A student asked how to shrink negative images using an enlarger. I had never done it, or considered it, so I did a little research.
There isn't much on the internet about it and I don't recall reductions being mentioned in any 'how to use an enlarger' guides I've perused over the years.
The best and most recent snippet I was able to find on the 'net was on the photrio.com forums here. Kudos to 'gorbas', 'ic-racer', and 'dan fromm' for their explanations, diagrams (which have been reproduced here), and questions.
It seems the basic idea is to flip the normal functions of using the lens stage control for focusing and the elevation control for size (e.g. use the lens stage for size and elevation for focus).
What was missing from the discussion, it seemed, was for many enlargers the easel must also be raised. 'ic-racer' seemed to have hinted at that with the comment "All enlarger head heights that are less than 4x focal length will not produce a focused image.".
'ic-racer's diagrams are below:
I personally do not have access to an enlarger with what seem like very long lens stage bellows in the above diagrams.
On my Beseler 23C II enlarger with 50mm lens I was able to create an image sized approx. 11/16 inch by 7/16 inch (50 by 36 points, or about the size of a American dime coin) from a 35mm negative.
I lowered the lens board as far it would go, lowered the entire head elevation as far as it would go, and raised the easel approx. 1 1/2 inch. From there a head elevation raise of approx. 1/2 inch brought the image into focus. A smartphone photo is below.
I was able to increase the size by raising the lens board about halfway, raising the easel another 1/2 inch and re-focusing by raising the head elevation. This produced an image slightly smaller than what a contact print would produce (1:1).
I may attempt to flip the lensboard in the future to see how that affects the size. Hopefully this information helps others.