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.

Tilting the easel

Is mainly used for correcting those converging verticals in pictures of buildings. The reason you get sloping upright lines is because the camera has not been held vertical to the view.  View cameras (large format) are the only cameras that can keep the film plane vertical and look slightly upwards at the same time with out this distortion. You can buy a shift lens which has the same ability or do it in Photoshop.

When enlarging you negative you can correct this by using a tiling easel.  Simply lift the easel by the appropriate amount and rest it on a block. Remember that you need to lift the easel on the side that has the converging lines. When doing this you should set the smallest possible aperture providing the greatest depth of field. When making the enlargement, part of the picture may be lost, it is just a case of cropping the picture to make it square. After processing the exposure may be different between the top and bottom this can be corrected by dodging to even up the overall exposure.

Related posts:

Scheimpflug's rule.

Photographys. Bigger again?

To make enlargements over 50 x 60 centimeters requires a different working practice and a large darkroom.

If your negatives allow you to print them at 50 x 60 centimeters this is not the end of the story, you can go bigger with roll sized paper of 65, 100 and even 130 centimeters wide. But to go larger you will need a different method to produce them and a larger negative to start with which will increase the quality of the image projected by the enlarger. Your darkroom will need to be big enough to cope with the increased size of equipment. You will need an area of clear space that the enlarger can project the image onto whether it is the floor or a wall. Of the former the latter is the better way of doing things as you are less likely to get dust on the paper, it also makes it easier to dodge and burn if you need to.

Enlarging the negative. How big?

Depends on how crisp the negative is! If it is razor sharp there is no reason why you cannot enlarge it to its maximum. A thirty-five millimeter negative should be able to produce a 24 x 30 centimeter print with ease and in exceptional cases 50 x 60 cm. To check if the negative is of a good enough quality, enlarge part of the frame to 18 x 24 centimeters, if it looks a bit grainy the chances are that when the whole negative is enlarged to 50 x 60 cm and viewed from a meter away it will look good.

When enlarging to larger sizes you need to take into account the distance that people will see the picture from. Large photos are rarely viewed up close.