8mm Forum: How much light is lost?

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Posted by Barrie Didham (Member # 1741) on July 02, 2010, 10:53 AM:

I,ve been looking at the setups of the projectors here with scope lense,s and i was wondering how much light is lost between the front of the lens and the back of the scope lens,i suspect some light must bounce off the back of the scope lens.

Why dont i see a sleeve around these gaps to help keep as much light in as possible to the scope lens,or does this not matter?

There seems to be enough room on the end of most scope lense's to let a sleeve move up or down while you move the zoom from the main lens in or out.

I can see how a tight rubber sleeve would grip the front outer of the main lens so it could be turned with enough play on the end of the scope lens so it could rotate freely and move up and down with out coming off or sifting the scope lens.

Would this cut down on loss of some of the light from the bulb through to the scope lens.

Silly question i know,but someone had to ask it [Big Grin]

Posted by Martin Jones (Member # 1163) on July 02, 2010, 11:01 AM:

The light "scatter" at the front of the lens will always occur whether a scope lens is fitted or not, and is part of the normal very small loss in any set-up.
Sleeving the gap between lens and scope lens serves no useful purpose. If the inside of the sleeve is black it will absorb the light. If it is a light or shiny surface it will direct the scatter back into the scope lens, but not in an ordered fashion, so would actually detract from the projected picture.
Martin

Posted by David Kilderry (Member # 549) on July 03, 2010, 10:28 AM:

Barrie, it is the quality of the lens blooming or blue surface tint that aids in light transmission through the outer elements (lens glass) that is the key to light loss.

The reason you notice a loss of light when projecting scope is the simple fact that the picture size is twice as large. Scope is my favourite format, but it sure takes a lot of light.

David

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