Flare and contrast-lowering stray light - what is it, exactly?

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Auf Reisen Regular Member • Posts: 492
Flare and contrast-lowering stray light - what is it, exactly?

So. Flare and stray light. What is it, precisely? I know how it manifests in photographs whenever there is a bright light source either in or close to the frame. I categorise them as two different phenomena, but they might have the same cause:

  1. Bulb- or crescent-shaped artefacts in pictures, often green or blue, often duplicated as different elements in the lens are hit.
  2. Lower contrast across the frame. This is the phenomenon I am especially interested in.

There are a couple of things I am confused about. First of all: what exactly causes flare and lower contrast in these situations? I know that scattered light can make its way to the sensor by being reflected by the inner tube of the lens body. I suppose this is what a hood mainly safeguards against, as it diminishes the opening through which light can enter the tube of the lens. But what about light outside of the frame that hits the foremost optical element?

I ask because I often see photographers with square filters mounted in front of their cameras where the foremost filter element is hit by a lot of light coming from outside the frame of the photograph. I suppose that some of this light is scattered into the inner tube of the lens, but what about the light that is reflected outwards?

I suppose the following thought experiment illustrates my confusion: Suppose you have a camera, a lens, and a hood fitted to the lens. You know install high-quality flat optical glass in front of the hood. The glass is hit by light outside of the frame of the photograph. Does this cause image degradation, especially lower contrast? I know that any glass element in the optical path takes away from the final image, ignore this for a second.

Now compare this arrangement to an installation where the optical glass is fitted between the front element of the lens and the hood. Suppose that the instalment is completely tight. Does this make for a noticeable difference?

The answer here would make a difference for how I approach using CPL filters. I currently use a larger CPL on a step-up ring on many of my lenses. The arranges shields the front element of the lens pretty well in some cases, but the CPL itself is exposed to a lot of light. Because I use step-up rings I often can't use the lens's hood. I have a screw-in hood that I can screw on front of the CPL but that is not an ideal solution for several reasons.

Also, do lens hoods in the shade have any noticeable effect on image quality?

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