I also thought the same as you when I first read the post but then I looked at the photos of the Trinar lens, the rear mount of the lens looks to be too big for 25mm plus it looks exactly like the dozens and dozens of enlarger lenses that I sold during the 70'-80's. I then re-read the post and I now preceive that he meant that the enlarger opening is too small. Zehner21, can you measure the opening in your enlarger to give a better understanding of what diameter size lens you need, you may need a 25mm size mount lens or even some other size besides 39mm. Most hobby enlargers made in the last 30 years had the mount size as 39mm but some had no standard size at all. Some hobby enlargers used lens mounting plates while others were just odd sized openings. (odd sized by USA standards, the size may be perfectly normal in whatever country the enlarger was made/sold) A few photos of the enlarger opening would also help. It is entirely possible that I'm way off base with my assumptions, OP please correct me if I'm wrong. RandyB