There are two different problems that are called “pinholes” in negatives. They’re quite different from each other.
A true pinhole is caused by a small air bubble in the emulsion or a gap in the emulsion. After coating, when the emulsion had dried, it leaves a tiny circular area of no emulsion. Usually, the gelatin overcoat covers it. There’s no emulsion to expose in the spot, so the area stays clear after development. In the enlarger, the light pours through the clear spot with full force exposing a black spot on the paper.
The only true pinholes I’ve ever encountered happed with the Hungarian-made Fortepan 100 and 400 films. Pinholes occurred frequently. I have never seen a true pinhole in Kodak, Ilford, Agfa, Fuji, Konica, or 3M branded films.
The much more common “shadow pinhole” can happen in any camera, but is most often a problem in sheet film or glass plates. This happens in roll-film cameras very rarely when a particle—almost always from the interior of camera that isn’t routinely blown out with a squeeze bulb blower as part of regular maintenance. Particles can enter a camera anytime the body is open to the air: film changing, changing backs, changing lenses.
The turbulence of a reflex mirror snapping abruptly to the shooting position can create a cloud of particles that can stick to the emulsion. This is one of the reasons that many camera makers advise blowing out the camera as part of regular maintenance. Any such particle will block the projected image light from the lens and cast a shadow on the emulsion. When developed, the shadow is a clear spot on the negative. This is often referred to as a “pinhole” even though it’s quite different than a true pinhole.
Users of sheet film and glass plate cameras are particularly plagued with pinholes produced by the shadows of particles or even small fibers adhering to the emulsion at exposure because the film is handled unshielded in the darkroom during the loading of the holder.
Once formed, there are three ways to deal with pinholes of either type:
1. Use a pointed wooden toothpick to apply a tiny amount of opaque material to the top (base surface) of the negative to block the light from the pinhole. This is best done on a light table with strong magnifying eyeglasses or a loupe. The resulting white spot on the print is then spot dyed to match the surrounding area. I prefer to use red WATER-SOLUBLE poster paint because, if I do a poor job of placing the paint, I can wash the negative to remove the paint, dry it and try again. I think it’s a bad idea to place the opaque material on the emulsion side as the emulsion might get damaged. Obviously, if you use an indelible opaque material, you have only one chance to get it right. It’s much safer to use a water-soluble material so that you can wash it off and try again if the first try is unsuccessful. The size of the negative matters. The larger the negative, the easier it is to place opaque material onto the negative. 35mm negatives are too small for this to be practical (at least for me).
2. Make the print with the dark spot projected through the pinhole and use a photographic bleach applied with a toothpick to the black spot followed by washing. Afterwards, the bleached area will have to be spot dyed to match the surrounding area. Retouch Methods, the former maker of Spotone dyes used to make a 2-bottle spot bleaching kit called Spot-Off for bleaching black spots on prints. You can make your own with Farmer’s reducer.
3. On FIBER-BASED PAPER ONLY, use a curved-blade Exacto knife to gently scrape the emulsion just enough to remove the excess density of the black spot and then spot dye to match the surrounding area. There used to be specialized print-etching knives made specifically for this purpose, but the proper blade in the Exacto knife works reasonably well. Knifing the print will necessarily alter the surface. The surface can be made uniform by giving it several coats of light spray lacquer (the entire print gets coated, dull or gloss as you prefer). Knifing black spots is not practical on RC prints.
Of these three methods, #1 is usually the most practical, provided that the negative is large enough. Small negatives require method 2 or 3. In Ansel Adam’s book, The Print, a photo of one of Adams’ print finishers is shown knifing a tiny black spot in the light sky area of an otherwise perfect print. The accompanying text explains the process.