Fresnell lens

[LJH]

Both of my Shen Hao's have a recess on the front side for a Fresnel so as to keep the GG in its original location.

 

[Rudolf Karachun]

By the way, dear friends. This is a nice and short article with pictures, about the Fresnel lens, how it is made and work.

http://en.wikipedia.org/wiki/Fresnel_lens

Please read it and try to explain me how it works, when the lens installed after the ground glass (mean between the ground glass and photographer) and with grooved surface of the Fresnel facing the ground glass.

In my opinion it is understandable how the Fresnel works when installed before the ground glass (mean between lens and ground glass) and with grooved surface facing the ground glass. It just sending a parallel ray of light thru the ground glass. We all know that the ground glass by itself a diffuser. So with the Fresnel before, it seems like less light will be diffused by the ground glass, and image what photographer see on the surface of the ground glass will be much brighter then without the Fresnel.

But what the Fresnel really do when located after the ground glass with grooved surface facing the ground glass?
Thank you.

 

[AgX]

The fresnel as any lens is of NO effect on the dispersion of the ground glass. This a fixed characteristic of the groundglass.
A lens hower is of influence on the overall exit angle of the complete groundglass.

The brightness of a groundglass can't be influenced by any lens. A lens though will influence the brightness-spread when viewed under a given viewing angle with respect to the groundglass.

 

[Rudolf Karachun]

I was trying to understand something, but sorry, didnt.