There has been a lot of chat and controversy about the use of fresnel lenses in combination with the ground glass on large format cameras in attempt to brighten the image formed on the screen thus assisting focusing and composition.
I decided to do a series of experiments ion a 4x5 camera to find out what all the fuss was about.
My findings are as follows:
THE FRESNEL LENS
Most fresnel lenses are plastic and can be bought on ebay. These are designed as reading aids and have a focal length between 5 and 10cm. They can easily be cut to size using scissors or a craft knife.
PLACING THE FRESNEL LENS
There has been much speculation on this topic. After much experimentation trying all four possible posiitions,
1) Camera lens side. Grooves facing back of camera lens smooth side touching screen.
2) Camera lens side. Grooves touching screen smooth side facing camera lens.
3) and 4) Same as above but fresnel lens put on viewer side of ground glass screen.
Because of the machining process in the manufacture of fresnel lenses the lens will not lie completely flat to the ground glass screen and may be found to be bulging slightly in the middle or sides.
A further series of experiments with the fresnel lens being deliberately distorted and moved up to half a mm from the screen were done to find the effects of this on image an focusing.
THE RESULTS
The image using the fresnel lens was noticably brighter in ALL positions.
The accurate focus of the camera was NOT affected by any of the fresnel lens positions.
The bulging of the fresnel lens, up to half a mm, had no effect on focusing or image.
DOWNSIDES
Some fresnel lens grooves were noticable, under loupe magnification especially on lenses over 180mm focal length.



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