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.

Angles of view with different lenses..

The diagram shows angle of view or angle of acceptance.

I have been looking back through my college notes and came across this series of pictures. I used the colour film you find in the pound/ budget shops. I have had no problems with the way the film has performed.

These pictures show how much of the view in front of the lens is depicted at the negative. As the focal length of the lens increases the angle of view reduces but the object size gets bigger. Therefore as you go up the focal range so the depth of field lessens.

View at 35mm

View at 50mm

View at 80mm

View at 135mm and the cold is getting to me.

View at 210mm

Choosing an enlarger.

This is the most important tool in the darkroom after the enlarging lens. With this in mind you need to consider the following: 

1.   The number of negative sizes you may wish to print. It is a good idea to get a multi format enlarger even if you are only going to use 35mm.

2.   The most appropriate lens size and quality.

3.   What type of negative illumination. Diffuser or Condenser.

4.   The maximum size of print you are likely to make.

5.   Whether you may want to do colour at a later date. Colour headed enlargers can be used with multigrade papers. 

As mentioned in “3” the type of negative illumination you choose is worth  a lot of consideration -  there are positive and negative points to both. Here are some of the pros and cons: 

Diffuser enlarger: 

Diffuser light box

This type of enlarger design is used with colour and multigrade heads. The light travels through a mixing box and semitransparent screen above the negative. To counteract the drop off in light, these enlargers use a powerful quartz-iodide bulb.  This multi directional light passes through the negative and down to the paper. The affect of this will produce a gentler, softer quality to the  light, producing a less contrasty grade for grade photograph. As a result damaged and flawed negatives lose or soften some of their faults. 

There are tonal differences between diffuser and condenser produced photographs because of the way light passes around the silver particle's. This is negated when using Chromogenic monochrome and colour films as they rely on dyes to capture the light. 

Condenser enlarger: 

Condenser light box

Uses a plano-convex lens which spreads a bright hard illumination  evenly across the negative. Supplied from a opal tungsten lamp. The harshness of this light produces a contrast enhancement that appears to make  fine detail more exaggerated. This crisp appearance to the photograph has the negative affect of bringing into sharp focus the grain, any scratches, flaws and dust from the negative, meaning more time spent on retouching. These enlargers are subject to the Callier affect this is where the highlights in the negative scatter the light more than the shadow areas creating the increase  in contrast.