Newton's rings. Updated

This effect can happen when using medium format and larger negative carriers as they use glass to keep the negative flat. Most carriers today use a frosted piece of glass above the negative known as anti Newton glass. 

The rings occur in the areas where two clear smooth flat pieces of glass lie next to each other but do not quite touch. The interference happens because the light rays are refracted, this produces irregular shaped dark rings in the projected image. This will happen more readily if any of the surfaces are wet. You can get round this by using a paper mask with the negative in the carrier. This will block out the light in the places where there is a gap between the glass surfaces. You will need to check the projected image to see if they are still visible.

Types of Enlarger Negative carriers

There are two main types of negative carrier, with and without glass. There are arguments for and against each type. Lets be honest there are three types, the adjustable.

Glass-less carriers avoid the Newton rings effect (a post for a later date) but also have the disadvantage of allowing the negative to buckle when they warm up, this is more pronounced with 6x6 negatives. To minimize this you should be very careful when focusing and use a smaller aperture to increase the sharpness across the negative.

Another disadvantage of a glass carrier is dust; to be honest it is not the big deal that everyone makes it out to be. It is just a case of being methodical in your approach. The main advantage of glass carriers is that they keep the negative flat. Another plus is that you can experiment with unusual effects by using various materials such as flower petals, salt crystals and soap suds etc in the carrier.

There are adjustable carriers also, which are very handy as they allow you to mask out badly illuminated edges without having to increase the magnification, or you can adjust them to include the rebate of the negative.