View Full Version : Contact Printing

19-Aug-1999, 04:09
attention contact printers! when i put a negative in my printing frame and lock the back, there are Newton rings between the glass and the negative. lots of the m. lots of colorful little asymmetrical little designs. does anyone know what is causing this? i've cleaned the glass, first with glass cleaners and then with s oap and water; the rings are still there. i've put negatives which i've processe d at different times in, and the rings are still there. i've never seen these ri gns in my prints before; now they appear in my prints. does anyone have any sugg estions?

Struan Gray
19-Aug-1999, 06:50
Newton's rings are caused by interference between light waves reflected from the bottom of the glass and the top of the film. The cleaner and flatter your glass is, the more likely you are to see them, since they require smoothly-varying glass-film gap to really show up well.

There are four ways to get rid of them. The first is to make sure that the glass-film gap is constant across the whole frame. In practice that means increasing the clamping force of your frame and/or using a foam spacer behind the film/paper sandwich to ensure that the pressure is even.

The second soution is to coat the film and/or glass with a spacer powder which has a small, consistent grain size. The powders range from the custom and exotic to the simple and homely - cornflower is often recommended in the latter case, but God help you if you get the negative damp.

The third solution is to make the glass surface rough on a lengthscale comparable to the wavelength of light - this is what is done to make anti-Newton glass. In that case the areas of equal interference are no longer nice smooth contours and you don't see any rings. You do get a slight amount of diffusion though. Incidentally, the emulsion side of the negative is often rough enough in itself - which is why you mostly don't need AN glass on both sides of an enlarger negative carrier.

The fourth solution is to fill the glass-film gap with an oil that has a refractive index closely matched to the glass and/or film. This kills the reflection(s) and hence the Newton's rings. This is impractical for contact printing, but is done is high-end enlarging of sheet film.

Some people recommend using a diffuse light sources to combat Newton's rings, as the many-angles of incidence jumble the interference pathways, but for contact printing it is not effective enough. The easiest way to assess the various anti-Newton measures is to use Newton's original method of producing the rings: a shallow glass dish placed on a flat glass surface. Look down through the dish and you'll see the rings surrounding the contact spot between the two pieces of glass. Now try to make them go away.

Peter Olsson
19-Aug-1999, 08:10
Rather peculiar it seems that Tmax films gives Newton rings more easily than more old-fashioned films, at least in a glass film carrier and perhaps in contact printing too?

Struan Gray
19-Aug-1999, 10:13
The emulsion side of processed Tmax seems to be much smoother than other films, and I've seen reports of Newton's rings where the emulsion touches a glass carrier. I haven't seen it myself, but then I only enlarge from 35 mm and 6x6, where negative curl is more significant and presumably is enough to keep the emulsion off the bottom glass. Some people recommend using AN-glass top and bottom and living with the slight diffusion that results.

In contact printing there is at least a theoretical danger that you will get Newton's rings around points where the negative and paper come into contact. A matt-finish paper might help. I was recently given a splendid antique frame with 1.5 cm-thick glass and a spring of Schwarzenegger strength - no rings for me!

Jan erala
19-Aug-1999, 12:06
In contact printing I taped a clear negative preserver (or a plastic foil for overheads) on the glass. I can't see any degration of this in picture quality, and I got rid of newton rings. In the enlarger you can use a thin cardboard cut to film size, so the film will not touch the glass or, an antinewton glass on the upper side. You can't use an-glass in contact printing, the pattern of it will be visible.

Nze christian
21-Aug-1999, 14:44
you can avoid it easily by putting a mylar paper between your negative and the glass. take a 1mil mylar and you won't lose definition. and check the pressure of your contact frame