bright screens

[Karo from Moscow]

When I asced about superscreens I interested mostly how they work with extremely short superwide lenses (55-75mm) on 4"x5" film .Thanks everybody!

[N Dhananjay]

Fresnel screens act as field lens - they redistribute the light. They are at their best with wide angles. Most of the darkness on the ground glass is because light rays strike the ground glass at different angles - imagine rays of light emanating from the centre of the lens, these rays will be perpendicular to the center of the ground glass but will strike the ground glass at an angle in the corners. With longer focal length lenses, these angles are relatively small and the illumination on the ground glass is quite even. With wide angle lenses, these angles can be quite large which is why the centre looks bright while the corners will look dark. If you move your head to look towards the lens from the corner, you will find the corner is bright (because the light ray is now perpendicular to your eye again) but the center will be dark. This unevenness is illumination is mitigated to a certain extent with fresnel lenses. See http://lighthousegetaway.com/lights/fresnel.html and http://www.howstuffworks.com/question244.htm for more details on how fresnels help. Hope this helps. Cheers, DJ

[Chris Ellinger]

Bill Maxwell can explain the problem, and probably provide a screen optimized for short lenses.

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