Spotmatic - focusing

[Godfrey DiGiorgi]

On May 1, 2007, at 12:43 AM, mike wilson wrote:

> <snip>plain matte fresnel<snip>
>
> I'm not sure what you mean by this.  Fresnel lenses and matte  
> screens are two different things.  I know that some cameras used  
> Fresnel instead of normal lenses under the screens for various  
> reasons.  Is that it?

Focuing screens look like a thin piece of ground glass, and they were  
once upon a time, but they haven't been made like that for many many  
years. The problem with ground glass is that much of the light  
impinging upon it off center is scattered away from the ocular making  
the center bright and the edges/corners dark. A lens is needed above  
the screen to redirect the light towards the ocular from the corners  
and edges.

There is limited room in an SLR viewfinder for a thick correcting  
lens of this sort if you want a compact SLR body. So a clever idea  
was used ... A fresnel lens is essentially a curved lens surface that  
has been sectioned into a set of concentric rings, flat on one side,  
to provide the same refractive properties. A secondary benefit of the  
fresnel lens is that it reduces spherical aberration.

So most focusing screens made since the middle 1960s are essentially  
a very finely sectioned fresnel lens with a matte finished flat side.  
The focal length of the lens is tuned to provide an even illumination  
to the ocular across its width and height. Focusing aids were  
embedded into the screen (split image rangefinders, microprism  
arrays, etc). Thus the term "plain matte fresnel focusing screen" for  
a simple screen without the focusing aids.

Other designs to achieve the same thing have also been used.  
Hasselblad's "acute matte" screen is a development of technology that  
Minolta introduced first with their XD series cameras in the 1970s:  
it uses a set of moulded microlenses to achieve the required  
collimation. Other screens have been constructed with clear fresnel  
lenses (a non-focusing surface, but very bright), all microprisms  
(very fast to focus manually, but almost useless if the focal length  
of the lens being used deviates out of a narrow range for this the  
microprisms have been tuned), etc etc.

Godfrey