about diffraction effects. Changing bulbs in my enlarger is a real pain so... My solution to this was to add two 1 stop ND filters. Since I use below the lens filtration for VC work my filter tray is empty. I describe here how I cut a couple of ND filters to fit the filter tray. I can now add up to 2 stops of ND in addition to the aperture. Since the lumens difference between 75 watt and 150 watt bulbs is about 1.25 stops I have good overlap and so can actually produce less light than swapping to a 75 watt bulb.

This is my system for staying within the constraints of reasonable exposure times and keeping the aperture in the sweet-spot for best image sharpness.